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CLOSE THIS BOOKFood from Dryland Gardens - An Ecological, Nutritional, and Social Approach to Small Scale Household Food Production (CPFE, 1991)
Part II - Garden management
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5. How plants live and grow
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT5.1 Summary
5.2 The vascular system in plants
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT5.2.1 Roots
VIEW THE DOCUMENT5.3 Photosynthesis
VIEW THE DOCUMENT5.4 Transpiration
VIEW THE DOCUMENT5.5 Coping with heat and drought
VIEW THE DOCUMENT5.6 Salt tolerance
5.7 Seasonal constraints to plant growth
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VIEW THE DOCUMENT5.7.1 Daylength Requirements
VIEW THE DOCUMENT5.7.2 Temperature Requirements
VIEW THE DOCUMENT5.8 Resources
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6. Growing plants from seeds
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VIEW THE DOCUMENT6.1 Summary
6.2 Sexual reproduction in plants
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VIEW THE DOCUMENT6.2.1 Life Cycles
VIEW THE DOCUMENT6.2.2 Flowering
VIEW THE DOCUMENT6.2.3 Pollination
VIEW THE DOCUMENT6.2.4 Fertilization
VIEW THE DOCUMENT6.3 Seed germination and dormancy
6.4 Suggestions for planting seeds under dryland conditions
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VIEW THE DOCUMENT6.4.1 Preparing the Seeds
VIEW THE DOCUMENT6.4.2 Preparing the Planting Site
VIEW THE DOCUMENT6.4.3 Planting the Seeds
VIEW THE DOCUMENT6.4.4 Planting Density
VIEW THE DOCUMENT6.4.5 Covering the Seeds
6.5 Caring for newly planted seeds and young seedlings
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VIEW THE DOCUMENT6.5.1 Watering
VIEW THE DOCUMENT6.5.2 Mulching and Shading
6.6 Diagnosing seed planting problems
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VIEW THE DOCUMENT6.6.1 Testing Seed Germination
VIEW THE DOCUMENT6.7 Thinning
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7. Vegetative propagation
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VIEW THE DOCUMENT7.1 Summary
7.2 Cuttings
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VIEW THE DOCUMENT7.2.1 Trees
VIEW THE DOCUMENT7.2.2 Perennial Herbs
VIEW THE DOCUMENT7.2.3 Cassava.
VIEW THE DOCUMENT7.2.4 Sweet Potatoes
VIEW THE DOCUMENT7.3 Tubers, tuberous roots, and bulbs
VIEW THE DOCUMENT7.4 Offsets
VIEW THE DOCUMENT7.5 Suckers
7.6 Grafting
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VIEW THE DOCUMENT7.6.1 Compatibility for Grafting
VIEW THE DOCUMENT7.6.2 Effects of Stock and Scion on the Grafted Tree
VIEW THE DOCUMENT7.6.3 Approach or Attached Scion
VIEW THE DOCUMENT7.6.4 Budding
VIEW THE DOCUMENT7.6.5 Apical Grafting
VIEW THE DOCUMENT7.6.6 Topworking
7.7 Layering
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VIEW THE DOCUMENT7.7.1 Simple Layering
VIEW THE DOCUMENT7.7.2 Air Layering
VIEW THE DOCUMENT7.8 Resources
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8. Plant management
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VIEW THE DOCUMENT8.1 Summary
8.2 Nursery beds and container planting
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VIEW THE DOCUMENT8.2.1 Nursery Beds
VIEW THE DOCUMENT8.2.2 Container Planting
VIEW THE DOCUMENT8.2.3 When Direct Planting is Better
VIEW THE DOCUMENT8.3 Planting sites and the sun
8.4 Transplanting
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VIEW THE DOCUMENT8.4.1 Timing
VIEW THE DOCUMENT8.4.2 The Site
VIEW THE DOCUMENT8.4.3 Water
VIEW THE DOCUMENT8.4.4 The Transplant
8.5 Plant interactions
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VIEW THE DOCUMENT8.5.1 Mixed Planting
VIEW THE DOCUMENT8.5.2 Allelopathic Plants
VIEW THE DOCUMENT8.5.3 Crop Rotation
8.6 Weed management
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VIEW THE DOCUMENT8.6.1 Resource Use
VIEW THE DOCUMENT8.6.2 Effects on Pest Populations
VIEW THE DOCUMENT8.6.3 Timing
VIEW THE DOCUMENT8.6.4 Methods of Weed Control
8.7 Pruning
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VIEW THE DOCUMENT8.7.1 Reasons to Prune
VIEW THE DOCUMENT8.7.2 Guidelines for Pruning Trees
VIEW THE DOCUMENT8.8 Trellising
VIEW THE DOCUMENT8.9 Resources
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9. Soils in the garden
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VIEW THE DOCUMENT9.1 Summary
9.2 Soil and land-use classification
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VIEW THE DOCUMENT9.2.1 Indigenous Classification Systems
VIEW THE DOCUMENT9.2.2 The USDA Classification of Soils in Drylands
9.3 Physical properties of soils
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VIEW THE DOCUMENT9.3.1 Soil Texture and Structure
VIEW THE DOCUMENT9.3.2 Soil Porosity and Permeability
VIEW THE DOCUMENT9.3.3 Soil Color
VIEW THE DOCUMENT9.3.4 Soil Temperature
VIEW THE DOCUMENT9.4 Soil profile and depth
9.5 Soils and plant nutrients
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VIEW THE DOCUMENT9.5.1 Soil pH and Plant Nutrition
VIEW THE DOCUMENT9.5.2 Nitrogen
VIEW THE DOCUMENT9.5.3 Phosphorus and Potassium
VIEW THE DOCUMENT9.5.4 Other Nutrients
9.6 Organic matter
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VIEW THE DOCUMENT9.6.1 Animal Manures
VIEW THE DOCUMENT9.6.2 Composting
9.7 Preventing soil erosion
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VIEW THE DOCUMENT9.7.1 Decreasing Runoff
VIEW THE DOCUMENT9.7.2 Decreasing Raindrop Impact
VIEW THE DOCUMENT9.7.3 Increasing Soil Resistance to Erosion
VIEW THE DOCUMENT9.7.4 Reducing Wind Erosion
9.8 Building garden beds
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VIEW THE DOCUMENT9.8.1 Sunken Beds
VIEW THE DOCUMENT9.8.2 Raised Beds
VIEW THE DOCUMENT9.9 Resources
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10. Water, soils, and plants
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT10.1 Summary
VIEW THE DOCUMENT10.2 Dryland garden water management
10.3 Water, soils, and plants
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VIEW THE DOCUMENT10.3.1 Water Storage in the Soil
VIEW THE DOCUMENT10.3.2 Water Movement in the Soil
VIEW THE DOCUMENT10.3.3 Evaporation
VIEW THE DOCUMENT10.3.4 Water Uptake and Transport by Plants
VIEW THE DOCUMENT10.4 Soil water and garden yield
VIEW THE DOCUMENT10.5 How much water?
VIEW THE DOCUMENT10.6 Measuring water applied to the garden
VIEW THE DOCUMENT10.7 When to water
10.8 Mulches, shades, and windbreaks
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VIEW THE DOCUMENT10.8.1 Surface Mulches
VIEW THE DOCUMENT10.8.2 Vertical Mulches
VIEW THE DOCUMENT10.8.3 Windbreaks, Shades, and Cropping Patterns
VIEW THE DOCUMENT10.9 Resources
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11. Sources of water for the garden
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VIEW THE DOCUMENT11.1 Summary
VIEW THE DOCUMENT11.2 Water quality for plants
VIEW THE DOCUMENT11.3 Water quality for people
11.4 Rain
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VIEW THE DOCUMENT11.4.1 Rainfall Records
VIEW THE DOCUMENT11.4.2 Measuring Rainfall
11.5 Harvesting rainwater for dryland gardens
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VIEW THE DOCUMENT11.5.1 Patterns of Water Harvesting
VIEW THE DOCUMENT11.5.2 Building on Local Knowledge
VIEW THE DOCUMENT11.5.3 Catchments and Runoff
VIEW THE DOCUMENT11.5.4 Estimating the Catchment to Garden Area Ratio
11.6 Harvesting stream flow and floodwater
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VIEW THE DOCUMENT11.6.1 Water Spreading
VIEW THE DOCUMENT11.6.2 Flood Recession Gardening
11.7 Groundwater and wells
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VIEW THE DOCUMENT11.7.1 Groundwater
VIEW THE DOCUMENT11.7.2 Locating a Well
VIEW THE DOCUMENT11.7.3 Hand-Dug Wells
VIEW THE DOCUMENT11.7.4 Small-Diameter Wells
VIEW THE DOCUMENT11.8 Water storage
VIEW THE DOCUMENT11.9 Resources
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12. Irrigation and water-lifting
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VIEW THE DOCUMENT12.1 Summary
VIEW THE DOCUMENT12.2 Irrigation efficiency
12.3 Surface irrigation
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VIEW THE DOCUMENT12.3.1 Transporting Water to the Garden
VIEW THE DOCUMENT12.3.2 Basin Irrigation
VIEW THE DOCUMENT12.3.3 Furrow Irrigation
VIEW THE DOCUMENT12.3.4 Trickle Irrigation
12.4 Root zone irrigation
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VIEW THE DOCUMENT12.4.1 Pitcher Irrigation
VIEW THE DOCUMENT12.4.2 Water Table Irrigation
VIEW THE DOCUMENT12.5 Sprinkler irrigation
12.6 Irrigation problems
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VIEW THE DOCUMENT12.6.1 Waterlogging
VIEW THE DOCUMENT12.6.2 Salinity
12.7 Water-lifting
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VIEW THE DOCUMENT12.7.1 Lifting with Human and Animal Power
VIEW THE DOCUMENT12.7.2 Lifting with Other Power Sources
VIEW THE DOCUMENT12.8 Resources
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13. Pest and disease management
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VIEW THE DOCUMENT13.1 Summary
13.2 An ecological approach
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VIEW THE DOCUMENT13.2.1 Pest and Disease Management by the Crop Plant
VIEW THE DOCUMENT13.2.2 Environmental and Mechanical Management of Pests and Diseases
VIEW THE DOCUMENT13.2.3 Pest and Disease Management Using Other Organisms
VIEW THE DOCUMENT13.2.4 Pest and Disease Management with Chemicals
13.3 Examples of pest and disease management
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VIEW THE DOCUMENT13.3.1 Insects
VIEW THE DOCUMENT13.3.2 Nematodes
VIEW THE DOCUMENT13.3.3 Large Animals as Pests
VIEW THE DOCUMENT13.3.4 Diseases
13.4 Diagnosing pest and disease problems
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT13.4.1 Wilts (Table 13.1 and Figure 13.26)
VIEW THE DOCUMENT13.4.2 Leaf Problems (Table 13.2 and Figure 13.27)
VIEW THE DOCUMENT13.4.3 Abnormal Growth (Table 13.3 and Figure 13.28)
VIEW THE DOCUMENT13.4.4 Fruit Problems (Table 13.4 and Figure 13.29)
VIEW THE DOCUMENT13.5 Resources
VIEW THE DOCUMENTReferences

Food from Dryland Gardens - An Ecological, Nutritional, and Social Approach to Small Scale Household Food Production (CPFE, 1991)

Part II - Garden management

The interaction of many factors influences the health and productivity of plants in the garden. Among these are the microclimate, soil, water, microorganisms, neighboring plants, and the genetic inheritance of the plant itself.

There are also many ways to propagate and care for plants, and more than one technique can produce the specific results desired. For example, a gardener wanting to keep her garden free of weeds could spray an herbicide, mulch heavily, or remove the weeds by hoeing. If she wants to start seedlings for transplanting she could purchase specially manufactured, imported seed boxes, use locally made containers, or ones she made herself from free materials. The different techniques in these two examples will all produce the desired short-term results: weeds can be controlled by spraying, mulching, or hoeing; seeds can be started in specialized boxes, local containers, or home-built ones. Because there are many ways to accomplish these and other garden management tasks, the choice of technique must be based on other criteria as well.

If the main goal of garden management is to maximize production and profit, and the required resources are available, then the industrial agriculture model may be effective (Part I). Large-scale, industrial agriculture has been very successful in increasing production and yields by greatly increasing the use of machinery and the fossil fuels to run them, chemical fertilizers and pesticides, and irrigation water. Compared with small-scale, indigenous agriculture it has much higher returns to labor, but much lower returns to energy (section 3.2.1). Industrial agriculture is based on increasing centralization of management and marketing, and on increasing control over nature, rather than working with nature.

Genetic diversity in crops, ecological diversity in fields and regions, and social diversity in management has been drastically decreased in the drive to increase production (section 14.2). In turn, this lack of diversity results in decreased sustainability, because industrial systems are less and less capable of maintaining their high levels of production when challenged by drought, shortages of irrigation water, a break-down in the fertilizer distribution network, increasing oil prices, or outbreaks of pests and diseases.1 The typical response of industrial agriculture to such problems is to attempt to increase control over nature and to centralize the system even more.

Experience has shown that the industrial approach to food production often results in increasing inequity because the capital and resource requirements are beyond the means of many Third World households. This approach has also been found to be harmful to the environment and to human health.

The criteria for selecting garden management techniques which we use in this book are self-reliance and local control of the food system; equal distribution of food for improved human health and nutrition; preservation of biological and cultural diversity; and conservation and protection of natural resources. This is why the approach we take to garden management in the Chapters of Part II is quite different than the approach of industrial agriculture, and reflects a growing interest in sustainable agriculture.

The term sustainable agriculture is widely used today to describe agriculture that has the goals of conserving the environment for the future and providing nutritious food for all people equitably (section 1.2). There is increasing awareness in both industrial nations and the Third World of the need to conserve resources for the future. Decreasing profits resulting from environmental degradation such as groundwater depletion and soil erosion, and consumer pressure for a more healthy food supply, are pushing industrial agriculture toward sustainability (section 3.2). For example, the United States’ National Academy of Sciences has published a major book titled Alternative Agriculture which advocates moving that country’s agriculture away from high inputs of chemical pesticides and fertilizers.2 Agriculture is increasingly being studied from an ecological perspective.3 However, because the concept of sustainability has become so popular, it is sometimes used in ways that distort its meaning and make it subservient to production economics.4

In the Third World there is also interest in reorienting agricultural development away from the industrial model of the green revolution and toward sustainability.5 Detailed descriptions of indigenous agricultural systems contribute to a growing appreciation of their ecological (environmental) and social sustainability.6 However, in many of the World’s poorest communities population pressure, social disruption and incorporation into the world economic system have made indigenous agriculture environmentally destructive, socially inequitable, or both. A redistribution of resources from the rich, industrial sector is essential for those poor communities to create sustainable agricultural systems. Ultimately, a sustainable agriculture must be one that supports an end to growth of the human population, to our increasing levels of consumption, and to cultural and environmental destruction.

The Chapters in Part II discuss methods of dryland garden management based on a striving for ecological and social sustainability in its fullest sense. This means making the most of the indigenous knowledge, ecological and social diversity, and locally adapted biological resources which characterize many dryland food systems, while using the knowledge and techniques of Western science to enhance sustainability.

References

1 Cleveland and Soleri n.d.c.

2 NAS 1989a.

3 E.g. Carroll, et al. 1990; Cox and Atkins 1979; Gliessman 1990.

4 Cleveland 1991; Orr 1988.

5 E.g., AGRECOL/ILEIA 1988; Dupriez and De Leener 1983.

6 E.g., Lagemann 1977; Richards 1986: Westphal, et al. 1981, 1985.


Figure 5.1 Plant Anatomy - the Chili, a Dicot

5. How plants live and grow

Knowing how plants live and grow enables gardeners to adjust management practices according to specific local situations and helps them solve problems in the garden. For our discussion of how plants live and grow we use the terminology of Western science. However, many local systems exist which serve the same purpose, using terms and concepts developed through people’s experiences. These local systems are also valid; appreciating and attempting to understand the local system is essential for working with gardeners.

5.1 Summary

This chapter begins with an illustration of basic plant anatomy (Figure 5.1). The vascular system transports food, water, minerals, and other essential substances throughout the plant. Photosynthesis and transpiration provide the plant with the food energy necessary to live, grow, and produce a harvest. Under hot, dry conditions the rate of water loss from the plant increases and can lead to water stress that reduces yields. Plants have evolved a variety of responses to help them survive under these dryland conditions. Some plants also have a tolerance of salty soil, a common problem in drylands. Many crops or crop varieties also have daylength and temperature requirements that can limit their growing seasons.

5.2 The vascular system in plants

The vascular system is the network of plant cells responsible for the movement of water, minerals, food (sugars), hormones, and other vital substances inside plants.

Water in the soil is taken up by the roots through a combination of osmosis and cohesion. Osmosis is the pattern of water movement across a water-permeable membrane such as the cell membrane. If two liquids are separated by such a membrane, water will move out of the more dilute solution, the one with a lower concentration of solutes like salt, and into the more concentrated solution (Figure 5.2). This movement will continue until both solutions have the same concentration of solutes per volume of water. If the concentration of solutes is greater in the root cells than in the soil, water will move into the roots. Water loss from transpiration increases solute concentration in the leaves and so water continues to be pulled up through the plant by osmosis.


Figure 5.2 Osmosis

Cohesion is the tendency of like substances to stick together. The cohesion of water molecules, together with transpiration and osmosis, causes a continuous flow of water to move up the plant. Once the soil moisture is depleted to the wilting point (section 10.3.1) osmosis and cohesion will no longer be strong enough to move water out of the soil and into the plant.

Dicots and monocots are the two major groups of garden plants. Their vascular systems are arranged differently. Dicots are those plants such as beans, cucurbits, amaranths, and many fruit trees which have two cotyledons, or seed leaves, in their seeds, and branching leaf veins. Monocots have only one cotyledon and usually the veins in their leaves are parallel to each other, running the length of the leaf as in maize, onions, date palms, and most cereals. In larger seeds the difference between a monocot and a dicot is obvious. For example, a bean seed can be easily split into two halves, the cotyledons. A maize seed, however, does not split because it has only one small cotyledon.

The xylem is the part of the vascular system that carries water and nutrients from the roots to the leaves. In monocots the xylem tissues are scattered in bundles that run the length of the plant, throughout the leaves, stems, and roots. In dicots the xylem tissues occur in a discrete layer, which in the stem surrounds the pithy center. In dicot roots the xylem is the tissue at the core (Figure 5.3).

The sugars made by photosynthesis (section 5.3) and many growth-regulating hormones produced by plants’ growing tips flow through the phloem. Osmosis is also thought to be the source of movement for substances in the phloem. As the concentration of sugars produced by photosynthesis increases in the phloem, water from the xylem enters these cells, building up pressure within them. This forces movement of the solution to cells with lower concentrations and pressure until it reaches a place where the sugars are needed or can be stored for later use. Because most photosynthesis occurs on the outer and upper layers of the plant, those leaf areas exposed to sunlight, the movement of solutions in the phloem is primarily inward toward the main stem and downward to the roots where there is little or no photosynthesis. Sometimes the fluids in the xylem and phloem are called sap.

In monocots the phloem and xylem tissues are grouped together in vascular bundles running vertically through the plant. In dicots the phloem is a distinct layer separated from the xylem by a thin layer of cambial tissue (Figure 5.3). These continuous layers of phloem and cambial tissue make grafting and layering of dicots possible (sections 7.6 and 7.7), whereas with monocots these techniques are not possible.

The outer surface of green plant parts is the epidermis. Underneath the epidermis in green shoots and stems lies the cortex, tissue that surrounds the vascular system. In dicot trees the outer layer of the trunk and branches is called bark, a term that refers to all of the tissue from the cambium and phloem to the outer surface. In bark the cortex and epidermis are replaced by a more rigid, woody tissue called the cork, which includes a layer of dead cells on the outer surface.

5.2.1 Roots

Even though they are not usually visible, the roots are one of the most important parts of a plant. Roots provide structural support by anchoring plants in the soil, and they absorb water and nutrients in the soil and transport them to the shoot system, the above-ground portion of the plant. Root hairs are fine “hairs” that grow out of the root’s epidermis, just above the actively growing part of the root and root tip. The root hairs provide much of the root’s surface area and so they are very important for the absorption of water and nutrients. Some plants have large, fleshy roots that store energy and water for the plant. A number of these large roots are commonly eaten such as sweet potatoes, carrots, beets, and cassava.

There are two easy-to-identify patterns of root growth: fibrous and tap roots (Figure 5.4). Fibrous roots spread out and downward in a mass of fine roots, none of which dominate. Fibrous root systems include many secondary and tertiary roots, or lateral roots, those that grow out of an older root and therefore do not tend to grow straight down (refer to Figure 5.1 in section 5.1). Monocots like maize and sorghum commonly have fibrous root systems. Garden crops that are dicots, for example, carrots, okra, chilis, sweet peppers, and amaranth, have a tap root, a dominant vertical root with other smaller roots growing out from it. These tap roots can make use of water deep below the soil surface. Many dryland fruit trees such as carob and olive also have a tap root. When the tap roots of mature plants are cut off, for example, in transplanting, the plants may die. Some of these plants can recover by developing alternative roots in a pattern similar to a fibrous root system. However, this will only occur if the plant is young, vigorous and its shoot system is relatively small.

Plants’ root systems also vary depending on a number of factors including the soil, irrigation patterns, distribution of nutrients, plant density, and neighboring plants. Root systems have a great capacity for compensatory growth. That is, in areas of soil where the conditions are favorable the roots will proliferate, compensating for areas of the root zone that are less favorable. This is important to consider when irrigating young plants, because the root system will develop most strongly where there is consistent moisture. If irrigations are frequent and shallow, for example 10-15 cm (4-6 in), then the plant will develop a shallow root system. Under hot, dry conditions moisture in this surface layer is lost quickly by evaporation. Shallow-rooted plants will require more water applied in more frequent irrigations than plants that have received deeper and less frequent irrigations, encouraging them to develop a deep root system.


Figure 5.3 Stem and Root Structures of Monocots and Dicots


Figure 5.4 Root Types

Poor drainage and overwatering also cause shallow rootedness as the roots avoid waterlogged soil. Watering patterns that encourage shallow rootedness can lead to other problems such as salinity (section 12.6.2) or roots growing primarily in upper soil layers where temperatures are high, both of which can inhibit growth and kill the plant in severe cases. For these reasons, when watering established seedlings and older plants it is important to wet the soil down to at least 15-40 cm (6-16 in), and below this for trees, in order to encourage deep root growth. However, because compensatory growth is a gradual process, one should not switch abruptly from frequent shallow irrigations to less frequent deep irrigations without a transition phase of deep but less and less frequent waterings.

Root growth is also affected by soil texture and structure (section 9.3.1). Roots will grow where soil conditions are best, for example, where compost and manure have been added and where the soil structure allows easy penetration of roots, air, and water. Extremely heavy, clayey soils with little structure make it difficult for roots to grow and they can become thick and deformed from trying to push through the soil.

From the soil roots obtain nutrients such as nitrogen and phosphorus which are essential for healthy plant growth. In some cases this is made possible through mutually beneficial or symbiotic relationships between plant roots and soil microorganisms. Mycorrhizae (Box 9.5 in section 9.5) symbioses enable plants to use more of the phosphorus, zinc, or copper in the soil.1 Symbiosis between Rhizobium bacteria and roots of legumes makes nitrogen in the air available to the plant while also enriching the soil (section 9.5.2).

5.3 Photosynthesis

Photosynthesis is the process by which green plants change the energy in sunlight into energy stored in carbohydrates (CBHs), the food used for growth and reproduction. Chloroplasts are the structures in plant cells where photosynthesis occurs. They contain a green pigment called chlorophyll which uses sunlight to fuel a reaction with carbon dioxide (CO2) gas in the air, and water (H2O) in the plant. The products of this reaction are oxygen (O2), water, and carbohydrates, such as starches and sugars (Figure 5.5). Any plant part containing chlorophyll can conduct photosynthesis, but the leaves are the main areas of photosynthesis in most green plants.

The carbohydrates produced by photosynthesis are broken down into the simple sugar glucose, which then combines with oxygen to produce CO2, water, and energy. This process, called respiration, provides the energy necessary for the plant to live and grow.


Figure 5.5 Photosynthesis

5.4 Transpiration

For photosynthesis to occur carbon dioxide (CO2) must enter the chloroplasts, most of which are found in the cells under the plant’s epidermis. Most CO2 enters the plant through the stomata (singular is stoma), tiny holes in the epidermis which can close (Figure 5.6). When the stomata are open not only can CO2 reach the chloroplasts, but moisture from the inside of the leaf is able to evaporate into the environment. This movement of water vapor through the plant’s stomata is called transpiration. As water evaporates from the leaves during transpiration, the concentration of nutrients in the surface cells increases compared with that in adjacent cells, from which water then moves by osmosis. The same process is repeated all the way down to the roots. Because of the great cohesiveness of water and its adhesion (the attraction between dissimilar substances) to the cells of the passages along which it moves to the leaves, the water is pulled upward from the roots to the leaves. The energy that keeps this water moving upward is supplied by the sun which causes evaporation of water from the plant during transpiration.

Transpiration is important for two reasons: as just described, it provides the “pull” that keeps water and nutrients moving up through the plant from the roots (Figure 5.7), and, under hot, dry conditions transpiration cools the plant the same way evaporation cools our skin when we sweat. About 90% of all water absorbed by plant roots is released in transpiration. Under stressful (hot, dry) conditions, the amount of water needed by the plant, and thus the amount released in transpiration, increases.

Transpiration rates vary depending on plant types and environmental conditions. Photosynthesis increases with available sunlight, so under sunny conditions the stomata are open longer to supply the necessary CO2 thus increasing transpiration. Conditions that increase evaporation, such as low air humidity, heat, and wind, also increase transpiration.

The stomata in some plants such as grapes will shut under extreme water stress. However, this may not save the plant. When stomata close to prevent water loss, the cooling effect of evaporation also stops, which can cause problems with high leaf temperatures.

Under sunny, dry, hot conditions transpiration rates are extremely high. If the soil is unable to provide enough water to keep up with the rate of transpiration the plant will wilt. If this water loss is not replaced soon, the plant will die. By shading, mulching, and providing garden plants with protection from drying winds the gardener reduces the need for water and so the amount of water that will be lost to transpiration (section 10.8). Directing water down to the roots, for example, with vertical mulch (section 10.8.2), minimizes the amount lost by evaporation from the soil surface, and makes more water available to meet the plant’s needs. Gardeners also use plants with lower rates of transpiration and other characteristics that make them better able to survive and produce under dryland conditions.

5.5 Coping with heat and drought

Dryland garden plants must often produce food and other products under hot, dry conditions. For plants, drought is a condition in which there is insufficient water available in the soil to meet the plant’s needs. A consequence of drought can be water stress, or water deficit - that is, insufficient water inside the plant for it to maintain itself and grow. Distinguishing between drought, a condition in the environment (especially the soil), and water stress, a condition in the plant, makes it easier to understand how plants respond to dryland conditions.


Figure 5.6 Stomata

Drought-adapted plants either escape drought or resist it in some way (Figure 5.8). Drought-escaping plants have short, rapid life cycles, allowing them to take advantage of brief periods of adequate moisture and decreasing their chance of experiencing drought. Some “famine crops” like short-season millet varieties and tepary beans follow this strategy, maturing before late season drought sets in.

Drought-resistant plants use one of two strategies, either they avoid drought or they tolerate it. Drought avoidance means more efficient use of water so that the plant will not experience water stress. For example, during periods of drought cowpeas avoid water stress by changing the orientation and movement of their leaves in relation to the sun, minimizing the amount of sunlight and heat they receive. This, in turn, reduces the amount of water lost from the leaves due to excess transpiration.2

Physiological differences enable some plants to lower transpiration rates in other ways. C4 plants, named for the four-carbon molecule they produce and use, are able to use CO2 more efficiently in a special form of photosynthesis. Because of this these plants do not need as much CO2 and therefore their stomata need not be open as long as in other plants. Shorter periods with stomata open mean decreased transpiration. Some C4 dryland garden plants are maize, sorghum, sugarcane, and amaranths.


Figure 5.7 Transpiration

Another modification of photosynthesis is found in Crassulacean acid metabolism (CAM) plants. In these plants photosynthesis happens in two stages, one during the day and one at night. The stomata are open only at night when they receive CO2 and transpiration occurs. Due to the cooler, moister, dark nighttime conditions the rate of transpiration is much lower than it would be during the day. The CO2 is then stored for use during the day when light energy from the sun is available. CAM plants found in some dryland gardens are pineapple, prickly pear cactus, and agave. CAM and C4 plants are not necessarily the best ones for dryland conditions. For example, maize and sugarcane, both C4 crops, are high water users.


Figure 5.8 Plant Adaptations to Drought

Other plants may significantly reduce transpiration rates in different ways. Some other physical characteristics that cut down rates of transpiration, making plants better able to cope with drought conditions include:

· Small leaf surface area.

· Small number of stomata per unit of surface area.

· Majority of stomata on the more protected, underside of leaves.

· Thick, waxy or resinous layer or cuticle on the leaf surface.

· Light-colored leaves that reflect light, resulting in lower leaf temperatures and therefore less need for cooling by transpiration.

· Hairs on leaves also reflect light and provide additional surface area for cooling the plant and reducing air movement, leading to reduced evaporation.

· Self-shading canopy.

· Deep rootedness.

· Drought deciduousness (section 6.2.1).

Plants that can survive water stress are called drought tolerant. The ability to tolerate drought depends on the stage in the life cycle during which drought occurs. For example, if cowpeas experience a water deficit while they are flowering and forming seeds the yield will be significantly reduced. However, if the same water deficit occurs while the cowpeas are forming leaves, before the flowering stage, then the reduction in yield will be much less3 (section 10.4).

Heat tolerance refers to a plant’s ability to survive and produce under hot conditions. Plants commonly respond to hot air temperatures with increased transpiration to cool the leaf surfaces (section 5.4). Cooling through increased transpiration is an example of why heat tolerance and drought adaptation do not always occur together. A plant may be capable of withstanding high temperatures but if it does so only by greatly increasing transpiration it is not very drought adapted. However, a few of the physical characteristics listed above such as leaf orientation to the sun, hairs on leaves, and light leaf color reduce leaf temperature in ways that do not increase transpiration.

Distinguishing between heat tolerance and drought adaptation is useful. In most drylands hot daytime temperatures are very common and so heat tolerance is a desirable characteristic. However, in gardens that receive a regular supply of water, drought adaptation may not be necessary. This is especially true if other varieties or different crops will give a bigger and better harvest with the same amount of water and other inputs.

Gardeners in drylands recognize that heat-adapted crops may differ widely in drought adaptation. We saw an example of this at a new rural settlement in arid Sonora State in northern Mexico where villagers were planting fruit trees. The only source of water for the 30 households in the village was a well 2 km (1 mi) away, and each household had only a few small containers for carrying the water on foot. Orange, pomegranate, papaya, mango, guava, and lime trees were planted. All of these trees were growing vigorously in a nearby town which has a reliable piped water supply. But the harsh, dry conditions in the new settlement were killing all except the lime trees, which were growing slowly. According to the gardeners, lime trees are the best for coping with heat and drought. These villagers also have avocado seedlings in containers that they keep in the shade near their houses. They said they will not plant the seedlings out into their gardens until a more secure water supply is found, because they know that avocados would not survive the heat and sun exposure with the little water they could provide.

5.6 Salt tolerance

The accumulation of salts can be a serious problem in dryland gardens and agriculture. Discussion of salty soils and water, and related management techniques can be found in section 9.3.1, Box 11.1 in section 11.2, and section 12.6.2. Whatever the source of salts, when they become concentrated in the soil they have an osmotic effect on plants. This results in a slower uptake of water and changes in hormone production leading to lower rates of transpiration and photosynthesis and increased respiration. The browning of leaf edges described in section 13.4.2 is a sign of this (Figure 5.9). Eventually under saline conditions, insufficient energy is available for the plant to grow or even maintain itself, and it will die.


Figure 5.9 Salt Burn

Some plants are less sensitive to salt accumulations than others and are referred to as being salt tolerant. Halophytic (salt-loving) plants actually like salty growing conditions, producing more as salinity increases to low levels. Some salt-tolerant dryland garden crops are beets, asparagus, cowpeas, spinach, date palms, and some tomatoes.4 There are also many salt-tolerant indigenous crops and wild plants, and new salt-tolerant varieties of widely grown crops are also being developed.5 Plants that are particularly sensitive to salinity such as the stone fruit and citrus trees are called halophobic.

5.7 Seasonal constraints to plant growth

No matter how carefully the garden environment is improved and managed, there are times when certain plants will not grow. This may be due to their needs for particular daylengths or temperatures that do not occur during some seasons. Local gardeners know the appropriate growing seasons for their crops, but they may be unfamiliar with the needs of newly introduced crops. Understanding seasonal constraints to plant growth improves the chances for healthy, vigorous garden plants.

5.7.1 Daylength Requirements

Some plants have a photoperiod requirement for a certain number of hours of darkness before they will grow, flower, and produce fruit. Without this they will not complete their life cycle and will not produce fruit and seeds for gardeners to eat and to plant in the future.

Closer to the equator there is less difference between hours of darkness and hours of daylight, both daily and seasonally. It is not unusual to find crop varieties from the tropics and subtropics with longer darkness requirements than varieties from higher latitude areas. For example, when grown in the northern Sonoran Desert where we live, some beans from central and southern Mexico will not flower until September, even if they are planted in March. This is because we live farther north where the longer nights the beans need to flower do not occur until September, the beginning of the cool season (Figure 5.10). Because beans are warm-season crops they cannot be sown early enough in the year to take advantage of the long nights of late spring. Therefore the only time for us to plant these varieties is in the late warm season, late July or early August, and this does not give some beans enough time to mature before the freezing weather in November.

Onions and sesame are other garden crops whose production is controlled by photoperiod sensitivity, although precise requirements differ by variety. For example, long-night (more than 12 hours, sunset to sunrise) varieties of onions and sesame are required for semitropical savanna West Africa. If short-night (less than 12 hours) onion varieties adapted to higher latitude temperate regions are planted in this area of West Africa they will not form bulbs.6 Tomatoes are an example of a photoperiod-neutral garden crop.


Figure 5.10 Daylength Sensitivity

5.7.2 Temperature Requirements

Like daylength, plants’ temperature requirements vary greatly both between and within species. Most egg-plants, cucurbits, some pulses and peppers require soil temperatures above 15°C (60°F) for normal germination and seedling development.7 Some varieties of deciduous fruit trees require a minimum number of days at temperatures below 0°C (32°F) for dormancy in order to produce fruit (section 14.4.1).

A uniform problem among all plants of one variety that cannot be traced to any other cause - such as failure to produce flowers, fruit, or bulbs, or bolting (premature flowering) - may be a sign that the plant’s daylength or temperature requirements are not being met (Figure 5.11).


Figure 5.11 A Uniform Problem Such as Bolting may mean Growth Requirements are not Being Met

5.8 Resources

Through careful observation and long experience many farmers and gardeners understand a great deal about their crops. They are the best resource for learning about how local crops live and grow. For a Western science approach, basic principles of botany can be found in many school textbooks. The perspective of a botanist or ecologist is often different than that of an agronomist. While the first two approach the subject with a broad environmental outlook, the agronomist often tends to emphasize production economics. This results in different priorities and concerns, and most importantly, in asking different kinds of questions. Frequently the information in botany or ecology books is more relevant to small-scale, low-input food production, such as household gardens. This is especially true for drylands because agronomy often assumes a modified, optimal environment for crop production, instead of considering how best to cope in a marginal environment with limited resources.

On the other hand, there are many valuable agronomy texts and a growing number of agronomists whose approach to food production is appropriate for small-scale, marginal systems. Lessons 23-27 and 31-32 in Agriculture Tropicale en Milieu Paysan Africain (Dupriez and De Leener 1983) describe the needs of plants for resources such as water, air, and light. Part I of Crops of the Drier Regions of the Tropics (Gibbon and Pain 1985) includes a section on “Crop Factors” with a good discussion of drought and water use in crops. The Better Farming series of pamphlets from the FAO (1976-1977) contain simple discussions of botany and other topics relating to agriculture.

References

1 Feldman 1988.

2 Hall, Foster, and Waines 1979.

3 Hall, Foster, and Waines 1979:156-157.

4 Ayers and Wescot 1985:31-35; Cox and Atkins 1979:300-304.

5 NAS 1990:17-39.

6 Kassam 1976:79, 82,104.

7 Hartmann and Kester 1983:147.

6. Growing plants from seeds

Most annual garden crops are grown from seed, and so are some perennials. It is easy to grow crops from seeds, and seeds can be traded, transported, or stored. An important reason for using seeds from open-pollinated garden crops is to maintain genetic diversity. The variability that exists for many traits between individual, open-pollinated plants allows gardeners to continually select plants best adapted to changing needs and conditions. In Chapter 14 we discuss genetic diversity and what it means for the gardener, her garden, and for all of us. In this chapter we discuss how seeds are produced and present ideas for planting them in drylands.

6.1 Summary

Many garden crops reproduce sexually when male reproductive cells (contained in pollen) and female reproductive cells (contained in ovules) are joined together during fertilization, producing an embryo that will be contained within the seed. After the seed matures, it will germinate and grow if environmental conditions are right. Appropriate techniques for preparing and planting seeds and for watering, mulching, and shading seedlings conserve water and protect the seedling from the harsh environment. Diagnosis and remedy of seed planting problems may include a germination test to check the health of seeds. Once the seedlings have emerged, thinning them can improve vigor and production.

6.2 Sexual reproduction in plants

Seeds and the plants that grow from them are the products of sexual reproduction. Some garden crops can be propagated vegetatively through asexual reproduction, which is discussed in Chapter 7.

Sexual reproduction is the combination of genetic material from the reproductive cells or gametes: sperm contained in pollen from the male combines with the ovule in the female (Box 14.1). The result is a seed that carries characteristics of both parents. Flowers are the specialized plant parts where the gametes are produced, and those flowers with female parts are the site of pollination, fertilization, and seed production.

6.2.1 Life Cycles

Plants that produce seeds have two distinct phases of growth. During vegetative growth roots, stems, and leaves grow, and during reproductive growth the plant’s resources are focused on developing flowers, seeds, and fruit. A plant’s life cycle is defined as the time it takes to produce seeds. How long an individual plant lives is its life span. In some plants, life cycle and life span are the same length of time; in others they are not.

Annual plants are those that take 1 year or less to go through their entire life cycle: germination of the seed, vegetative growth, reproductive growth, and seed production, after which they die. That is, their life cycle and life span are equal. This is also true of plants that spend their first year in the vegetative growth stage, and enter and complete their reproductive growth stage and die in their second year. These plants whose life cycle and life span are both about 2 years long are called biennials. Perennials are those plants that live longer than 2 years, usually going through vegetative and reproductive stages each year after an initial period (1 or more years) of only vegetative growth. That is, their life cycle may be 1 year long, but their life span is much longer as in the case of olive trees, which can live for hundreds of years. On the other hand many agaves, whose swollen leaf bases, roots, and flower stalks are eaten, and leaf fibers used for weaving, have a life span of about 20 years. During this time they go through only one life cyle, producing a flower stalk once and then dying.

Most annuals and many biennials are herbaceous, that is their aboveground growth is green, pliable, and tender. Many perennials such as bananas and yams are herbaceous as well. However, some are woody in that their stems, trunks, or branches become hard, rigid, and covered with bark, as with olive and peach trees.

Some dryland perennials such as pomegranates, figs, the stone fruits, and jujubes are deciduous. That is they have a repeating seasonal cycle of losing their leaves, and becoming dormant, followed by a period of growth, leafing out, and flower and fruit production (Figure 6.1). Nondeciduous perennials are sometimes referred to as being evergreen. Both deciduous (e.g., fig) and evergreen (e.g., carob) trees may lose their leaves to reduce transpiration during extreme drought.1 Because they avoid drought in this way such plants are said to be drought deciduous. Cassava is a drought-deciduous, short-lived perennial that loses all but a few leaves on the ends of its stems during drought.2

6.2.2 Flowering

In plants male gametes, contained in pollen grains, are produced in the anthers, and female gametes, contained in ovules, are produced in the ovary. Some plants like okra have perfect flowers which contain both male and female structures. Monoecious plants have separate male and female flowers on the same plant as in most squashes and maize. Dioecious plants such as the pistachio and date palm bear female flowers on one plant and male flowers on another (Figure 6.2). The papaya is an interesting example of a tree that can be perfect, monoecious, or dioecious. Dioecious papaya plants may even change sex, and in savanna West Africa dioecious male papaya plants are cutback to the ground to encourage female shoot production.3

The flowers of many herbaceous garden plants last only a very short time. Squash blossoms, for example, wither and drop off after only 1 day. Stressful conditions such as high temperatures and drought may shorten the flower’s life as well, making hand pollination useful (Box 6.1 in section 6.2.3).


Figure 6.1 Yearly Cycle of a Deciduous Tree - the Pomegranate


Figure 6.2 Perfect, Monoecious, and Dioecious Flowers (1)


Figure 6.2 Perfect, Monoecious, and Dioecious Flowers (2)

6.2.3 Pollination

Pollination happens when a pollen grain lands on the stigma, the receptive surface of the female flower part where the pollen grain germinates, and grows down the style to reach the ovary. Flowers can be cross-pollinated or self-pollinated. Cross-pollination occurs when pollen from one plant pollinates the flower of another plant in the same species that is genetically different. When the pollen from a male date palm is blown onto the flowers of a female tree, cross-pollination has occurred (Figure 6.3). An example of cross-pollination of a monoecious plant is the pollination of maize when pollen from one plant is blown to the silks of other plants (Figure 6.9 in section 6.2.4).


Figure 6.3 Wind Cross Pollinates a Dioecious Plant

Self-pollination refers to the pollination of a flower on a plant that is genetically identical to the pollen donor. Two thyme plants started by cuttings from the same original plant may pollinate each other, because they are genetically identical this is self-pollination, not cross-pollination. Other examples of self-pollination are when a monoecious plant such as a squash or a plant with perfect flowers, like sesame, okra, or tomatoes, pollinates its own flowers, often with help from insects (Figures 6.4 and 6.5).

Knowing how plants are naturally pollinated improves the gardener’s understanding of how different genetic combinations occur. It also helps her control pollination for seed production, selecting parent plants with the most desirable traits (Box 6.1).

When pollen is carried by the wind to female flowers, as in Figure 6.3, they are said to be wind-pollinated. Examples of wind-pollinated crops are maize, dates, pistachios, olives, and the amaranths. Insect-pollination occurs when insects carry the pollen to the female flower parts, as in Figure 6.4. Some insect-pollinated crops grown in dryland gardens are the cucurbits, pulses, tomatoes, garlic and onions, the stone fruits, and mangoes. Box 6.1 discusses how wind- and insect-pollination can be controlled.


Figure 6.4 Ants Assist the Self-Pollination of a Monoecious Plant

There is no absolute rule for distinguishing wind-and insect-pollinated plants, however, the flower is often a good clue. Plants with many inconspicuous, small flowers lacking color or fragrance are often wind-pollinated. Their pollen is relatively dry, light, and easily blown by the wind.

Showy, fragrant, white, or brightly colored flowers usually rely on insect-pollination. Their appearance or fragrance attracts insects such as wasps, bees, ants, flies, and butterflies. Bats, rodents, and some birds also act as pollinators. Because the pollen in these flowers is frequently heavy, moist, and sticky it adheres to the insects or other animals which carry the pollen to another flower, pollinating it when they rub against the stigma.


Figure 6.5 A Perfect Flower Self Pollinated with Help from a Bee

Box 6.1
Controlling Pollination

If successful pollination of a crop by wind, insects, or other natural means is uncertain, then hand-pollination can be done. Examples are when the number of female flowers is limited (as in squash), when pollen supply is limited (as in date gardens), or to take advantage of environmental conditions most favorable for fertilization (as with cool mornings for maize).

When controlling pollination, the first step is to identify the plant’s life cycle and flowering characteristics. Flowers that are just about to open are best for hand-pollination. With flowers such as squash, which are usually pollinated by insects, the pollen can be rubbed on the sticky surface of the stigma (Figure 6.6). For wind-pollinated ones, the male blossoms can be shaken over the female flowers, dusting them with pollen (Figure 6.7).

Maize, for example, is a wind-pollinated crop that should be hand-pollinated in the cool of early morning because hot, dry conditions will kill maize pollen. The male flowers or tassels are shaken so that the pollen falls on the silks, which are the stigmas and styles of the female flowers. Maize should always be planted in clusters or blocks, not in single rows or as isolated plants, since it often needs cross-pollination between plants for good seed production (section 6.2.4).

Pollination may also be controlled to maintain the purity of a specific variety. In these cases steps have to be taken to prevent unwanted pollen from fertilizing the ovaries. In wind-pollinated plants, female flowers can be closed or covered with cloth or paper before and after being hand-pollinated. Wind-pollinated varieties can also be separated from each other in space (e.g., planting different maize varieties at least 0.5-1.6 km or 0.3-1.0 mi apart), and in time (staggering planting times so that different varieties will not be flowering at the same time). The Hopi Native Americans of southwestern North America have maintained a large number of very distinct varieties of maize for hundreds of years by planting the varieties in fields separated from each other.

Surrounding insect-pollinated plants with a frame of sticks, bamboo, or wire covered with a finely woven screen or netting may be enough to control pollination by large flying insects. If ants are pollinators, the female blossoms can be covered and tied shut. If the flowers are perfect, their anthers must be removed so that they will not self-pollinate. In monoecious plants the male flowers on the plant should be removed for the same reason.

Pollination can also be controlled when gardeners want to improve the drought resistance, taste, yield, or other qualities of their crops. This is done by selecting the male and female plants with the desired characteristics and crossing them to produce seeds.


Figure 6.6 Hand-Pollinating a Squash Blossom


Figure 6.7 Hand-Pollinating a Date Palm in Iraq

6.2.4 Fertilization

After pollination the pollen grain germinates and a pollen tube grows from it, down the style, into the ovary and finally the ovule (Figure 6.8). When the male gamete from the pollen grain joins with the female gamete in the ovule, fertilization has occurred. The fertilized ovule will develop into a seed, and in some plants the ovary will thicken around the seed or seeds. This thickened membrane is the fleshy part of a fruit.

Fertilization is important for two reasons: a) fruit and seed foods such as olives, jujubes, okra, tomatoes, and sesame will only be produced if fertilization occurs, and b) seeds are needed for growing many garden plants, especially annuals.

Fertilization will fail if either the pollen or the ovules are no longer viable. A cell, flower, seed, graft, or cutting is viable if it is capable of living. Maize pollen is only released for several hours around sunrise. The pollen is usually viable for about 24 hours but under hot, dry conditions this period is significantly shortened. This is why hand-pollinating maize in the cool of early morning improves chances of fertilization.4 Similarly, the pollen from tomato flowers may pollinate the stigma but hot, dry weather can kill the pollen during the approximately 50 hours it takes for fertilization to occur.5 This is why some tomato varieties stop bearing fruit under very hot conditions, and why shading can help increase production.

A few crops, such as some maize varieties are self-sterile or self-incompatible. This means that even though they are monoecious, flowers on the same plant cannot fertilize each other. While self-sterile plants are incapable of fertilization by self-pollination, genetically different individual plants of the same variety can be fertilized by cross-pollination (Figure 6.9). This is a good reason to have a cluster of plants of one variety growing in the garden.


Figure 6.8 Fertilization


Figure 6.9 Self-Sterile Plants can be Fertilized by Cross-Pollination

6.3 Seed germination and dormancy

Once the seed has matured it is ready to be planted and grow into a new plant. A mature seed contains a living plant embryo and food reserves to fuel the seedling’s growth until the root and shoot systems take over.

When water penetrates the protective outer seed coat, the seed swells, breaking the coat open. The coat may also be broken by extreme temperatures due to fire or freezing, the growth of soil microorganisms, or because of being eaten and digested by animals.6 Breaking of the seed coat is a physical process, not a biological one; even dead seed can do this, so it is not a sign that the seed is alive.

When a living seed has swollen with water and produces a root and shoot, it has germinated. Emergence is when the shoot first breaks the surface of the ground. Seeds with epigeal germination, for example, many legumes and cucurbits, push their cotyledons above the soil surface. This contrasts with hypogeal germination, for example, in peaches and maize, in which the cotyledon remains underground and the new shoot grows up above (Figure 6.10).

In some seeds a condition of dormancy prevents germination for a period after the seed is mature. A dormant seed will not germinate when exposed to water until other requirements are met. Some seeds contain chemicals that inhibit germination for a time after maturation. The fruits of citrus, tomatoes, and some cucurbits contain chemicals that usually keep the seeds inside them dormant. When the seeds are removed from the fruit, and either planted or washed several times, they will no longer be dormant.

Other seeds require environmental stimuli such as low or high temperatures to germinate. The seeds of some high-latitude or -altitude dryland fruit trees, such as peaches, require a period of chilling to break embryo dormancy (section 14.4.1). Some seeds require thorough drying before they will germinate, preventing germination while still on the parent plant.

Overall, compared with wild species, domesticated crops tend to have brief dormancy periods (0-6 months), and high germination percentages which drop significantly as seeds get older. This is because domesticated crops are selected for immediate germination and growth under relatively controlled, improved conditions. In contrast, seeds of wild plants have more requirements for germination, longer periods of dormancy, and staggered germination, reducing the risk that they all germinate and then die if conditions worsen.7


Figure 6.10 Seed Germination

6.4 Suggestions for planting seeds under dryland conditions

The suggestions given here for direct planting seeds can also be used with the nursery beds and containers discussed in section 8.2.

6.4.1 Preparing the Seeds

Soaking large or hard-coated seeds like maize, beans, and squash in water before planting helps break or at least soften their seed coats. This minimizes the time the seeds are in the ground before germination, when they are most vulnerable to being eaten by birds or insects, or attacked by disease. Presoaking also means that the garden does not need to be watered as much to keep the seeds moist until they germinate, and so saves water. During presoaking bad seeds, which are hollow due to disease or pests, or because the embryo never developed, will float to the surface and can be composted.

Presoaking for too long can kill the embryo. The larger and/or older the seed, the longer it will need to be soaked. Generally, seeds of herbaceous plants need no more than 8 hours of soaking. Large, hard-coated seeds of perennials can be soaked up to, but no longer than 24, hours.8 Once the seed is softened it must be planted. Seeds for planting should never be soaked and then left to dry out completely because this will kill the embryo.

Seed coats of some plants, like carob, are so tough that scratching them is recommended to speed germination.9 Scratching can be done easily by lightly rubbing the seed on a rough surface like a rock. The area where the seed was attached to the ovary, known as the hilum (Figure 6.11), should be avoided when scratching seeds this way, or the embryo may be injured.

The seed coats of many small seeds can be lightly scratched by putting them in a gourd, can, or jar with some small-sized gravel. Cover the container and shake the contents vigorously (Figure 6.12). In this case damaging the hilum is not a problem because the scratching is so gentle. Hard shells protecting seeds of some crops such as the stone fruits, olive, and many nuts can be carefully cracked just before planting to speed absorption of water and germination.


Figure 6.11 The Hilum of a Legume Seed


Figure 6.12 How to Lightly Scratch Seed Coats (1)


Figure 6.12 How to Lightly Scratch Seed Coats (2)


Figure 6.12 How to Lightly Scratch Seed Coats (3)

6.4.2 Preparing the Planting Site

The soil in the garden should be prepared before planting seeds (Chapter 9). Planting seeds in depressions such as furrows or basins concentrates and saves water (Figure 6.13). Seedling emergence and growth can be encouraged by special attention to the soil immediately around the seed (section 6.4.5). For example, adding some extra compost to soil in the planting depressions makes it rich in nutrients and light textured, encouraging root growth. The compost also breaks up the soil surface, making it easier for water to infiltrate. Good infiltration also helps avoid damping-off fungus (section 13.4). In heavier soils, mixing some extra sand into the soil around the seeds of root or bulb crops such as carrots, beets, or onions helps young roots and bulbs get a vigorous start.


Figure 6.13 Planting Depressions (1)


Figure 6.13 Planting Depressions (2)


Figure 6.13 Planting Depressions (3)

6.4.3 Planting the Seeds

Gardeners can control their seed planting density by sprinkling seeds into the planting depressions with their fingers (Figure 6.14). Pouring them can result in too many seeds being planted (Figure 6.15). This is a waste of seeds and time because many seedlings will have to be thinned later on. Mixing very small seeds such as amaranth or basil with sand or dry soil prevents them from sticking together, making it easier to sow them evenly.


Figure 6.14 Sprinkling Seeds

6.4.4 Planting Density

In indigenous, mixed gardens plants often grow much closer together than is recommended in many garden books. This close planting in a well-managed garden can increase production, but planting too closely may reduce production if soil nutrients, water, and sunlight are inadequate to meet all the plants’ needs.


Figure 6.15 Pouring Seeds

There is not a lot known about how planting density affects different crops and crop mixes. It has been found, for example, that closely planted apple trees with overlapping root areas produce many more downward growing roots in contrast to widely spaced trees which produce more horizontal roots.10 How much some crops can adapt their physical structure to different planting densities and which crops can do this is not really known. Each gardener must experiment with planting densities and combinations that work for them. (See section 8.5 for discussion of inter-cropping.)

The way plants grow and use space affects how densely they can be planted. Mixing plants of different ages and life cycles decreases competition for resources because they will not have the same requirements and will make use of different levels of space both above and below ground (Figure 6.16). For example, plants with shallow, widespreading roots can be more densely planted with deep-rooted plants than with other shallow-rooted ones. Vines can be interplanted with trees, bushes, and cereals whose leaves occupy different levels above the ground. Annual leaf or fruit crops can often be successfully interplanted with root crops (Figure 6.17). Seeds can be sown among mature annuals which provide shade and wind protection for the seedlings. As the seedlings grow and require more space the older plants are nearing the end of their lives and can be removed or cut back, allowing the younger ones to replace them.


Figure 6.16 A Mixed Garden with Annuals and Perennials

Starting with dense planting allows the gardener to thin, transplant, or harvest as the plants mature and if more space is needed. However, experimentation and careful observation are important because planting too densely can be a waste of seeds and can lead to disease and to competition for scarce dryland resources like water.

We do not feel that charts of precise planting densities are useful and have not provided them here. Such charts are usually based on industrial-style row gardens, not mixed gardens in dryland environments. They do not take into account variables such as the soil quality, water availability, temperatures, and the other plants including the many unique local varieties growing in the garden. The soundest approach is to observe and talk with local gardeners, keep basic principles in mind, and experiment.


Figure 6.17 Interplanting or Intercropping


Figure 6.18 Seed Planting Depth (1)


Figure 6.18 Seed Planting Depth (2)

6.4.5 Covering the Seeds

Seeds need to be covered with enough soil to prevent them from drying out, but not so much that the shoot has difficulty emerging. The thickness of this cover depends upon the type of soil and the size of the seed (Figure 6.18).

· In heavy clay soils the covering should be thin (approximately two to three times the diameter of the seed) because these soils dry out more slowly and are harder to penetrate.

· In light, sandy soils the cover should be thicker (approximately four to six times the diameter of the seed) because these soils dry out more quickly and are easy to penetrate.

Seeds with epigeal germination may benefit from being planted slightly shallower than suggested above. If these seeds are planted too deeply and are unable to push their cotyledons above the soil surface they will die.

Diverse local conditions mean there are, however, exceptions to the guidelines given above. For example, in their dry-farmed maize fields the Hopi Native Americans living in the high desert of southwestern North America plant seeds about 25-33 cm (10-13 in) deep (Figure 6.19). This unusually deep planting is an adaptation to local soil and water conditions. In these fields approximately the top 30 cm (12 in) or more of soil is fine sand with a clay-sand loam below. In the spring, water from melted winter snow percolates quickly through the sandy soil, but is held in the clay-sand loam. When the soil has warmed, the seeds are planted in this moist layer. By planting deeply the Hopi take advantage of moisture stored in the soil. The Hopi have selected maize varieties over many years for physical adaptations to this planting method such as a dominant, deep root and a shoot with an underground portion about twice as long as in other varieties.11 These characteristics and the sandy texture of the upper layer of soil make it possible for Hopi maize to emerge from such a deep planting hole and grow successfully.

Adding sand and organic matter to clayey soils, especially to soil covering the seeds, is a good idea. When ready to plant seeds in their terraced garden beds, Hopi carry up buckets of fine yellow sand from the valley floor below. They explain that they cover their garden seeds with this sand because the heavier, clayey soil in the terraces forms a crust when it dries out after irrigation. This crust can be so hard that it prevents seedlings from emerging.12

The soil covering the seeds should be pressed down firmly as it not only helps retain moisture but protects seeds from being removed by wind, insects, and birds. The surface can also be lightly mulched (section 6.5.2).

6.5 Caring for newly planted seeds and young seedlings

When caring for newly planted seeds and young seedlings under dry conditions the most important consideration is maintaining soil moisture. Protecting young plants from hot, dry winds and strong direct sunlight helps reduce water consumption and stress on the plant. (See section 13.4 for diagnosing problems with young seedlings.)


Figure 6.19 A Planting Hole in a Hopi Maize Field

6.5.1 Watering

The soil must be kept moist around seeds and young seedlings because they do not have an extensive root system for gathering and storing water. If the soil dries out they will quickly die. Seedlings experiencing water stress often have retarded shoot development and a greater proportion of root growth than unstressed seedlings. This is because under water stress, development of roots, which provide water, takes priority over shoot development for photosynthesis, which increases transpiration. Overwatering, on the other hand, can lead to soil saturation, forcing oxygen needed by the roots out of the soil (section 10.3.1). Lack of oxygen can kill the seed or seedling and encourages damping-off fungi which can also be fatal (section 13.4). Thus the soil should be kept moist but not saturated. Watering frequency will depend upon weather and soil conditions, planting methods, and plant types, and must be arranged to fit the gardener’s schedule.


Figure 6.20 Ways to Disperse the Flow of Water on Seeds and Seedlings

Breaking the force of flowing water when watering prevents seeds and seedlings from being washed away. This can be done by sprinkling or splashing the water by hand or pouring it through a bunch of leaves or stalks. A sprinkler can be made by putting holes in the bottom of a container such as a calabash or tin can, or making ceramic vessels with holes in their bottoms (Figure 6.20). Whether using a sprinkler or not the water should be poured as close to the soil surface as possible to avoid eroding the soil around seeds and young seedlings (Figure 6.21).


Figure 6.21 Watering Seeds and Seedlings

6.5.2 Mulching and Shading

Mulching and shading conserve moisture around seeds and seedlings when the weather is hot and dry. (For general information on mulching, shading/and windbreaks see section 10.8.) Special points about mulching and shading seeds and seedlings include the following:

· A thick mulch can harbor insects and encourage disease, both of which prey on tender seedlings and seeds. It can also smother and kill emerging seedlings. A light mulch such as a sprinkling of fine organic matter on the soil surface after watering will reduce evaporation. It is a good idea to periodically clear the mulch away and check for problems such as insect or fungal damage.

· Mulching and shading should allow enough sunlight to reach the plants. Pale, spindly seedlings with long stems are signs of insufficient sunlight (Figure 6.22).


Figure 6.22 Too Much Mulch can Harm Seedlings

6.6 Diagnosing seed planting problems

If seedlings fail to emerge the following questions should be asked (Figure 6.23):

· Was it too cold, hot, dry, or wet?

· Were seeds past their after-ripening (section 14.3) or dormancy period?

· Were the seeds old or moldy and therefore no longer viable?

· Was enough time allowed for germination to occur?

· Were the seeds carried away or eaten by insects, birds, rodents?

· Were the seeds buried too deeply for the seedling to emerge? Or were they planted too near the surface, causing them to dry out?


Figure 6.23 Why Seeds Fail to Emerge

Table 6.1 provides more information on diagnosing and treating seed planting and seedling problems. If the evidence is unclear or indicates a problem with the seeds, the following germination test can be used.

Table 6.1 Diagnosing and Treating Seed Planting and Seedling Problems

Observations

Possible causes

Suggested actions

Failure to emerge:




Seed did not germinate

Environment; old or nonviable seed

Investigate environment; try germination test, look for other seeds


Seed missing

Insects, small mammals, or birds

Plant again, try giving more protection, bird scaring, container planting


Seed germinated but dried up before it emerged

Planted too shallow, did not get enough moisture, got too hot

Plant again, deeper, water more often, mulch


Seed germinated and grew but did not reach soil surface to emerge

Planted too deeply

Plant again, shallower


Seed germinated and grew curled up under soil surface

Soil crust too hard

Add organic matter and sand to improve soil quality around seeds


Seed or seedling dead, normal roots with chew marks or clean cuts

Ants, beetle larvae, and other insects

Plant again, try using containers


Preemergent seedling dead; dark, soft, lesions and dark root

Preemergence damping-off

Add organic matter and sand to improve drainage, use containers with fresh soil

Parts of seedling missing

Probably eaten by insects or birds (wild or domestic), or possibly damaged seed stock

Look on/around plants, under mulch, especially at night, remove mulch, fence, bird scaring, plant in containers, find new seed stock

Spindly, pale seedling

Insufficient sunlight

Increase exposure to sunlight, remove surface mulch

Seedling deformed, abnormal

Damaged or infected seeds

Do germination test to check viability, find new source of seeds

6.6.1 Testing Seed Germination

The germination percentage is the percentage of a variety of seeds from a particular source which can be expected to germinate. The standard method for measuring germination percentage on a commercial scale involves four tests, each having a sample size of 100 seeds.13 For each test the number of seeds germinating within what is considered a reasonable amount of time for that particular crop are counted. Most domesticated annual crops germinate in a few days if in the proper temperature range, although some seeds, for example cilantro and chilis, take longer. The counts for each of the four tests are summed and divided by four to give an average germination percentage. For example, if the numbers of germinated seeds from the tests were 79 + 83 + 81 + 74 = 317, then the germination percentage is 317/4 = 79.3%.

This test is also useful for finding out how strong and healthy the seeds are. Nutritional deficiencies in the seed-producing crop, or poor postharvest handling may result in deformed seedlings. Seeds that are very slow to germinate in this test, assuming the correct conditions are provided, are said to lack vigor. Old seeds lose vigor over time as the embryo becomes weaker; eventually they can no longer germinate and they die. When planted, seeds that lack vigor are more likely to die under stressful conditions or produce weak seedlings.

The seeds of most annual garden plants have a germination percentage of over 70%, so approximately three-quarters of a seed sample should germinate in a test. However, germination in the garden is usually slightly lower than germination tests and this difference increases as the germination percentage drops.14 For example, if the germination percentage in the test is 95%, in the garden it may be about 90%, but if the test shows 70%, in the garden it may be only 50%.

Household gardeners usually do not have the resources to devote 400 seeds to a germination test, and can use a smaller sample. If a group of gardeners obtain seeds from the same source they could each contribute some seeds for a cooperative test. Using a smaller sample size increases the possibility of test results being affected by sampling error (section 4.5.3). Because of this the results of a smaller test are not meant to compare with those of the standard germination percentage test described above. However, a small-scale test can still help the gardener check the viability and vigor of the seeds she is using. The goal is not to determine an actual germination percentage, but to see if there is a significant problem with the seeds. In other words, is the germination rate closer to 80% or 10%? If it is closer to 10% there is a problem with the seeds and it probably is not worth using them.

When doing the test, small samples of seeds should be selected from various parts of the storage container, ensuring a representative sample of the seed stock. A piece of cloth with each side measuring about two hands in length (35-45 cm, 14-18 in), is wet and the seed samples scattered on it, keeping them about 1 cm (0.5 in) apart. The cloth is carefully rolled around a stick and tied, making a snug but not tight roll15 (Figure 6.24).


Figure 6.24 Testing Seed Germination

Keeping the roll moist is critical; sprinkling it with water and protecting it from sun and wind will prevent drying. However, if the roll is kept dripping wet the seeds will rot. If they are seeds normally planted at the time of year when the test is done the temperature in the shade should be fine.

The cloth is left rolled for the amount of time germination is thought to take for that particular crop. For most annual garden crops seven to ten days is plenty of time. The cloth is then unrolled to check for signs of germination. If no germination occurs within the expected length of time it is possible that certain conditions were not appropriate. Did the seeds dry out? Did they drown? Were they too hot? If there are no obvious problems the cloth and seeds can be rolled back up and the test continued. If that does not succeed the test can be tried again. If there is still no germination, or it is poor, or seedlings are abnormal or diseased it is best to look for another source of seeds or try a different crop or variety. But if normal germination does occur then the problem must be sought in the seeds’ environment (refer to Table 6.1).

6.7 Thinning

Thinning is done so that plants have enough space to obtain sunlight, water, and nutrients for vigorous and productive growth. Thinning is common with plants grown from seed. Vegetative propagation involves larger plant parts which are usually properly spaced on first planting.

Ideally, plants should be left dense enough so that when healthy and mature their leaves will shade the soil, reducing soil temperature and evaporation. But, if plants are too crowded they compete for water, light, and soil nutrients; they lack vigor and cannot produce nutritious, good tasting fruit, leaves, or roots. Because these plants are weaker they are also more vulnerable to stress and disease.

We describe three thinning techniques; pulling, cutting, and transplanting, because each is more appropriate for certain situations. Many seedlings that are pulled or cut can be eaten, including leaf crops and legumes like beans and peas. But some, like tomatoes, are poisonous. Usually local gardeners and others can identify which seedlings can or cannot be eaten. Because transplanting is an important technique used not only for thinning it is described in the Chapter on plant management, section 8.4.

Pulling and cutting are done when transplanting is not desired. The less vigorous plants - those that are smaller, paler, or weaker - are chosen. Seedlings should be cut when they have extensive root systems and are growing so closely together that pulling would disturb adjacent plants. For cutting, a knife or other sharp tool can be used to cut the plant off at or near ground level. If plants are small they can be pinched off with the fingers. Cutting is not appropriate for some perennials, bulbs, or other plants like onions and Jerusalem artichokes which can store energy in their roots and send up new shoots from this reserve. These plants must be pulled or transplanted to thin them out. To thin by pulling, plants are grabbed low on the stem and pulled straight up and out. After pulling the soil should be pressed down so no holes or gaps remain which expose the roots of living plants.

6.8 Resources

Local gardeners and farmers have often developed special techniques for controlling the pollination of their crops and planting the resulting seeds. They are the best sources for locally appropriate insights and methods. Botany and biology textbooks are good resources for explaining the process of sexual reproduction in plants. Items #6-2, #7-3, #7-4, #9-3 and #10-4 from Developing Countries Farm Radio Network (DCFRN) cover topics discussed in this chapter such as seed and fruit formation and planting seeds.

References

1 Evenari, et al. 1982:208.

2 Purseglove 1974:174.

3 Samson 1986:256.

4 Purseglove 1983:315.

5 Purseglove 1974:534.

6 Hartmann and Kester 1983:129.

7 Mayer and Poljakoff-Mayber 1975:25.

8 Hartmann and Kester 1983:145.

9 Hartmann and Kester 1983:171-172.

10 Atkinson, et al. 1976.

11 Bradfield 1971:5; Collins 1914.

12 Soleri 1989.

13 Hartmann and Kester 1983:164-166.

14 FAO 1961:102-103.

15 DCFRN #4-1.

7. Vegetative propagation

Propagating plants from plant parts other than seeds is called vegetative propagation. Plants grown from seed are the product of sexual reproduction and contain a mixture of genes from two parents. Plants grown by vegetative propagation are reproduced asexually, and have only one parent. The genetic characteristics of the new plant will be identical to those of the parent plant, although responses to different environments can make them different in some ways.

For example, date palms can be propagated both from seed and vegetatively. Using seed, the sex of the new date tree and the qualities it will have are unknown until it flowers, 8 to 10 years after planting. The seedling will be a unique combination of the characteristics of its two parents. However, if propagated using vegetative methods, in this case an offset, the new seedling will be a clone, or a genetic copy of its parent: the same sex, growing characteristics, and fruit qualities.

Knowing the characteristics of a new plant is one of the main reasons for using vegetative propagation. This is particularly important for trees that do not produce fruit or nuts for several years. (Refer to Figure 7.1 to identify plant parts used in vegetative propagation of trees). Some crops such as bananas, sweet potatoes, and cassava are usually propagated vegetatively because it is a fast and easy method, and because they produce few or no viable seeds.

7.1 Summary

There are several methods of vegetative propagation. The choice of which method to use depends primarily on the crop being propagated. Although we list a few examples of dryland garden crops that can be propagated using each method, local gardeners often know methods that work best with crops in their area. The explantions given in this chapter will help the field worker understand how indigenous methods work, how they might be improved, and what new methods might be introduced.

7.2 Cuttings

Cuttings are plant pieces, usually stems or branches, capable of growing new roots, called adventitious roots. To grow these new roots cuttings must rely on stored energy or energy that they can produce. However, the cutting can only provide this energy if it is carefully protected from stress like heat and drought. Some of the dryland garden plants that may be propagated by cuttings are deciduous trees such as the stone fruits, fig, mulberry, and pomegranate. Olive and carob are two non-deciduous trees that can be started from cuttings. Cassava, sweet potatoes, and some perennial herbs can also be propagated from cuttings.

In sections 7.2.1 through 7.2.4 we give some examples of how cuttings are used to propagate different dryland garden crops.

7.2.1 Trees

Depending on the tree, cuttings can be of either green, new growth produced during the current season, called softwood cuttings, or older, woodier shoots from the previous season or earlier, called hardwood cuttings. Shoots that are producing flowers or fruit should not be used for cuttings. In general, cuttings from younger plants are easier to root. However, if the parent plant is old, heavy pruning will encourage lots of new growth that can then be used for cuttings.

Cuttings should not be taken during times of great environmental stress such as drought or very cold or hot temperatures. Otherwise, the best time to take a cutting depends on the kind of tree and the kind of cutting. In deciduous trees hardwood cuttings are taken during the dormant period before any buds have begun to swell or develop.1 For nondeciduous trees the time for taking hardwood cuttings is not as easy to predict and experimentation with cuttings taken just before a period of rapid growth may be the best place to start. Softwood cuttings are taken during the early part of the growing season, and are pliable but not woody. They should snap if bent in half.2 All but the top few leaves are removed from softwood and evergreen hardwood cuttings.


Figure 7.1 Terms Used in Describing the Vegetative Propagation of Trees

Hardwood deciduous tree cuttings should be dormant and leafless, and the tip is usually removed. Cuttings range in diameter from about 0.6 cm to as much as 5 cm (0.25-2 in). All cuttings should have at least three nodes (Figure 5.1 in section 5.1). A relatively young, vigorous shoot with many leaf nodes that is growing in full sunlight is best. It should be a lateral shoot, one growing horizontally or at an angle in relation to the ground, or a basal shoot from around the base of the trunk (or crown). Rapidly growing vertical shoots with large spaces between the nodes are called water sprouts and are not good for cuttings (Figure 7.2). The growth habit of a vegetatively propagated plant will often resemble that of the cutting from which it is grown.3 For example, a vertical water sprout may produce a tall tree with fewer side branches than a tree grown from a lateral shoot which will tend to have fuller, more widespreading growth.


Figure 7.2 Choosing Shoots for Cuttings

Sometimes shoots are prepared for hardwood cuttings by girdling them several weeks in advance. Girdling is the technique of constricting or cutting the stem bark, which blocks the downward flow of carbohydrates, hormones, and other substances through the phloem. These substances accumulate at the base of the cutting where they stimulate increased rooting.4 Girdling can be done by lightly scoring the bark or tying string or wire around the shoot base.

All cuttings should be cut off from the parent tree just below a leaf node or bud. If girdling is used, the cut is made either on the girdled area or slightly below it, that is, closer to the parent plant. When making hardwood cuttings it is often best to include the shoot base and the area just below it because in some trees this is the area most likely to sprout, since most carbohydrates are stored here. In many cases like that of the olive, the cuttings are made with a small piece of bark and cambial tissue from the main trunk still attached to ensure that the base is included (Figure 7.3). This piece is sometimes called a heel.


Figure 7.3 A Cutting with a Heel

A sharp tool such as machete or knife makes it easier to get good clean cuts, minimizing the damage to both tree and cutting. If possible the tool should be washed with soap, bleach, alcohol, or vinegar to remove any disease-causing microorganisms. If the wound on the parent plant is large or exposed to harsh conditions, shading the wound will help protect the plant from shock.

Softwood cuttings should be planted as soon as possible after being removed from the parent tree. If there is a delay between removal and planting these cuttings must be kept cool and moist, by wrapping them in a damp cloth for example. Putting the cut end in water is not recommended as this can leach out needed nutrients and hormones.5

Hardwood cuttings taken during the dormant season should be kept cool and moist until planting time. One way to do this in temperate regions is to bury them outdoors in the sand.6 The cuttings should be buried horizontally, or vertically with their tips pointing downward. Burying them prevents the cuttings from sprouting in storage and then dying due to lack of water and cold temperatures. If the cool-season temperatures are very cold (at or below 0°C, 32°F) then the cuttings need to be buried in a protected area, or in a box of sand indoors.

Cuttings are planted so that at least two leaf nodes are below the soil surface. The soil around cuttings must be kept moist but not saturated. However, because cuttings - especially softwood ones - are susceptible to rotting good drainage must also be provided. Therefore, the area immediately around the cutting should be sand, or a mixture of sand and garden soil which drains well. If any containers are used they must also have good drainage. (See section 8.4.2 for more suggestions on preparing a permanent planting site for the tree’s future growth.)

Once planted, cuttings need to be protected from drying sun and wind with mulch, shades, and wind-breaks (section 10.8). Sometimes cuttings are covered with mud or a thin clay and water wash, or wrapped with straw to prevent sunburn and to reduce transpiration.7

7.2.2 Perennial Herbs

Young stem cuttings can be used to propagate a number of perennial herbs such as the mints, oregano, marjoram, and rosemary. The best time to take cuttings from these herbs is at or just before the period of most rapid growth. The cutting should be young, healthy, non-woody growth. The length of the cutting will vary with the plants’ size and type but should include at least five leaf nodes.

The bottom one-half to one-third of the cutting, including several leaf nodes, is submerged in a container with water. Since root production is sometimes inhibited by light, keeping the submerged portion in the dark may help. This can be done by putting the rooting end in an opaque container and covering the area around the stem with a piece of cloth. The top, leafy part of the cutting should be exposed to normal day and night light cycles because it must photosynthesize to produce energy for root growth.8

Roots will develop from the cutting in several days or weeks. Cuttings root most easily when it is warm and humid. When roots have developed, the cutting can be planted in moist soil, shaded, mulched, and watered probably once or twice a day during the first few days.

7.2.3 Cassava.

Cassava, grown for its edible roots and leaves, is propagated from stem cuttings called sticks.9 The sticks are cut when the plant is mature and the roots are being harvested. If not planted within a few days of cutting, the sticks can be stored dry and in the shade for up to 8 weeks. If stored, their ends must be recut just before planting. The sticks should:

· Be from a plant free of cassava mosaic virus (see section 13.3.4 for a general discussion of plant viral diseases).

· Have at least three leaf nodes.

· Be taken from the central area of the plant, about 15 cm (6 in) above the ground, to get pieces mature enough that they will not rot10 (Figure 7.4).

· Be 30-45 cm (12-18 in) long.


Figure 7.4 Taking a Cassava Cutting

Sticks are planted by burying two-thirds of the lower (older) end in the soil. The buds point up to the upper, younger end of the stick. In parts of both East and West Africa cassava sticks are frequently planted at approximately a 45° angle11 (Figure 7.5).

7.2.4 Sweet Potatoes

Sweet potatoes are also grown for their swollen tuberous roots. They can be propagated from vine cuttings at least 20 cm (8 in) long, taken from healthy growing tips of mature plants. The cutting should have at least seven nodes and can be planted vertically or at an angle, one-half to two-thirds of its length. If the soil is kept moist, roots can develop from the nodes in as little as five days12 (Figure 7.6). The tuberous roots can also be planted in a nursery bed and when shoots grow to 23-30 cm (9-12 in) long they can be separated and planted.


Figure 7.5 Planting a Cassava Cutting


Figure 7.6 Planting a Sweet Potato Cutting

7.3 Tubers, tuberous roots, and bulbs

Tubers are enlarged stems that grow underground. Hausa potatoes, tiger nuts, Jerusalem artichokes, potatoes, and yams are dryland garden plants that can be propagated from their edible tubers. The tubers are selected at harvest time and stored until the next planting season.

Tubers chosen for propagation should be:

· Mature.
· Free of disease (sections 13.3.4, 13.4).
· Not blemished with cuts or bruises.
· Of good eating quality.

Potatoes, Hausa potatoes, and yams are usually grown from setts, or pieces of tubers. Setts are made by cutting up the tuber so that each piece has least one or more eyes or leaf nodes. Tiger nut tubers are soaked in water for a day and then planted whole.13 Jerusalem artichokes are also planted whole.

Tubers are usually planted horizontally. In hot drylands setts can be protected from heat, which inhibits shoot or slip production, by planting them with the cut side down. Both whole tubers and setts are covered with a layer of soil three to four times their thickness.

Swollen edible storage roots, or tuberous roots, resemble tubers, but are actually part of the root of the plant and not of the stem. Most garden root crops such as beets and carrots are grown from seed, but sweet potato can be grown by planting the root itself, as well as from shoots (section 7.2.4).

Bulbs like garlic and onions are formed by enlarged leaves which grow underground. Each of the cloves of a garlic bulb can be planted to produce new plants. Thus 10 cloves of garlic can produce 10 new bulbs. To grow garlic the cloves of a mature bulb are separated, and each one planted separately, with the narrow tip pointing upward (Figure 7.7). Some onions called multiplier or bunching onions grow in clusters similar to garlic bulbs and are propagated the same way.

Many nonbunching onions are biennials because their full life cycle takes 2 years to complete. In the first year seeds are planted which produce plants with edible bulbs. During the second year a bulb is planted and grows a shoot that flowers and produces seed (Figure 7.8). Because part of this cycle involves the use of seeds, nonbunching onions are not really vegetatively propagated.

In some areas of savanna West Africa nonbunching onions are propagated indigenously in the following way:14

a) The selected bulb is cut approximately in half horizontally and the top is eaten.

b) The bottom half is planted with the cut side up in a nursery bed, and side buds or offsets (section 7.4) sprout from the base and grow into shoots.

c) When the shoots are 10-15 cm (4-6 in) tall the bulb is dug up and the shoots separated and transplanted into their permanent growing site.

d) Each of these shoots will flower and produce seeds used to grow bulbs for eating and planting.


Figure 7.7 Planting a Garlic Clove


Figure 7.8 Planting an Onion - a Biennial Bulb (1)


Figure 7.8 Planting an Onion - a Biennial Bulb (2)

7.4 Offsets

Offsets are lateral shoots used for plant propagation. They develop around the base of a monocot’s stem or trunk, and are allowed to grow and develop their own root system before being separated from the parent plant and planted elsewhere.15 When natural production of offsets is slow, cutting back the main stem, as described for nonbunching onions in West Africa (section 7.3), can stimulate their formation. A few dryland garden plants propagated by offsets are certain onions, date palm, banana, and pineapple. In bananas these offsets are called suckers, but because they grow from the corm, which is an underground swollen stem, they are in fact offsets. Box 7.1 gives an example of propagation with an offset.

Box 7.1
Propagation by Offset - the Date Palm

The following is an example of indigenous offset propagation of the date palm, an important dryland garden tree.16

a) The planting site is prepared for the transplant (section 8.4.2).

b) Date offsets are not removed until they have a root system and some of the dark green foliage characteristic of a mature plant. This usually takes 3 to 10 years after the offset first appears.

c) Thoroughly watering the area around the offset makes it easier to dig up.

d) The offset’s leaves are tied and often cut back to as little as one-half of their original length. The remaining leaves are then wrapped in a moist cloth to decrease water loss.

e) The offset’s longer roots are dug up and cut to a length that can be transplanted.

f) A chisel or other sharp tool is used to separate the offset from the parent plant (Figure 7.9). In cases of difficult separation it may be best to cut more from the parent plant than the offset because the larger plant is better able to recover from such cutting. However, this should be done very carefully because a large, healthy, productive tree should not be jeopardized to remove an offset. In some areas it is common to coat the exposed surfaces of both plants with wet clay or soil to prevent sunburn and dehydration.

g) The offset is transplanted as soon as possible or temporarily placed in water. Figure 7.10 shows date palm offsets soaking in a small irrigation canal at an oasis in western Egypt.


Figure 7.9 Separating Offsets from a Date Palm


Figure 7.10 Date Palm Offsets Soaking in an Irrigation Canal in Western Egypt

7.5 Suckers

Shoots originating underground from the roots of dryland trees like olives and jujubes are called root suckers (Figure 7.11). Drought encourages root suckering in jujube trees. Suckers can be dug up and separated during the dormant or “pregrowth” season and transplanted. Often a small portion of the parent root is transplanted with them. Suckers growing from a tree which was grafted onto root stock will produce plants with the same qualities as the stock, not the scion of the parent plant (Figure 7.12).

7.6 Grafting

Grafting is the technique of connecting pieces from two different plants so that they will develop and grow as one plant. The stock is the part used as the base and roots of the new plant. The scion is the piece used as the top, fruit-producing part of the new plant. The union between the stock and scion, each from different plants, is formed from the contact of the cambium, a layer between the phloem and xylem in dicots. (Figure 5.3 and the discussion on the vascular system in section 5.2 are helpful for understanding how grafting works.)


Figure 7.11 A Root Sucker Growing from a Jujube Tree


Figure 7.12 Suckers Growing from the Stock of a Grafted Tree

Parenchyma cells are thin-walled plant cells that perform many functions. They are produced in the cambium of both stock and scion to heal the graft wound. The parenchyma cells, which line up with the cambial cells of stock and scion in the healing process, later become new cambial cells. In turn, these new cambial cells form new xylem and phloem cells, establishing connections between the vascular system of the stock and the scion. This connection is only possible in dicots, and is essential if the scion is to receive the water and nutrients necessary for it to survive.

Grafting is used when one variety is hardy and resistant to root disease, but produces low yields of poor-quality fruit, and another variety is sensitive to drought and root diseases but has good fruit production (Figure 7.13). In such cases the scion of the fruit producer may be grafted onto a stock of the hardy variety. Sometimes a hardy, nondomesticated variety is used for stock. For example, the atlantica pistachio (Pistacio atlantica), which bears inedible fruits and grows wild in the Negev Desert, is used as root stock for domesticated pistachio scions.17 Grafting is also useful for changing fruiting varieties of a tree or for repairing a damaged tree.

7.6.1 Compatibility for Grafting

Grafting is limited by the compatibility of stock and scion. There are no simple or absolute guidelines for determining if two plants are compatible, but using plants in the same genus is often, though not always, successful. For example, the stone fruits - almonds, peaches, and apricots - are all in the genus Prunus. Almonds and apricots graft well onto peach root-stocks, but an apricot scion is not compatible with almond stock nor is an almond scion with apricot stock.18


Figure 7.13 Scion that Produces Good Fruit is Grafted onto Vigorous, Hardy Stock

When the graft fails soon after being made the reason could be either incompatibility or a problem with the graft itself, such as infection, pest damage to the graft tissue, drying of the graft wound, or poor alignment of cambial tissues. Incompatibility between scion and stock may be obvious shortly after grafting but it can also take years to show up. It may be due to a failure to produce new xylem or phloem tissues or the production of a chemical or virus by one part of the graft which is poisonous to the other.19 If, years after grafting the scion breaks off the stock in a clean line where the graft wound was it is almost certainly because of incompatibility. The clean break shows that the graft failed to join into an integrated, strong union. Indicators that a graft combination may be incompatible are:20

· Failure to form a graft union.

· Yellowing leaves at the end of the growing season, early seasonal leaf loss in deciduous trees, slow growth, shoot dieback and general lack of vigor.

· Differences in the growth of stock or scion.

· Differences between stock and scion in the time when their new growth for the season begins or ends.

· Premature death of trees.

Experimentation and consultation with local gardeners and other experts in the area are the best ways to determine which grafting combinations are compatible.

7.6.2 Effects of Stock and Scion on the Grafted Tree

The effect of stock and scion on each other depends on their characteristics, as well as on the growing conditions. The stock should be adapted to local soil moisture, salinity, pH, and drainage. Although these conditions affect both stock and scion, the impact on the stock has greater influence on the tree’s health. In some cases tree size is determined by the stock and “dwarfing stock” is used to produce small, compact trees.

The influence of the stock on fruiting is complex and variable. Time of blossoming and fruit set, number of years a tree will produce fruit, fruit size, and total yield can all be affected. For example, one experiment found that when scions of orange, grapefruit, and tangerine are grafted to sour orange (Citrus aurantium) stock, they produce fruits that have smooth skins, are juicy and sweet, and store well. But the same scion varieties grown on rough lemon (Citrus Union) rootstock frequently produce fruits with thick skins, having poor taste, texture, and storage qualities.21

Scion varieties are chosen primarily for their fruiting characteristics such as flavor, texture, size, quantity, and storage qualities. In some cases scions may influence the vigor and growth pattern of the stocks. For example, on a grafted citrus tree with a vigorous rootstock and a weak scion the scion will dominate and slow the tree’s growth.22

There are many ways of grafting using different cuts, some more adapted than others to particular plants and conditions. Gardeners in many areas have a long tradition of grafting their trees and are often very knowledgeable about techniques and the best tree varieties to use. Working with them is the best way to learn about grafting and grafting problems in the local area. In the following sections we give a brief introduction to four basic grafting techniques. The first three are usually used for young trees, the last one for more mature trees.

7.6.3 Approach or Attached Scion

The approach or attached scion grafting method is useful for many species that are difficult to graft. It is also used for grafting during stressful environmental conditions such as drought. Overall it is the most reliable grafting method although it does require more time and attention than others. With this technique both stock and scion are growing rooted plants during the grafting process which reduces the risk that either will die. Mangoes and guavas are two dryland garden trees that are grafted using this method.

There are many different cuts that can be used for this method; the one shown in Figure 7.14 is the most common, sliced-approach grafting. Stock and scion plants whose stems are the same size are selected. Smooth, flat, vertical cuts are made on both stock and scion to expose cambial tissue. The wounds are matched, bound together with string or other fiber, and sealed with mud or wax. It takes approximately two or more months for an approach graft to become successfully joined. When the union has formed, the root of the scion and the top of the stock are gradually cut back and removed. This should be done over several weeks, first nicking the scion about 2.5 cm (1 in) below the graft union. A week later the cut is deepened, and during the third week it is completed. At the same time, between half and all of the stock above the graft union should be removed.23

7.6.4 Budding

This is a popular grafting method in which a bud is grafted onto the stock plant. There are many types of cuts used depending on the kind of tree and local conditions and practices. In hot drylands the bud should be placed on the side of the stock plant that is most protected from sun and wind.

Budding is done during periods of active growth for the stock plant, when the cambial cells are rapidly dividing.24 North of the equator budding is done in spring (March-April), June (late May-early June) and fall (late July-early September). South of the equator budding time is spring (September-October), December (late November-early December) and fall (January-early March). Fall budding is most common and is done before the end of the growing season when the rootstock seedling has grown large enough to support a bud. In the spring, rootstock growth above the graft is removed and the bud produces the new top. Spring budding is done right after the rootstock starts growing again. About 2 weeks after grafting the rootstock growth above the bud is removed and the new top grows from the bud. In places with long growing seasons December (or June in the northern hemisphere) budding may be used. A rootstock seedling started in the spring of the same year is used, as are buds of the current season’s growth. Soon after grafting, the root-stock growth above the bud is removed to allow the bud to produce the new top growth.

Some dryland garden trees that can be bud grafted are mango, guava, citrus, jujube, and the stone fruits.


Figure 7.14 Approach Grafting (After Dupriez and De Leener 1987:234) (1)


Figure 7.14 Approach Grafting (After Dupriez and De Leener 1987:234) (2)

T-BUDDING The T-bud or shield is one of the quickest, easiest, and most reliable budding methods. However, if the stock plant is stressed by drought, pests, or other problems it will not work. This is because it will not be possible to separate the bark from the cambium and xylem (often referred to as the wood), as is necessary for this method.25 If this is the case, chip budding may be more appropriate because it does not require separation of the bark and wood.

In T-budding the stock is prepared by making a vertical cut about 3 cm (1.25 in) long through the bark and a horizontal cut, extending about one-third of the distance around the tree, intersecting the vertical cut to form a T (Figure 7.15).26 The scion bud is cut out by making a horizontal cut through the bark and into the wood about 2 cm (1 in) above the bud. Then an upward slice is made to meet the horizontal cut, starting about 1.5 cm (0.5 in) below the bud and slightly into the wood to ensure that the bud is included. This bud piece is then pushed down, under the flaps of stock bark until the horizontal upper edges of stock and scion are flush. The bud piece should fit snuggly and be covered by the two flaps of bark. Then the wound is bound and tied shut, avoiding any direct pressure on the bud.


Figure 7.15 T-Budding

CHIP BUDDING As already mentioned, chip budding does not require separation of the bark from the stock, and so can be used when the bark will not separate, for example, when the weather is dry. Instead, this method relies on the contact of two flat surfaces. The stock seedlings used are frequently still quite young; mangoes, for instance, are only three weeks old.

The stock is prepared by making a horizontal slice below the bud that is to be removed. A second slice starting above that bud, running around and behind it, and meeting the first cut will free the piece from the stock. This piece can be composted. This exposes an area of woody tissue ringed by cambial tissue and bark. Continuing the vertical cuts slightly below the horizontal one on the stock will create a lip on the lower end of the stock cut. A scion bud can be cut out following these same steps and should be of matching size with the exposed area on the stock. The scion is placed on the stock so that their cambial tissues are in contact, with the lower lip of the stock helping to keep the scion in place (Figure 7.16). The graft is then tied and carefully wrapped to cover all exposed cuts.

CARE FOR THE BUDDED SEEDLING The budded seedling should be shaded and watered during the healing process. Tying some leaves above the grafted bud will give it protection from the hot sun (Figure 7.16). About 10 days after grafting, whatever material has been used to wrap the graft should be cut to avoid constricting the bud. Once the graft has been successfully joined, which can take about 2 to 4 weeks, the focus of the plant’s growth must be shifted from stock to scion.

In many plants a condition of apical dominance exists; that is, the top or terminal bud is the center of growth, supressing growth in any lower buds. To overcome apical dominance the top of the stock may be bent over and tied or pegged at a level lower than the graft. This focuses growth on the scion while still using the stock as a source of food until the graft is more established, at which time the stock can be cut off just above the graft. Other methods are to bend and partially break the stock, or simply cut it off above the graft immediately after grafting.27


Figure 7.16 Chip Budding


Figure 7.17 Apical Grafting

7.6.5 Apical Grafting

Apical grafting is a technique in which there is no stock above the grafting point. It is used on dryland garden trees such as olive, peach, and citrus. For apical grafting the scion should include several buds. Matching areas of cambial tissue on stock and scion are joined (Figure 7.17). If the stock is larger than the scion, only a portion of the cambial tissue can be joined (Figure 7.20 with Box 7.2). The graft is then tied into place. Because of the relatively large amount of surface area exposed in this kind of graft it is important to cover it with a protective seal, such as the clay and dung mixture described in Box 7.2.

7.6.6 Topworking

Topworking is done on trees that are more mature than those used in the three grafting methods already described. Citrus, olive, avocado, and mango are examples of dryland garden trees for which topworking may be appropriate. There are many different ways to do topworking. One method used when younger trees are the stock is to remove all branches except the structural limbs and lateral branches 7.5-10 cm (3-4 in) in diameter. Scion cuttings with at least three buds and with lower ends cut into a wedge shape with one side longer than the other are inserted into cuts made near the base of the stock tree’s lateral branches (Figure 7.18). The ends of the stock branches on which a graft has been placed are then cut back at an angle just above the graft. The tips of other stock tree limbs are cut back so they are not higher than the highest scion.

The advantage of topworking is that the established stock tree provides the root and branch structure; this allows the scion to start fruit production more quickly than in other forms of grafting. Shading is very important for topworking as with other types of grafting, though it may be more difficult because it involves protecting a whole tree. Sometimes a thin coating of a clay and water mixture is applied to protect the scions from sunburn (Box 7.2). Leaves or palm fronds tied around the main limbs also provide shade. As soon as the scions start to leaf out and grow they will provide some shade. After the graft is well joined, the branch angle spreading technique described in section 8.7.2 can be used. To ensure the establishment of the grafts, all new stock growth such as suckers and other shoots should be removed as soon as they appear.

7.7 Layering

Layering is a method of vegetative propagation that encourages the growth of adventitious roots from branches or shoots, eventually producing a viable plant that may be separated from the parent and transplanted elsewhere.


Figure 7.18 Topworking a Tree with Stub Grafts (1)


Figure 7.18 Topworking a Tree with Stub Grafts (2)


Figure 7.18 Topworking a Tree with Stub Grafts (3)

By bending, girdling or cutting, the movement of food and hormones through the phloem down to the roots is interrupted. Root development is encouraged where these substances accumulate, just above the cut or bend. The flow of water and minerals from the roots to the layered part continues through the intact xylem. Rooting is also encouraged when the stem is hidden from light by being wrapped or covered with soil.

If simple layering is to be done, layering material such as shoots and low-hanging branches should be allowed to grow. Severe pruning one year before layering encourages the growth of young shoots around the plant’s base which can be used for layering.32

Deciduous plants such as figs and grapes are layered during the dormant season and so are prepared to take advantage of the increase in vegetative growth that follows. Evergreen trees like the cashew, carob, and mango can be layered just before or during a period of rapid vegetative growth. Perennial herbs such as mint, thyme, and oregano can also be propagated in this way.

Box 7.2
Guidelines for Grafting

· When choosing stock and scion keep in mind that:

- Plants in the same species are usually but not always compatible.

- Differences in maturity between stock and scion are not usually a problem, although young seedlings for stock and growth less than 2 years old for scion, graft more readily.28

- Both stock and scion should be alive and healthy.

· The best scion material:29

- Is young, less than 2 years old, with short inter-nodes.

- Has healthy vegetative buds, not flower buds. Vegetative buds are narrower and more pointed than flower buds. The buds should not be in their most active growth phase; if they are the scion will leaf out immediately and the leaves will transpire and use up the water available to the scion rapidly, causing it to dry up and die.

- Is from the upper part of the tree but includes only the bottom two-thirds of the shoot.

· The best stock are young, vigorously growing plants. For topworking however, they are often older than for other grafting techniques. Although rooted cuttings or layered plants can be used, seedlings produce the strongest rootstock plants.

· Both stock and scion should be washed clean of dirt and debris before the cuts are made.

· For surface cuts in bud grafting, making the starting and ending slices before doing the entire cut avoids a sloppy cut which can strip off bark (Figure 7.19).


Figure 7.19 Cutting Grafts to Avoid Tearing the Bark

· The cutting tools should be sharp to give a controlled, clean cut without crushing the plant’s tissues and leading to infection.

· Cleaning cutting tools with soap, bleach, alcohol, or vinegar between cuts avoids spreading diseases.

· The scion pieces should be oriented properly. The upper (farther from the roots) and lower (closer to the roots) ends of the scion should be identified and kept in the same position in the graft. This is easy to do because the buds always grow up, with their pointed ends aimed away from the ground. If the scion is turned upside-down the graft will fail.

· Matching the cambial tissue of stock and scion of similar size is relatively easy. If stock and scion are of different sizes or stages of development the inner edges of their bark must be matched to ensure contact of cambial tissues (Figure 7.20).30

· Once the graft is in place, it can be secured with plant fibers, leather strips, rubber bands, pieces of plastic, or string. These ties must be removed when the union has healed so they will not constrict the growing tree.

· Under hot, dry conditions it is particularly important to see that the graft and any exposed cut surfaces do not dry out, killing the cells and preventing a union. A mixture of two parts clay and one part fresh cow dung can be used,31 and adding some plant fibers or animal hair to the mixture will make it stronger and less likely to crumble away as it dries. Heated beeswax, painted over the ties that hold a graft in place, can be an effective seal. Melted paraffin wax can also be used although it has a lower melting temperature than beeswax. However, melted wax should be applied carefully to the area of the wound to avoid burning the graft tissues. In some extremely hot drylands wax seals may not work because they will melt off. Other useful materials include plastic, which is wrapped around bud grafts on mango trees in Mali, moist cloth, and possibly local tree resins.


Figure 7.20 Matching the Cambial Tissue at the Inner Edge of the Bark (After Garner, et al. 1976:86) (1)


Figure 7.20 Matching the Cambial Tissue at the Inner Edge of the Bark (After Garner, et al. 1976:86) (2)


Figure 7.20 Matching the Cambial Tissue at the Inner Edge of the Bark (After Garner, et al. 1976:86) (3)


Figure 7.20 Matching the Cambial Tissue at the Inner Edge of the Bark (After Garner, et al. 1976:86) (4)


Figure 7.20 Matching the Cambial Tissue at the Inner Edge of the Bark (After Garner, et al. 1976:86) (5)


Figure 7.20 Matching the Cambial Tissue at the Inner Edge of the Bark (After Garner, et al. 1976:86) (6)

7.7.1 Simple Layering

For simple layering (Box 7.3), vigorous shoots from around the parent plant’s base work well because they bend easily and tend to root quickly. The shoots used should be about 1 year old and sufficiently long to produce new roots far enough away from the parent that digging them out will not harm the parent’s own roots. Branches are also used but because they are older tissue than shoots, they are stiffer to bend and slower to produce roots.

7.7.2 Air Layering

Another kind of layering called air layering, or marcottage, is based on the same principles as simple layering but is done on shoots that are not buried in the soil (Figure 7.22). Young shoots, about 1 year old with leaves and leaf buds, growing in an exposed part of the plant are best. The shoot is girdled, removing a ring of bark approximately 2.5 cm (1.3 in) wide near the base. Care should be taken not to injure the xylem as this will weaken the shoot and interfere with the flow of water and nutrients to the shoot tip. The girdling wound is surrounded with a light rooting mixture which can be made of compost, sand, sawdust, and soil. This mixture is moistened and then wrapped in plastic, cloth, or bark and leaves, and bound in place. During the rooting period the soil mixture must be kept moist; shading can help. Rooting can take from 30 to more than 100 days after which the shoot should be slowly cut from the parent plant and transplanted, preferably during a period of slow growth. Air layering is frequently used with avocado, cashew, and mango trees.

Box 7.3
Guidelines for Simple Layering

The following are guidelines for simple layering in drylands.

a) Vigorous shoots with numerous leaf buds that can be easily bent to the ground are selected.

b) The area around the parent plant should be cleared of weeds and other debris.

c) Organic matter is worked into the soil where the layering will be done. Adding sand to clayey soils improves drainage.

d) A small depression is dug in which to place the shoot and to concentrate moisture.

e) All leaves are removed from the layering material except those on and around the tip, which is gently bent to the ground, and all buds on the upper surface are removed. In the case of trees, some sort of wounding is usually done to encourage the production of roots in the buried portion of the shoot. This can be done by girdling or making several cuts through bark and cambial tissue on the shoot’s lower surface. Filling these cuts with soil prevents the wound from growing shut (Figure 7.21).

f) The layering material is staked or weighted down so that it forms a U-shape or at least a right angle, with the growing tip extending up vertically. If layering trees, wooden or metal stakes, rocks, or other heavy objects, can be used to hold down the branches or shoots. Freshly cut wooden stakes can sometimes encourage the growth of fungi and other materials are better. The shoot is buried with compost-enriched soil, leaving the tip exposed above ground.

g) The layered shoot should be watered generously, and the area kept moist. However, it must not get waterlogged as this will cause the shoot to rot. Under hot, dry conditions the shoot should be mulched and shaded to protect it from drying and high temperatures, both of which inhibit rooting.

h) After the growing season or when the shoot tip appears to be growing, well-established and vigorous, the shoot can be cut off from the parent plant. This can be done all at once if the shoot is very vigorous, has well-developed roots, and the environmental conditions are not stressful. However, making the separation gradually, first nicking the parent branch, cutting more and more and then completing the cut over a few weeks minimizes shock to the shoot. Cutting as close as possible to the original rooting point without damaging the new shoot’s roots is best (Figure 7.21). Once cut, the new plant is left in place two to three weeks before transplanting, allowing it to recover from the separation.

7.8 Resources

For information on grafting, talk with local gardeners, farmers, and extension workers who are experienced grafters. For a more formal, printed, source of information on grafting, layering, and other means of propagating fruit trees, The Propagation of Tropical Fruit Trees (Garner, et al. 1976) is a useful book. While it focuses on the commercial production of a limited number of common tree crops, it still provides some useful basic information about propagation. However, the technical information is often not relevant for the gardener or small farmer. Plant Propagation by Hartmann and Kester (1983) is a widely used source on plant propagation in the English language. The background information on the anatomy and physiology of different propagation methods is its most valuable contribution to gardeners. The methods described are frequently only relevant for large-scale commercial propagation and many of the specific examples are of temperate-region ornamentals. Aumeeruddy and Pinglo (1989:36-48) give a number of very brief examples of indigenous techniques of vegetative propagation.


Figure 7.21 Simple Layering of a Fig (1)


Figure 7.21 Simple Layering of a Fig (2)


Figure 7.22 Air Layering, or Marcottage, of a Cashew (1)


Figure 7.22 Air Layering, or Marcottage, of a Cashew (2)

References

1 Nokes 1986:38.

2 Nokes 1986:42.

3 Hartmann and Kester 1983:205-206.

4 Hartmann and Kester 1983:260.

5 Garner, et al. 1976:61.

6 Hartmann and Kester 1983:304.

7 Garner, et al. 1976:65.

8 Hartmann and Kester 1983:278.

9 Kassam 1976:54.

10 Acland 1971:36.

11 Kassam 1976:56.

12 Kassam 1976:66.

13 Irvine 1969:185-187.

14 Kassam 1976:83.

15 Hartmann and Kester 1983:485.

16 FAO 1982b; Popenoe 1973.

17 Evenari, et al. 1982:364.

18 Hartmann and Kester 1983:368.

19 Hartmann and Kester 1983:374-376.

20 Hartmann and Kester 1983:371.

21 Hartmann and Kester 1983:380.

22 Hartmann and Kester 1983:382.

23 Garner, et al. 1976:108.

24 Hartmann and Kester 1983:449ff.

25 Hartmann and Kester 1983:448.

26 Hartmann and Kester 1983:456-459.

27 Garner, et al. 1976:124.

28 Garner, et al. 1976:84.

29 Hartmann and Kester 1983:427.

30 Garner, et al. 1976:85-80.

31 Garner, et al. 1976:90.

32 Garner, et al. 1976:39.

33 Garner, et al. 1976:42-48.

8. Plant management

Young plants are tender and especially vulnerable to stress and a plant of any age needs special protection for a period after being transplanted, especially under hot, dry conditions. However, once a garden plant becomes established in its final growing site it needs less attention. As the plant matures and the root system grows, it is better able to withstand heat, drought, disease, and pests, and will not need to be watered as frequently. At this point the main work in the garden is maintaining the plant’s health to ensure a good harvest.

8.1 Summary

Seeds or plant parts for vegetative propagation can be planted in nursery beds, containers, or directly in the garden. Starting plants in places other than their permanent growing site can offer such advantages as convenience for the gardener and special conditions that encourage and protect young plants. Eventually all plants started in temporary growing sites must be transplanted. The choice of a permanent growing site for garden crops depends on the gardener, the crop, and the growing environment. Transplanting requires care especially where hot, dry conditions add to the stress that the plant experiences. Weeding, pruning, and trellising are management practices that help maintain and improve the health and productivity of the garden.

8.2 Nursery beds and container planting

Seedlings and recently planted cuttings require extra care and attention. To make this work easier they are sometimes started in nursery beds and containers, temporary growing places from which healthy young plants can be transplanted into the garden. Whether nursery beds or containers are useful or not depends on the type of plant being propagated, convenience for the gardener, and environmental conditions such as temperature, wind, rainfall, pests, and disease.

8.2.1 Nursery Beds

Nursery beds are garden beds devoted entirely to starting plants, both from seed and by vegetative propagation. They may be in the garden itself or elsewhere, but wherever they are located, nursery beds are planned and planted as the temporary location of the plants, not their final growing site (Figure 8.1).

One indigenous example comes from central Mexico where a system of canal-fed gardens is used and special nursery beds are constructed for starting grain amaranth seedlings during the dry season.1 A rectangular bed measuring approximately 2 m × 15 m (6.5 ft × 50 ft) and 4-5 cm (1.5-2 in) deep is filled with rich, muddy soil scooped up from the canal bottom. The next day as the soil dries it is cut into 3 cm × 3 cm (1.25 × 1.25 in) squares with a special slicing rake called a cuchilla. Using a small stick or their fingers, the gardeners make a small hole 1 cm (less than 0.5 in) deep in the middle of each square and then drop amaranth seeds in. The bed is sprinkled with manure which is then swept off so only that which falls into the planting hole remains, covering the seeds. Twenty to 30 days later the seedlings in their soil cubes are transplanted into the garden beds. Where clayey soil is available gardeners can try making soil cubes to start transplants (section 8.2.2).

Nursery beds allow the gardener to concentrate and save resources such as water and time, and to take advantage of favorable microclimates for early- or late-season germination. For example, in temperate drylands warm-weather plants can be started in sunny, protected areas before the end of the cool season. Moving nursery beds seasonally or yearly helps avoid problems with nematodes which can build up in continuously cultivated soil (section 13.3.2).


Figure 8.1 Nursery Beds for Starting Seedlings for the Garden

Steps in selecting and planting a nursery bed are:

a) A site is selected that is convenient for daily care, easy to protect from wind, sun and pests, and has enough room for plants to grow to the stage at which they can be transplanted.

b) Soil and bed are prepared as described in Chapter 9 except that the soil need not be as deep because plants will not grow to maturity here. For most annuals 15-20 cm (6-8 in) is deep enough. When establishing a nursery bed for trees, the length of time required for them to become strong enough to transplant needs to be taken into account. This will affect where the nursery bed is located and how deeply the soil should be prepared.

c) Planting depressions are made (see section 6.4.2).

d) Seeds may be sown more thickly than in a permanent bed because plants will be removed and dispersed before maturity. However, enough room must be left so that individual plants can be removed for transplanting without major damage to the roots.

e) The sown seeds are covered with soil and possibly mulch. The soil is kept moist by gentle watering as needed. Shades, windbreaks, and bird-scaring devises are often used.

8.2.2 Container Planting

Containers offer many of the benefits of nursery beds with the added advantage of being easily moved to adjust to changes in sunlight, shade, temperature, and the gardener’s schedule. Containers are often used for starting cuttings or seedlings for transplanting. Some plants, whose leaves or small fruit are continually harvested in small quantities for seasoning or medicinal purposes, are kept permanently in containers. Examples include mint, basil, oregano, marjoram, rosemary, epazote, and chili peppers.

Containers may be the only option for urban gardeners with little space and no access to land. For them, permanent container plantings on rooftops, in windows, or on balconies can provide herbs, condiments and some greens for the household meals (Figure 8.2).

CONTAINERS A wide variety of locally available and free or inexpensive containers can be used for planting including calabashes, pots and pans with holes in the bottom, baskets, steel cans, wooden boxes, trays, cardboard cartons, plastic bottles, and other made, found, or discarded materials (Figure 8.3). Galvanized steel containers are not good because this material gives off zinc salts toxic to plants.2

Baskets made of leaves or other plant fibers make excellent containers for eventual transplants. Often there is no need to remove tree seedlings from a basket container when transplanting, because the basket will eventually decompose in the soil, thus avoiding disruption of the tree’s root system. However, the basket fibers should be cut through and pulled open in several places before planting the tree. This is especially true if the basket is tightly woven or made of a tough, woody material. Planting containers should not have been used for paint, fuel, pesticides, or other substances that can harm the seedling, contaminate the garden where it is transplanted, or poison people and animals.


Figure 8.2 Container Gardening in Mexico City

Soil cubes are good for starting transplants because the seedling’s roots are not disturbed by removing the soil from a container. The soil cubes described in section 8.2.1 are slices of a special type of nursery bed, but soil cubes or blocks can also be made and used like containers. One way to make the blocks is to press the wet soil into a form such as a container or even a hole in the ground. Then as it dries, it is sliced into squares which will be cubes when removed from the form. However, as already mentioned, the soil used must have the right clay and humus content to hold together in blocks until it is time to transplant. Timing of transplanting is also important, because if the seedlings’ roots grow out of the soil block they have no protection and will dry up or be injured when transplanted.


Figure 8.3 Containers (1)


Figure 8.3 Containers (2)


Figure 8.3 Containers (3)


Figure 8.3 Containers (4)


Figure 8.3 Containers (5)


Figure 8.3 Containers (6)

CONTAINER SIZE Containers can be of any size and may hold one or more plants, but should be small enough so that the gardener can move them without difficulty. However, very small containers, such as a cup, are not appropriate for most plantings as they offer little room for the plants to grow and insufficient soil from which to receive nourishment. Also, the larger the container the more soil there is for holding water, and the less frequent the waterings need be. Containers should be deep enough to allow the seedling’s roots to develop to the stage when they can withstand being transplanted. To allow transplanting, the top opening must be as big, but preferably bigger, than the bottom, making it easy to remove seedlings without damaging their roots (Figure 8.4). Vertical ridges on the inner surfaces of containers prevent roots from growing in a spiral pattern which results in root-bound seedlings (section 8.4.4). A community garden or tree nursery project that includes local manufacture of containers can design them with a cone shape and vertical ridges on the inner surface (Figure 8.5).


Figure 8.4 Container Openings (1)


Figure 8.4 Container Openings (2)

TEMPERATURE REGULATION The relatively small amount of soil in containers can dry out rapidly and the soil temperature can rise and fall to extremes of hot and cold, all of which are harmful to plant growth. Containers with thick walls provide better insulation, reducing temperature fluctuations. Under sunny, hot conditions light-colored containers are good because they reflect sunlight, and thus heat, away from the soil. In the cold season or in cold areas, dark-colored containers that absorb sunlight and heat may be better. A container can be shaded and insulated by wrapping it with cloth, or by mounding soil, compost, or leaves around it.


Figure 8.5 Ridges on the Inside of Containers

WATERING AND DRAINAGE Containers must allow drainage of excess water or else waterlogging (section 12.6.1) and damping-off (section 13.4) will occur. Holes can be made in containers of most materials except glass and fired clay, which are so brittle that holes cannot be made in them without breaking. In addition to holes, a few small stones or pieces of broken clay pots in the bottom of containers can improve drainage.

Salts in water or soil can accumulate on unglazed ceramic containers, appearing as a white patina or coloring on the outside surface. If this occurs, the empty container can be submerged in water for several days and scrubbed to remove the salts.3 Using rainwater will reduce salt buildup in locations where this is a problem with other water sources.

SOIL MIXTURE Filling the container about three-quarters full leaves room for adding enough water to soak down to the plant’s root zone (Figure 8.6). In Chapter 9 soil qualities good for the garden are discussed. The same qualities are good for container soil mixtures but with an even greater emphasis on good drainage. The planting mixture must be able to hold some moisture but both soil and container must allow good drainage because water-saturated soil encourages disease problems. How the mixture is made depends on the local soil. Two helpful guidelines are:

· Well-composted organic matter is the best soil amendment both to open up the structure of a heavy clay soil and to improve water-holding capacity of sandy soil.

· Keeping the top 2-3 cm (1-1.5 in) of the soil a sandy texture allows quick infiltration and keeps water from gathering around the stem base where it encourages disease. In heavier soils, adding some sand as well as organic matter improves drainage.


Figure 8.6 Soil Depth in Containers (1)


Figure 8.6 Soil Depth in Containers (2)


Figure 8.6 Soil Depth in Containers (3)

If soil-borne diseases are a problem, heating small amounts of soil, called soil sterilization, can help. Heating moistened soil to 71°C (160°F) for about 30 minutes kills most bacteria and viruses.4 In areas with high daytime temperatures, moist soil could be put in a covered metal pot or closed plastic bag and left to heat up in the sun for several days. Because heating the soil also kills beneficial microorganisms it should only be used when absolutely necessary.

Before planting, the soil in the container should be leveled and wet throughout. After this, drainage can be tested by adding water up to the container’s rim. If there is still water standing on the soil surface after about 15 minutes, the drainage needs to be improved. The drainage openings in the container may need to be enlarged or more organic matter or sand added to the soil.

PLANTING In large containers the gardener can make planting depressions for seeds using her fingers or a stick. Small containers concentrate water on seeds so there is no need for planting depressions. In small containers seeds can simply be sprinkled across the soil surface (Figure 8.7), covered with a layer of dry garden soil (section 6.4.5), pressed down firmly, and the planting area gently watered.


Figure 8.7 Planting in Containers

PLACEMENT AND CARE Containers should be kept in a convenient location with adequate shade and sunlight. Sometimes the drainage holes become clogged by the surface on which the container is resting. If this happens putting gravel under the container can help by making air spaces that water can pass through to drain out of the wet soil in the container. The soil surface should be allowed to dry slightly between waterings (section 6.5.1).

8.2.3 When Direct Planting is Better

Sometimes planting seeds directly in the garden is more appropriate than container or nursery bed planting. The primary advantage of planting directly in the garden is that the plants will not be moved, and therefore their growth will not be disturbed by hardening-off (section 8.4.4) or transplanting shock. For example, seedlings of cucurbits and some root crops may die or their growth may be severely slowed by transplanting. Direct planting also eliminates work for the gardener. The gardener’s schedule or the garden location may make it convenient to care for the seedlings right in the garden with less work, and the garden environment may be good for starting seedlings.

These advantages must be weighed against the possible disadvantage of direct planting: increased vulnerability to pests and the elements because management and resources are not as concentrated as with nursery beds or containers.

8.3 Planting sites and the sun

The location of plants in relation to the sun should be considered when direct planting or transplanting. Garden plots and crop mixes can be planned to protect tender plants from the harsh midday and afternoon sun (section 10.8.3). For example, sunflowers, sorghum, okra, or other heat-tolerant, tall crops can shade melons, tomatoes, chilis, and other more sensitive crops. Some garden crops like prickly pear cactus do better when transplanted with the same orientation to the sun that they had before being moved, so that the north-facing side is still facing north.

Trees and other perennials will shade larger and larger areas as they grow bigger, and this should be planned for. For example, in subtropical drylands the winter sun warms the garden, house, and work area. If a non-deciduous tree such as a carob, olive, or loquat is planted in the southern part of a northern-hemisphere garden or the northern part of a southern-hemisphere garden, it may block the low winter sun, keeping surrounding areas in shadow. A deciduous tree would lose its leaves in the cool season, allowing the sun to warm the area, but then leafing out and providing shade in the hot season (Figure 8.8).

8.4 Transplanting

Transplanting can be done while thinning plants, or to move seedlings from containers or nursery beds to permanent locations in the garden. It may also be used for moving mature plants. The chances for successful transplanting can be improved if timing, the transplant site, water availability, and preparation of the transplant itself are planned.

8.4.1 Timing

Deciduous trees like figs, the stone fruits, pomegranates, and jujubes are best transplanted when they have lost their leaves and are dormant. This minimizes stress and takes advantage of the surge of growth after dormancy to help the transplant become established. If moved during this time, young deciduous perennials can be bare-root transplanted, that is, they can be transplanted without their root ball covered with soil.

Nondeciduous trees are best transplanted before a period of growth, for example, at the start of the rainy season. Avoid transplanting any plant just before or during a time of environmental stress such as the middle of the hot, dry season. Time of day is also important for transplanting. When it is hot and dry the evening is the best time for transplanting because it allows the plant a cooler, dark adjustment period when transpiration rates are lowest. Cloudy days are also good times for transplanting.

Of course, the most important consideration for timing transplanting is the gardener’s schedule. Food preparation, field work, and marketing may take priority over gardening, so transplanting must be planned around those activities.

8.4.2 The Site

Planning ahead for the growth of garden plants helps in choosing the location for a transplant. A sketch of the garden layout is a good tool for thinking about different transplant sites and for anticipating the effect on space, shade, and sunlight. This is especially true for perennials. When the site is chosen, holes are dug according to the needs of each transplant. The bigger the hole the better, because the less energy the plant must use to develop a healthy root system and obtain nutrients, leaving more energy available for producing edible parts.


Figure 8.8 Planting Sites and the Sun South of the Equator

Seedlings of annuals such as tomatoes or peppers are often transplanted into a spot in an existing garden bed. In this case, the hole need only be made large enough to receive the transplant’s root ball because the soil is already prepared deeply enough for the plant’s future growth (Figure 8.9). The main considerations in this situation are the other plants growing in the same bed or nearby (see section 6.4.4 on planting density).

For transplants not placed in existing garden beds, or seedlings of trees and other large plants, the soil at the planting site will have to be prepared (Figure 8.9). Annuals need a hole at least 45 cm (18 in) deep and 30 cm (12 in) in diameter. The exact size will depend on soil quality and depth (Chapter 9). If the soil quality is poor the hole should be made bigger than it would be in better soil to protect the plant from negative effects of the poor original soil. This hole is then refilled with compost and good soil just as would be done when preparing a garden bed (section 9.8).

When transplanting it is best to use no more than one-third compost or other organic matter for refilling the hole. The remainder should be soil, or soil and sand. Exact proportions are not as important as making sure that there is not an abrupt and extreme change in soil texture between the soil in the transplants root ball and the soil in the planting hole. For example, mixes containing much more organic matter may be so much looser than the transplant’s root ball that this major change in soil texture slows the flow of water (section 10.3.2) and discourages the transplant’s roots from growing out of the root ball, causing it to become root-bound after transplanting. Planting holes should not have smooth sides, as this also encourages root binding. The sides of the holes should be loose and rough textured. Any manure used in the planting hole should be well rotted or composted or it may burn the transplant’s roots.

For trees, the size of the hole depends upon the soil and the type of tree being planted. For example, in dense, hard soil with thick layers of caliche or iron-stone, a hole 1.5 m (5 ft) deep and 1 m (3.3 ft) in diameter is a minimum size. Once again, the deeper the hole the better because more soil is loosened and prepared making root growth easier. If its roots cannot penetrate hard, dense soil a tree will be stunted and may lack the hardiness needed to survive harsh dryland conditions. In addition stunted shallow roots do not anchor a large plant well and so cannot prevent it from being blown over. Holes dug in sites where soil is of better quality and less dense need not be as big because the roots will be able to penetrate beyond the planting hole more easily.

Planting holes should also provide good drainage, because plant roots will not survive long periods in water-saturated soil. An impermeable soil layer like dense clay or caliche can prevent water from draining below the root zone. Roots become suffocated and salts that would otherwise be washed below the root zone accumulate (Figure 12.5 in section 12.6.1). (A test for drainage in a tree hole is described in section 9.4.)


Figure 8.9 Transplanting Holes (1)


Figure 8.9 Transplanting Holes (2)


Figure 8.10 Watering a Transplant Hole to Settle the Soil

A large hole is refilled with good quality soil and organic matter using coarse organic matter near the bottom and finer material near the surface. The hole is then watered thoroughly to settle the soil. When it is watered and the organic matter is compressed, the soil level in the hole may sink significantly. Allowing a day or two for this to happen before planting will avoid later problems with sinking or shirting soil. Before planting more soil can be added (Figure 8.10).

8.4.3 Water

There must be enough water at the planting site to wet the refilled hole before transplanting and to thoroughly water the transplant afterward. In addition, enough water should be available for daily watering during the period of adjustment after transplanting. A basin is made with a diameter about two to three times larger than the root ball to hold water with the plant in the center on a slightly raised area which protects the trunk or stem from standing in water (Figure 8.11).


Figure 8.11 Protecting the Transplant’s Trunk from Water

An example of indigenous transplanting basins comes from the dry Oaxacan valley of southern Mexico. Large vegetable plots are carefully laid out with planting basins or cepas prepared for each chili or tomato transplant.5 Hand-dug wells among the cepas in each plot provide water for hand irrigation.

For the people of Dar Masalit in central Sudan, the major work in establishing fruit tree transplants (mango, guava, and lemon) in seasonally dry stream beds and terraces is watering while the trees become established. This must be done daily at first, tapering off to about once a week after 3 years.6 The trees also have to be protected by thorn fences against livestock; and thorn branches are no longer easy to find. One-half of the fruit trees are lost to termites, flooding, and other causes. However, it seems that in good locations (those with a high water table yet protected from flooding), the trees require little work once they begin bearing fruit.

8.4.4 The Transplant

Preparing plants for transplanting begins about 3 to 7 days in advance for annuals, and even longer for perennials. It is done by gradually exposing the plants to more sun and heat and reducing their water intake. This process of exposing them to controlled stress, called hardening off, reduces the rate of transpiration and photosynthesis and causes the plant tissue to become more dense because it contains less water. Hardening off also encourages food storage in plant tissues because growth is slowed.7 Some plants may wilt slightly when the hardening process begins but usually recuperate at night. As long as their central stalk and growing tip remain green and firm they are not being harmed. After several days the plant will stop wilting, unless the hardening is too severe. Just before and then immediately after transplanting the plant should be well watered. Drought-hardened plants seem to be better able to cope with subsequent drought and are more productive under dry conditions than plants that have not been hardened.8

Younger plants have more vigor and resilience and are generally better for transplanting. For example, tomato transplants 5 weeks old were found to be far more productive than 7-, 9- or 11-week-old transplants.9 Transplants should not have flowers or fruits because these use the energy they need for surviving and becoming established after transplanting. If the plant must be transplanted when it is flowering or fruiting, flowers and fruit should be removed. Annuals are transplanted after the first two to four true leaves (not including the cotyledons) have developed. Perennial transplants can have more aboveground growth than this, although younger plants have a better chance of surviving, and they are smaller and easier to move.

Before planting, the transplant should be checked to make sure it will not introduce pests or diseases into the garden. It should have a vigorous root system with no galls (unusual swellings or growths due to pathogens, sections 13.4.1 and 13.4.3) or soft brown lesions and no harmful insects or their eggs on the stems or leaves. Plants weakened by disease, predators, or other sorts of stress will also have more difficulty surviving transplanting than those in good health.

Except for bare-root transplanting of deciduous perennials and untangling a root-bound transplant, the roots should be disturbed as little as possible. This is because damaged roots reduce a plant’s ability to obtain water and nutrients, and make it more susceptible to disease and environmental stress.

A seedling should be handled as little as possible, and then by gently holding its soil-covered root ball. The plant should not be held by its stem because young, tender sterns bruise easily and diseases often develop on these bruises.

No matter how carefully the transplanting is done, some root hairs will be damaged. Though small, these root hairs constitute the majority of the total root surface area and are vital for the intake of water and nutrients. Loss of root hairs, and thus a reduction in root surface area and water absorption, causes the wilting common in transplants. This is why extra watering and protection are needed after transplanting, especially under hot, dry conditions.

The roots should be pointing downward when the plant is placed into the transplanting hole. If pushed upward during transplanting roots will be closer to the soil surface where higher temperatures and less moisture will slow growth (Figure 8.12).

A seedling that has been left too long in its container will not have sufficient room for its roots, or adequate soil from which to obtain water and nutrients. The roots spiral around each other in the restricted space, eventually choking the plant (Figure 8.13). These plants are root-bound, often appear unhealthy, and frequently wilt, even with regular watering. This problem occurs frequently with perennial transplants left in the container for 1 or more years. Vertical ridges on the inside of containers as described in section 8.2.2 help avoid this pattern of spiraling root growth.

The root ball of root-bound plants should be gently loosened and the roots untangled. Briefly soaking the root ball in water helps loosen the soil. If untangling them is impossible, the gardener may try cutting them in a few places to pull the spiraled roots apart and direct them out and away from the root ball. Obviously all this handling and cutting make a root-bound transplant extremely vulnerable to water and heat stress and it will need shade and a lot of water until it becomes established. However, if the roots are not untangled and slightly spread, the plant will remain root-bound even after transplanting because its roots will continue to follow the same spiraling pattern. A root-bound plant is poorly anchored in the ground and easily blown over.


Figure 8.12 Transplant Roots


Figure 8.13 A Root-Bound Transplant

Tomatoes can be transplanted slightly deeper than they had been growing because adventitious roots will grow from nodes on the stem, strengthening the plants and improving their ability to obtain water and nutrients (Figure 8.14). All other plants including perennials should be transplanted to the same depth at which they were already growing.


Figure 8.14 Transplanting Tomatoes

When transplanting a grafted seedling (section 7.6), it is best to keep the graft union at least 20 cm (8 in) above the soil surface. If close to or under the surface the union may remain wet when the seedling is watered, encouraging the growth of microorganisms which could destroy the graft union and rot and kill the plant.

Under hot, dry conditions some perennials and mature annuals are pruned to decrease the total leaf surface area and thus the amount of water lost through transpiration (Figure 8.15). When transplanting woody fruit trees their aboveground growth should be pruned back about one-third to compensate for root damage. By diminishing total plant size with pruning, the plant can focus valuable resources such as water on survival. Drought-deciduous plants self-prune under extreme drought for the same reason.


Figure 8.15 Pruning the Transplant’s Leaves

Box 8.1
Steps in Transplanting

The basic steps for transplanting in drylands are reviewed below. Figure 8.16 illustrates transplanting an annual.

a) The transplant is hardened off and pruned if needed.

b) A hole is dug corresponding to the soil conditions and the transplant’s needs.

c) Before transplanting both the transplant and the transplanting hole are watered thoroughly.

d) Transplanting is best done in the evening or on a cloudy day.

e) Transplanting is done quickly, leaving the plant out of the ground for as little time as possible. If the transplant is not in a container but must be moved some distance to the planting hole the root ball can be wrapped in moist cloth. This prevents it from falling apart, protecting the roots. Once at the site a root-bound root ball can be carefully loosened; the roots untangled and cut.

f) The transplant is placed in the hole at the same depth at which it was growing before, with the roots directed downward and slightly spread so that they will not bind each other. The hole around the transplant is filled with soil which is packed firmly as it is added.

g) A ring-shaped trough around the plant makes a basin in the soil to hold water. The trunk or stem is in a central raised area which helps keep it dry.

h) The transplant is watered deeply, mulched, and shaded if necessary.


Figure 8.16 Transplanting an Annual into a Garden Bed


Figure 8.17 A Mixed Garden in Northern Mexico

8.5 Plant interactions

Plant interactions in the garden can affect how well the plants produce and how much care they will need. Many combinations of crops seem to improve garden production, and in indigenous dryland gardens a wide variety of crops and crop varieties are often grown in mixtures (section 14.2.2).

8.5.1 Mixed Planting

Mixed planting in a household garden may take many forms. It can be a combination of various trees and plants such as the garden in Durango, Mexico, in Figure 8.17. It can be fruit trees surrounded by squash vines or garden beds containing alternating rows of different crops. However it is organized, the goal in a mixed garden is a greater average harvest of diverse garden produce for the least amount of labor and resources.

Many gardeners recognize the interactions between different crops in their mixed gardens. These interactions change over time, even from year to year when large perennials are mixed with annuals. For example, a gardener we visited in Durango, Mexico, pointed out that soon she would not be able to plant annual vegetables under some of her peach trees because it was getting too shady. For her the peaches were more valuable and so the trees were not thinned. Also she was finding other areas such as the edges of her garden where annuals could be grown, and vines like chayote were encouraged to climb up fences and over rooftops where they got plenty of light.

In addition to the variety of goods produced, mixed planting takes advantage of limited resources such as space, water, soil fertility, and the gardener’s time (section 6.4.4). Some other benefits of mixed planting are discussed below.

DIVERSITY Species and varietal diversity can help reduce pest and disease problems and their impact on the garden’s productivity (section 13.2.2). Growing only one or a few crops or varieties encourages the growth of pest populations and disease. Having different plants with diverse forms and life cycles discourages this.

There are a variety of ways in which crop mixes can reduce pests: 1) by providing a habitat for birds or insects like parasitic wasps which prey on insect pests in the garden, 2) by providing alternative host plants for pests, and 3) by protecting vulnerable crops by visually or chemically hiding them.10 Several dryland garden crops are thought to reduce populations of damaging nematodes in the garden (section 13.3.2).

REDUCING WEEDS Dense crop mixtures create a living mulch that reduces weed growth by shading emerging weed seedlings from the sun. Mixtures that combine crops with upright growth forms with those that sprawl or vine control weeds especially well. This is because these two plant forms can be sown together densely enough that the soil is covered by the mature plants, but not so densely that they are harmed by competition for soil, water, and light. For example, it has been shown that planting cowpeas and mung beans among sorghum or pigeon peas suppresses weed growth just as much as two hand weedings would; and watermelons and sweet potatoes planted among yams or among a yam, maize, and cassava mixture suppress weed growth as much as three hand weedings would.” The squash plants commonly grown mixed with maize and beans in southern Mexico send out rapidly growing vines with large leaves that quickly cover any exposed soil.12

CREATING MICROENVIRONMENTS Some plants can be used to create an environment favorable for other plants. Trees or other large plants in indigenous mixed gardens provide shade and wind protection to smaller, more tender ones. This mimicks a situation frequently found in nature. For example, in the Sonoran Desert of North America large “nurse” plants appear to protect smaller, more sensitive plants such as wild chiltepines (Capiscum annuum var. aviculare or glabriusculum) from frost or extreme heat. By the way they lay out their gardens and mix different species of garden plants, some gardeners have created environments that mimick this pattern found in nature.

In the Middle East, intercropping patterns in the smaller date gardens illustrate the effect of the date palm tree on its environment. When the trees are young alfalfa is often grown around them. As the trees grow they create a partially shaded area protected from desert winds. The alfalfa is then replaced by vegetables and trees such as citrus, fig, mango, pomegranate, and jujube.13

Research on mixed cropping and its effects on plants, pests, and resource use is limited and focused mostly on large-scale agriculture in temperate and humid tropical areas. A better understanding of mixed cropping systems, especially under the conditions of small-scale, low-resource agriculture would be extremely valuable. However, the very complexity characteristic of these systems makes them difficult to examine using conventional methods. Perhaps this dilemma will force researchers to adopt a new approach to understanding agriculture.

8.5.2 Allelopathic Plants

Many crops produce chemicals that in some way inhibite the growth of other plants. This is called allelo-pathy14. For example, sunflowers, asparagus, eggplant, and sorghum produce allelopathic chemicals that affect other plants of the same species and sometimes other species.

The squash (Cucurbita pepo) grown in southern Mexico that covers the soil surface discouraging weeds (section 8.5.1), may also reduce weed growth through allelopathy. The leaves of these squash plants appear to suppress growth of many common local weeds, but not that of the maize and bean crops among which they are planted.15

Sometimes allelopathic plants are obvious because no other plants will grow around them, and they are often well-known among local gardeners and farmers. Plants that inhibit the growth of others need to be identified and taken into account when planning a garden. For example, placing a garden in the shade of a eucalyptus tree (Eucalyptus spp.) would be a waste of the gardener’s time and energy because volatile oils in the leaves of many eucalyptus varieties inhibit germination and growth of some plants. Hot, dry conditions stimulate the release of these oils.16 In Nigeria Eucalyptus spp. are popular trees to grow as boundary markers for fields and as dooryard ornamentals.17 Their leaf litter was found to suppress germination and early growth of tomatoes and chilis, but affected maize only slightly and cowpeas not at all.

Leaf litter from tamarisk trees (Tamarix aphylla) creates such salty soil conditions that most garden plants cannot survive.18 Walnut (Juglans spp.) and pine (Pinus spp.) trees also have allelopathic qualities. The extent of the root system and the area that would be affected by leaf drop of these trees should be noted when establishing a garden. Root pruning, described in section 8.7.1, may be helpful.

Walnut wilt is a disease affecting garden plants, like tomatoes, that are growing in the root area of a walnut tree. The woody pith tissues in the garden plant’s stems turn brown, the plant wilts, and eventually dies. The cause of walnut wilt is an allelopathic chemical produced by the walnut roots. This poison remains in the soil even after the tree is dead, so putting a garden near where a walnut tree is or was growing is not a good idea.

In addition, parts of some plants may not be useful as mulch or compost because of their chemical properties. However, thorough composting (section 9.6.2) is thought to eliminate allelopathic qualities in amaranth weeds (Amaranthus retroflexus) and goosefoot (Chenopodium album) which otherwise may have negative effects on garden crops.19 These “weeds” are also widely favored as green vegetables. Local farmers and gardeners are often aware that particular plants are poisonous and should not be used in the garden or compost. If there is some uncertainty experiments can be conducted, being careful not to contaminate the whole garden or compost pile.

8.5.3 Crop Rotation

In addition to interacting with each other directly, crops can be affected by what was previously grown in the garden. Gardeners can take advantage of the positive effects of this interaction by rotating the location in the garden or field where different crops are planted.

Some soil-borne pathogens and some disease-causing nematodes feed on or parasitize certain crops or crop families and do not harm others (sections 13.3.2 and 13.3.4). Crop rotation can help control or eliminate these diseases and pests by removing the crops they feed on for 1 or more years. For example, if clubroot fungi (which only affect members of the crucifer family) infest cabbage growing in one garden bed, kale planted in that bed the following year will probably also be affected. Rotating crops from other families, such as chilis from the solanaceous family, will control the pathogen.

Soil improvement can be another benefit of crop rotation. By rotating crop mixes dominated by nitrogen-fixing legumes with those emphasizing non-leguminous crops it is less likely that nitrogen will be depleted (section 9.5.2).

8.6 Weed management

Plants growing in the garden other than those that were purposefully planted are called weeds. Weeds are hardy, establishing themselves and surviving without any attention. They quickly take advantage of disturbed habitats such as a garden, which in drylands is an oasis of favorable growing conditions.

Even though weeds are not intentionally planted they are not necessarily unwanted. It is now recognized that weeds are often useful to the gardener and her household.20

Some uses for dryland weeds:

· food and condiments for people and animals
· medicine
· mulch
· green manure and compost
· insect traps
· bedding for animals
· erosion control
· crafts.

Some weeds are plants that people are slowly domesticating. In the high-altitude drylands of Chihuahua, Mexico, the Tarahumara Native Americans allow the jaltomata to grow among their maize, on the edges of their maize plots or in patches of fertile soil such as where animal manures or refuse are dumped.21 The jaltomata produces dark, sweet edible berries. The plants allowed to grow among crops and other disturbed areas had a higher number of berries per plant and a lower number of seeds per berry than those growing in the wild. This may be because the Tarahumara have selected for plants that have the best fruit and that respond to improved growing conditions.

Weedy relatives of garden crops can contribute new genetic material to the crops through cross-pollination.

In the short-run this may reduce garden yields somewhat, but the long-term effect is to increase the genetic diversity and thus the adaptability of the garden crop. An example of this is the crossing of the “weed” teosinte (Zea spp.) with maize which is carefully managed by farmers in an area of Jalisco, Mexico.22

However, weeds may also have a negative impact on the garden if they attract or spread disease and pests and compete with garden crops for essential resources. In some cases, weeds like leaf amaranth are hardier than most garden crops, easily outgrowing them. There are solanaceous weeds that carry diseases harmful to solanaceous garden plants. For example, in southwestern North America Datura spp. weeds can introduce curly top virus (section 13.4.1) to gardens, harming the tomatoes and chilis growing there.

There are few documented examples of traditional weed management. Even though the following example is about farmers in the humid tropics it is worth mentioning because it is rare documentation showing the complexity of a traditional weed management system. Small-scale farmers in Tabasco, in humid southern Mexico, classify weeds as either “good” or “bad” plants, “buen” or “mal monte”.23 More than 40 weeds are classified this way based on their direct use to people and animals, their effect on the soil and crops growing there, and how difficult they are to control. Many different management methods such as living mulches, burning, cutting, and cultivation are used depending on the particular field or plant.

Which plants are weeds and how they are managed will depend on many things including the crops in the garden and the usefulness of the weed. Yet, often it is not the plant itself which is either good or bad but rather the timing of its emergence and growth in relation to garden resources and crops.

8.6.1 Resource Use

The gardener must decide whether the benefits provided by a weed are worth the resources it will use. For example, under dry conditions all available soil moisture and nutrients may be needed by garden crops and yields may be reduced by weeds that outcompete them. If there is little competition, however, weeds may produce food while helping shade garden crops and the soil.

If products provided by the weeds are desirable enough, the resources they use can be an acceptable loss. For the Tarahumara, quelites, or wild greens that emerge in their maize fields, are an essential food source.24 The quelites, amaranth (Amaranthus retroflexus), Brassica spp., and goosefoot sprout and grow with the rains in May. This is during the hungry season (April to July) when food reserves are low or depleted. Leaves from quelite seedlings between 2 to 6 weeks old are harvested and cooked, and make a significant contribution to the Tarahumaran diet. The quelites are removed from the field when they are about 6 to 8 weeks old, by which time their roots start to compete with the deeper-rooted maize crop and their leaves become tough and unpleasant tasting.

8.6.2 Effects on Pest Populations

As with other crops in a mixed garden, weeds can have an effect on insect populations in and around the garden (section 8.5.1). Weeds can mask the presence or smell of crops from insects, offer these insects more desirable food sources, and provide an environment in which beneficial, predatory insects will become established.25 In southern Zimbabwe the common weed, wild marigold (Tagetes minuta), has been found to significantly reduce populations of root knot nematodes in soil where it grows, and is left to grow among crops26 (section 13.3.2).

Some weeds are trap plants for insects keeping them off crops, while others may encourage beneficial insects. Managing these weeds, for example, cutting them at a particular time, can move those beneficial insects onto the crops. However, weeds may also attract or encourage a concentration of harmful insects and provide places for them to reproduce, as happens in the fields of the Tohono O’Odham Native Americans of the Sonoran Desert (section 8.6.3).

The effect of weeds on pest populations depends on many changing factors such as the garden crops, the garden microclimate, pest life cycles, migrations, and predator-prey relationships between pests and other animals. Local experience and knowledge combined with experimentation are the best ways to learn about the relationship between weeds and pest populations in a particular area.

8.6.3 Timing

Timing is the key to easier weed management. Weeding the garden can consume a lot of time, and may compete with other tasks, for example, weeding fields. The time demands of weeding may influence when gardens are planted, garden size, when weeds are harvested for food, and when they are removed.

The Tohono O’Odham Native Americans of the Sonoran Desert manage weeds in a way similar to that of the Tarahumara described in section 8.6.1.27 Amaranth seedlings are allowed to flourish for several weeks, providing tender, leafy greens. When the plants get taller than about 30 cm (12 in) they are removed to avoid competition with crops and because they start attracting pests such as cucumber beetle larvae and grasshoppers.

Once garden plants are well established they will shade the soil surface, discouraging new weeds from growing due to lack of sunlight. In some date gardens of the Middle East, Bermuda grass (Cynodon dactylon, Pers.) is an aggressive and harmful weed around young trees.28 However, when the trees are larger and more established some gardeners encourage the grass as a fodder for animals. The shady conditions created by the tall trees keep the sun-loving grass from getting too vigorous and competitive.

8.6.4 Methods of Weed Control

Knowing the life cycle of weeds and how they reproduce is essential for controlling them. Removing undesirable weeds before they produce seeds or spread vegetatively will save work in the future. Putting seeds in the compost or mulch could spread the weeds throughout the garden. Pre-irrigating empty garden beds to germinate weed seeds, and then removing the weeds, is effective if there is plenty of water. Weeds easily propagated by vegetative means like Bermuda grass should not be used for compost or mulch.

There are four methods of weed control useful for dryland gardens: heavy mulching, cultivation, hand weeding, and burning, each of which has advantages. The weeding method chosen will depend on a number of factors including the gardener’s experience and schedule, the garden layout, the time of year, and the soil type (Table 8.1).

· Heavy surface mulches control weeds by blocking sunlight from the soil. Use of these mulches is described in section 10.8.1. As described in section 8.5.1, plants like squash vines, which cover the soil surface, act as living mulches and are also effective for discouraging weed growth. The main drawback to this method of weed control is that mulches may hide pests that are harmful to young plants.

· Hoeing or some other method of cultivation is relatively quick and kills most weed seedlings by disturbing their roots. However, cultivation may be awkward in a mixed garden and is not appropriate where wind or rain erosion can be a problem.

· Weeding by hand is effective but requires more labor than the other three methods.

· Controlled burning before planting or of weeds removed by other methods is quick, effective, and does not require a lot of labor. Burning is not appropriate in areas with alkaline soil where ashes would raise the pH, or in very small, mixed gardens where fire would harm plants already growing.

Table 8.1 Controlling Weeds in Dryland Gardens

Method

Timing

Positive features

Things to think about

Heavy mulching

Best after crops established to avoid pest problems.

Low labor, added benefit of controlling water losses, long lasting; pulled weeds can be used as mulch.

Mulches can harbor pests harmful to young plants.

Cultivation with hoe, stick, or other tool

Before planting and/or while plants growing, best before seed production.

Kills seeds and seedlings by burying, cutting, or exposure to sun; breaks up soil surface reducing evaporation; weeds can be used in compost.

Be careful not to damage garden crops; if windy can expose topsoil to erosion; difficult in a dense garden or under trees with surface roots.

Hand weeding; pulling and cutting

Before or while garden plants growing; before weeds produce seeds.

Thorough and selective, works well in mixed garden; saves weeds for eating, composting, or other uses.

Most labor-intensive; timing essential to eliminate weeds before seed production.

Controlled burning

Before planting, or burn weeds removed by other methods.

Relatively quick, can be low labor, kills both plants and seeds.

Ashes increase soil alkalinity; lose organic matter useful as compost or mulch.

8.7 Pruning

Pruning is the selective removal of plant parts to promote different patterns of growth and development. Pruning may also be done as part of transplanting or grafting to reduce water loss through transpiration. The products expected from the plant and the way the plant grows determine which parts are pruned. For example, flowering is undesirable in crops such as basil or leaf amaranth because it diverts energy and resources away from leaf production. On these crops growing tips are harvested before they flower, or flower heads are pruned off to encourage more leaf production (Figure 8.18). In fruit-bearing plants such as tomatoes and eggplants, however, flower production is essential for producing the fruit, so some vegetative growth may be pruned to encourage flowering.

8.7.1 Reasons to Prune

Following are a variety of reasons to prune plants in dryland gardens:

· To delay flower and seed production and encourage leaf production. Examples: leaf amaranth and mint.

· To reduce vegetative growth and produce an earlier, more concentrated harvest. Example: tomato.

· To improve the quality and size of fruits by removing some of the immature fruits. For example, on papayas pruning out some of the fruit makes room for those remaining to grow. On trees like olives, lemons, and peaches an extremely heavy fruit crop one year will be followed by a very small one the next, and in some cases no crop at all. Thinning the fruit in the year of heavy production reduces the demands on the tree and stimulates flowering for next year’s crop, evening out the amount of fruit borne year to year. Examples: stone fruits, loquat, olive, and citrus trees.

· To selectively thin out branches of perennials creating a strong structure and reducing the risk of wind damage. Examples: stone fruits, olive, fig, and pistachio trees.

· To thin out dense foliage that inhibits air circulation, thus reducing conditions that encourage fungal disease. Examples: tomato and basil.

· To shape the plants for specific purposes, for example, to provide space for other crops (Figure 8.19). If a gardener plans to use layering as a method of propagation, pruning should be done to leave some shoots or low-lying branches appropriate for this. In West Africa, trees whose leaves are collected are sometimes pruned into a ladder-like form with short branches making it easy to climb up the tree and reach the leaves growing on its higher branches.29 Example: most perennials.

· Remove flowers or fruit on very young plants to reduce stress and direct the plant’s energy into becoming established and vigorous. Example: most flowering plants.

· To diminish leaf surface area therefore reducing water loss and concentrating resources during times of stress such as grafting or transplanting. In some cases with perennials like mangoes, when transplanting a young seedling the leaves are cut halfway back (Figure 8.15 in section 8.4.4). Examples: many perennials and annuals.

· Harvesting of nonedible plant parts. Examples: date fronds for shading or building, banana leaves for shading and wrapping foods, pollarding of neem trees in West Africa to collect wood for building materials.

· To control tree height by removing apical dominance and encouraging lateral growth, and so making it easier to harvest fruits from the tree. Examples: stone fruits and jujube.

· Root pruning of trees to stimulate deeper, downward growth and discourage root growth into the area of other trees, into garden beds, or under buildings. Root pruning is done by digging a trench where the roots are to be cut. The trench should be at least 1 m (3 ft) away from the tree’s trunk, or 50 cm (20 in) beyond the drip line (section 13.4) if this is farther. If root invasion is a recurring problem it may help to leave the trench open without refilling it, taking precautions so that people or animals do not fall into it. Examples: trees with invasive root growth, including nongarden trees like Eucalyptus spp. whose roots may be intruding into the garden.

· Perennial herbs that have become woody can be pruned back severely to encourage fresh, full growth and greater leaf production. Examples: oregano, marjoram, mint, and lemon verbena.


Figure 8.18 Pruning to Encourage Leaf Production


Figure 8.19 Pruning to Shape a Plant

8.7.2 Guidelines for Pruning Trees

This section discusses some simple suggestions for pruning applicable to all fruit trees. Table 8.2 summarizes pruning advice for some widely known dryland garden fruit trees.

SHAPING THE TREE Many trees do not need pruning since their natural form is strong and productive. However, pruning these trees is still useful for removing dead or diseased wood, and branches that are crossing or rubbing each other. Branches in a direct vertical line, one above the other, block each other’s light and create an unbalanced, weak tree. Pruning one off makes a healthier, more stable tree (Figure 8.20).

Pruning some garden trees to a particular shape can make them stronger and more productive. A shape for the tree should be chosen before pruning begins to guide decisions about which growth to remove. Two basic forms for pruned trees are open center and central leader (Figure 8.21).

Table 8.2 Pruning Suggestions for Some Dryland Garden Trees

Fruit tree/vine

Deciduous

Pruning suggestions

Almond

yes

Pruning to remove secondary or smaller side branches stimulates formation of new fruiting spurs.

Apricot

yes

Prune as recommended for almond.

Avocado

no

Minimal pruning to control height and open up dense growth for improved air circulation and sunlight.

Carob

no

Prune to shape.

Cashew

no

Prune when young to shape and remove shoots around base of trunk unless these are desirable for layering.

Citrus

no

Prune to shape, to open up dense canopy, remove basal suckers; occasionally thin fruits.

Fig

yes

Prune to shape, to open up dense canopy, reduce chance of wind damage; occasionally thin fruits.

Grapes

yes

During dormancy prune heavily leaving trunk and two to four major branches only to stimulate new growth for fruit bearing during the coming growing season.

Guava

no

Prune to shape and remove basal suckers.

Jujube

yes

Prune to shape and control height.

Loquat

no

Minimal pruning to shape, and reduce wind damage; occasionally thin fruits.

Mango

no

Minimal pruning when young to shape or remove heavy flower set.

Olive

no

Prune to shape and open up structure; occasionally thin fruits.

Peach

yes

Prune heavily to stimulate lots of new growth to bear next year’s crop.

Pistachio

yes

Prune to shape.

Pomegranate

yes

Prune to shape; occasionally thin fruits.

Open center trees have any central trunk above 0.5-1.0 m (1.5-3.3 ft) removed when the trees are very young. This eliminates apical dominance and produces an open, bowl-like form. As a result fruit can be more easily reached. Spreading the tree’s canopy allows light to penetrate into the inner branches, encouraging fruit production and ripening.

The central leader form allows the tip of the main trunk to continue growing and dominating the tree. All branches grow out from this trunk giving the tree a cone or cylinder shape. This shape can be very strong and vigorous and its canopy will take up less room than an open center tree.

BRANCH ANGLES The angle at which branches grow out from the trunk or larger branches affects their strength. Branches growing at small angles (less than 40°) are weaker and more likely to split or break than those with angles of 45° to 65°. This is because there is usually a fissure or crack in the wood’s growth pattern when the branch and trunk meet in a narrow angle (Figure 8.22). If it is not possible for an adult to press her index finger into the place where the branch joins the trunk then the angle is too small.


Figure 8.20 Pruning Out a Competing Branch


Figure 8.21 The Open Center and Central Leader Tree Forms

If a branch on a young tree is growing at a narrow angle it can be widened by wedging something between the branch and trunk to spread the angle (Figure 8.23). Ropes and weights can also be used to pull the branch down, into a more open angle. The spreading should be started at the beginning of the growing season and continued until the desired angle can be maintained by the tree. Putting some cloth padding between the tree and the rope or spreader will prevent damage to the tree. Farmers in Italy use lengths of cane with a V-shaped notch cut in each end to spread branches of fruit trees.


Figure 8.22 Branch Angle and Strength


Figure 8.23 Spreading a Branch

In addition to forming a weak connection, branches growing at a narrow angle from the trunk are more vertical, leading to apical dominance, and vegetative growth is stimulated more than reproductive growth. When the branch’s angle of attachment is wider, the tip will soon hang down, overcoming apical dominance (Figure 8.24). This encourages the new growth at the high part of the branch to be vegetative, while reproductive growth is favored at the lower tip.

HOW FRUIT IS BORNE BY THE TREE An important consideration when pruning is how it will affect the harvest of fruit, leaves, or nuts. Some trees, like the mango, bear fruit only on the tips of the current year’s growth at the ends of branches; this is referred to as terminal bearing (Figure 8.25). Trees like peaches produce fruit only on shoots grown the previous year (Figure 8.26). Trees like apricots and almonds bear fruit on short flowering shoots called spurs which grow laterally from branches and last 3-5 years (Figure 8.27).


Figure 8.24 Branch Angle and the Type of Growth it Encourages


Figure 8.25 Terminal Fruit Bearing in the Mango

Pruning is most important for trees bearing fruit on this or last year’s growth. Heavy dormant-season pruning will stimulate new growth, providing more sites for fruit production. For example, heavy dormant-season pruning is important for grape vines because they bear fruit only on the current year’s growth. Fruiting spurs are productive for a limited time, depending on the kind of fruit tree. For example on apricot trees, fruiting spurs are productive for about 3 years and on almonds for about 5 years. Pruning secondary or side branches during the dormant season helps to stimulate new fruiting spurs.


Figure 8.26 Peaches Bear Fruit on Last Year’s Shoots

WHEN TO PRUNE Pruning tree branches is best done when the shoots or branches are small, unless, of course, particular products are wanted. In temperate regions with cold winters, deciduous trees should be pruned at the end of the cold season. Pruning tends to increase cold sensitivity and so pruning at the beginning or during the cold season could make the tree more vulnerable to damage and disease. Root pruning is best done in the dormant season or at the start of the rainy season so as not to stress the tree.

Planning the desired form and anticipating the plant’s growth lets the gardener prune before consuming resources that could be used in other, more desirable forms of growth. Compared with branches, small shoots and buds are easy to remove and these wounds heal quickly with little danger of disease.


Figure 8.27 Apricots are Borne on Fruiting Spurs

MAKING THE CUT The plant part being pruned should be as completely eliminated as possible. At the same time, it is important not to damage the plant as this can disturb growth and leave a wound vulnerable to infection. In some older branches the branch collar or area of tissue surrounding the branch’s base is evident. The collar should not be cut off because this tissue forms a rapidly growing, disease-resistant callus over the pruning cut. If a branch collar is not evident, the pruning cut should be made away from the main branch (Figure 8.28).

When pruning many plants the gardener can easily pinch or pull the part off with her fingers. If the branches or shoots are too big or there is a danger of ripping the remaining tissue then a machete, knife, saw, pruning shears, or other sharp tool can be used.


Figure 8.28 Where to Prune a Branch

When cutting large branches it is easiest to start by removing most of the branch so only a small stub remains. Cutting the underside of the branch first will prevent tearing the bark and avoid pinching the tool blade. After most of the branch has been removed the final pruning cut can be made without having to handle the awkward, long, heavy branch (Figure 8.29).

When pruning the ends of branches, making the cut at a slight angle with the high point just above a desirable shoot or bud will promote the growth of that shoot or bud (Figure 8.30). All cuts should be made cleanly, leaving a smooth surface and without tearing the bark. If a diseased area is pruned either by hand or with a tool, washing the hands with soap or thoroughly cleaning the tool with soap, bleach, alcohol, or vinegar before touching another plant avoids spreading the disease.

For best healing, pruning cuts should not be covered or treated with anything. Exposure to the air promotes healing, but under hot, dry conditions it is a good idea to shade large wounds.

NOTCHING Notching refers to the technique of wounding the phloem tissue above or below a dormant bud to encourage a particular kind of growth. Notching is based on the same principle as girdling discussed in section 7.2.1. Wounding the tissue interrupts the flow of carbohydrates down the phloem from the leaves hack to the roots, thus concentrating it above the cut. If the tissue is broken above the bud not only is this flow cut off but so is the downward movement of hormones from the terminal bud which supresses growth below it. Therefore the bud will develop into a shoot. Notching below a bud encourages development into a flowering bud because of the concentration of food.


Figure 8.29 Pruning a Large Branch

Notching is usually done on the young shoots and branches of deciduous trees at the end of dormancy. It is best to make the cut at least 2 cm (0.8 in) wide to prevent it from closing up rapidly. The notch should only penetrate the bark into the cambial tissue, and should extend about halfway around the branch or shoot (Figure 8.31).31


Figure 8.30 Tip Pruning

8.8 Trellising

Trellises support the growth of plants up and away from the ground. Plants like passion fruit, grapes, and many cucurbits, peas, and beans, are climbing vines whose growth and production can be improved by trellising. Many tomato varieties also benefit from trellising. Another advantage of trellises is that by encouraging vertical instead of horizontal growth, more room is made available in the garden. Trellises keep fruits such as tomatoes, cucurbits, and peas off the ground, making them less vulnerable to many pests and to rotting from contact with moist soil. Trellised plants can also shade people and other plants. However, where there are strong, drying winds, trellising may not be a good idea because it exposes more leaf surface area, leading to increased transpiration.


Figure 8.31 Notching to Influence Growth (After Kourik 1986:206) (1)


Figure 8.31 Notching to Influence Growth (After Kourik 1986:206) (2)

Trellis designs take into account special climatic conditions. For example, in high-altitude or high-latitude drylands with cold nighttime temperatures during part of the year, planting beans, tomatoes, or other cold-sensitive plants against a south-facing rock pile is a good idea. This will serve as a trellis for these plants and the heat stored in the rocks during the day create a warm microclimate for the plants as the air temperature drops at night.

Some types of trellises are:

· Other trees and plants in the garden which are living trellises (Figure 8.32),

· Constructed trellises made of adobe, wire, string, sticks, cane, branches, and stalks (Figure 8.33). Plant fibers, vines, leather, string, wire, or cloth are useful for tying the parts together and tying the vines to the trellis.

· Existing structures such as houses, walls, or rock piles. In savanna West Africa pumpkin and luffa vines often cover thatch-roofed houses, providing extra shade and cooling from their transpiration,


Figure 8.32 Maize Provides a Living Trellis for Bean Plants


Figure 8.33 A Constructed Trellis for Grape Vines in Egypt

8.9 Resources

Local gardeners and farmers are the most knowledgeable sources of information on plant management in their areas. Observing, working, and talking with them is the best way for outsiders to understand local techniques, resources, and problems.

Developing Countries Farm Radio Network (DCFRN) has brief scripts for radio programs about plant management topics such as nursery beds (#7-4), transplanting (#7-5, #10-5, #10-6), intercropping (#3-6, #7-1a), and weeds (#5-1a).

Regional books on wild or native plants such as Traditional Food Plants (FAO 1988), The Weed Flora of Egypt (Boulos and el-Hadidi 1984) and La Culture des Lgumes Indignes au Nigria (van Epenhuijsen 1978) can help an outsider learn to identify important local plants and can be very useful when working with gardeners on weed management. Many gardening books have some information on pruning and there are also some specialized publications on that subject.

References

1 Early 1977:42-44.

2 Hatmann and Kester 1983:166.

3 Garner, et al. 1976:10.

4 Hartmann and Kester 1983:37-38.

5 Wilken 1987:164-167.

6 Tully 1988:130-131.

7 Hartmann and Kester 1983:192.

8 Arnon 1975:36; Larson 1975:159.

9 Horticultural Abstracts 1984 55(5):283.

10 Altieri and Liebman 1986.

11 Altieri and Liebman 1986.

12 Gliessman 1986.

13 FAO 1982b:97-98.

14 Rice 1984:1-2.

15 Gleissman 1986:89-90.

16 Rice 1984:173-178.

17 Igboanugo 1986.

18 Duffield and Jones 1981:158.

19 Bhowmik and Doll 1984.

20 Ruthenburg 1980:84.

21 Davis and Bye 1982.

22 Benz, et al. 1990.

23 Chacn and Gleissman 1982.

24 Bye 1981.

25 Altieri and Liebman 1986.

26 Rice 1983:52.

27 Nabhan 1983.

28 FAO 1982b:140.

29 Dupriez and De Leener 1987:90.

30 Kourik 1986:206.

9. Soils in the garden

Loss of productive land to the growth of cities and roads, flooding from dams, waterlogging, salinity, soil erosion, and over-cultivation is a serious problem in industrialized countries as well as in the Third World. The social organization of production, not merely technology, is a major factor in soil degradation.1 An emphasis on short-term production, mechanization, reliance on chemical fertilizers, and extensive mono-cropping contribute to serious losses of soil resources in industrialized agriculture. Many indigenous production systems which at one time conserved soil resources no longer do so because of disruption of their social organization, population pressure, and environmental changes.

Sustainable production from dryland gardens should be based on management techniques that improve and protect the soil using local resources, and that are appropriate for the local social and economic situation. In this chapter we emphasize the importance of reducing wind and water erosion, and of maintaining adequate soil organic matter to ensure fertility and water-holding capacity.

9.1 Summary

Dryland soils can differ a great deal from one region to another and even from one side of a village or garden to the other. The valuable contributions that gardens can make to the household mean that locating the garden where there is good soil and/or improving the soil is worth the effort.

There are many different ways of classifying soils; some based on the way the soil can be used, others on specific physical properties of the soil. Texture, structure, porosity, permeability, color, temperature, profile, depth, pH, salt, and nutrient content are characteristics that help in choosing and managing soil. Often local systems of classification based on gardeners’ experience will be the most useful.

Soil is made up of minerals (in sand, silt, and clay), water, air, and organic matter from dead plants and animals. In addition, there are usually many living things in dryland soils, including plant roots, algae, fungi, bacteria, and animals such as moles, beetle grubs, earthworms, and nematodes.

Plants use carbon dioxide and sunlight to make energy, but must obtain water, nitrogen, potassium, phosphorus, and other nutrients from the soil. The best way to make sure these nutrients are available is by regularly adding organic matter such as compost, animal manure, and green manure. Organic matter also improves the ability of the soil to hold water. Increasing and maintaining organic matter and water in the soil are the most important goals of soil management in drylands. Even most poor soils can produce abundantly when improved and managed.

Wind and periodic heavy rains can cause severe erosion problems in drylands where vegetation is sparse. Simple ways to prevent loss of fertile garden soils are decreasing rainwater runoff, modifying the slope, and improving the soil’s resistance to erosion.

Card en beds concentrate improved soils into growing areas that can be easily managed and maintained. Beds may be raised or sunken, but for most drylands sunken beds are best because they help moderate soil temperatures and make the most of scarce water.

9.2 Soil and land-use classification

Past and present climate, the rocks from which the soil developed, and topography all affect soil characteristics. Soils in much of the drylands are geologically young and relatively fertile, but are alkaline, and can have a high calcium content with layers of caliche (Box 9.3 in section 9.4). Soils in other parts of the drylands are geologically old and have been leached of basic mineral nutrients such as potassium (K+), calcium (Ca++), magnesium (Mg++) and other cations. They tend to be acidic and may contain impermeable horizons of plinthite (Box 9.4 in section 9.4). These soils cover large areas of sub-Saharan Africa, South Asia, Southeast Asia, and South America.

There is a great deal of local variation in dryland soils. For example, in valleys the soil deposited by seasonal river flooding has a much higher clay and nutrient content, and higher water-holding capacity than adjacent soils (Figure 9.14 in section 9.7.1). Because they are often near a water supply in the dry season, these soils are ideal for gardens (section 11.6). Local variation in dryland soils can also be seen in the patches of younger, more fertile volcanic material pushing up into older, leached soils.

People use soil classification systems to help themselves organize different soil types for food production, and most gardeners and farmers have a system that they use in selecting and managing soils. Soils can be classified in many different ways based on their physical properties, profile and depth, plant nutrients, organic matter content, and how easily they are eroded. These topics are covered in following sections of this chapter. The diverse systems for using these characteristics to classify soils reflect the culture, environment, and needs of those who created them.2 Soil classification systems used by soil scientists in the Third World are often influenced by those of former European colonial rulers. For example, there are differences in the systems used by neighboring Anglophone and Francophone countries in West Africa.3

Land-use classification includes not only soil type, but other factors that affect how the land can be used by people, such as tenure systems, social preferences, proximity to water supply, exposure to sun and wind, and yields under different cultivation systems.

For selecting or improving soil in garden sites, understanding local classification systems is the most appropriate place to begin. These local classification systems probably exist in all communities that grow crops, except perhaps the newest or most transient.

9.2.1 Indigenous Classification Systems

Indigenous systems often combine soil and land use classification. The system in southwestern Tlaxcala state in Mexico, for instance, is based on both inherent characteristics and how the soil responds to management.4 Color and texture are the main characteristics used, but structure, saltiness, or depth may also be included. Inferior or superior ratings reflect anticipated yield, although farmers feel that the true nature of the soil can be known only after working it for several seasons. The ratings take into account the difficulty of the different management practices needed, including irrigation, fertilization, and cultivation, all of which depend on the characteristics listed above.

In semiarid areas of northeast Brazil farmers classify land using 15 specific terms for land types referring to location, soil type, and crop history, as well as many finer distinctions that govern planting of crops.5 These terms are organized according to two major criteria: strong (fertile) versus weak (infertile) and hot (dry) versus cold (wet).

People often use the distribution and growth of plants and animals as indicators of soil quality, since these are affected by the water-holding capacity, water content, and fertility of the soil. For example, in northern Kenya, Turkana gardeners use the presence of small termites and certain species of trees as indicators of good garden sites.6

An excellent place for an outside worker to begin is by developing an understanding of indigenous knowledge including local soil and land-use classification systems (section 4.3.4). Compiling a list or catalog of the names and terms of the system and their meanings helps a field worker from outside the community learn the local system and the criteria on which it is based. This may also be valuable for giving field workers who are community members a new perspective on, and appreciation of, the local classification system.

9.2.2 The USDA Classification of Soils in Drylands

The United States Department of Agriculture (USDA) is promoting its system, the “Seventh Approximation,”7 for classifying soils in the Third World. The USDA system organizes all soils into 10 different categories, or orders, based on how the soil is formed and under which climatic conditions. The orders are broken down into suborders, the suborders into groups, the groups into families, and the families into series, of which thousands have been defined. Because it is so widely used, we briefly describe the main soil orders found in drylands according to the USDA system: Aridisols, Alfisols, Entisols, Vertisols, and Oxisols.

Aridisols occur primarily in arid areas and are the most abundant USDA soil order in the warm drylands including the Sahara and part of the Kalahari Desert in Africa, deserts of southwestern North America, and from the eastern Mediterranean to India. They are the most common of the 10 major soil orders worldwide, accounting for 18.8% of the land surface.8 During most of the time when the soil is warm enough for plant growth, water is unavailable to many plants (tension ¬15 bars, discussed in section 10.3.1), is too salty, or both.9 Because they developed under arid conditions, nutrients have not been leached out, and many Aridisols are relatively fertile, although they are alkaline with salt and caliche deposits. The Aridisols also have the sparsest natural plant and animal populations.

Next in abundance to Aridisols in the warm drylands are Alfisols and Entisols. Alfisols are slightly to severely leached, acidic in upper layers, and occur throughout much of savanna Africa, India, and northeast Brazil. Entisols, which characteristically lack distinct soil horizons, comprise most of the Kalahari Desert. Significant areas of Vertisols exist in western India, eastern Sudan, and eastern Australia. They have high clay content, especially montmorillinite, and so shrink and swell a great deal, making them difficult to cultivate. In drylands the low pH, intensely weathered, infertile Oxisols occur mainly in southern Tanzania and northern Mozambique.

9.3 Physical properties of soils

Physical properties of soils include texture, structure, porosity, permeability, color, and temperature. Understanding the physical properties of soils helps in selecting new garden sites, and in improving soils in existing gardens.

9.3.1 Soil Texture and Structure

The proportion of different-sized mineral particles (sand, silt, and clay) in the soil is known as texture. Texture is important in the garden because it influences movement and storage of water in the soil, the ease of bed preparation and cultivation, the amount of air in the soil, and soil fertility. Knowing the soil texture helps in deciding how the soil can be improved.

The smallest soil particles, less than 0.002 mm (0.00008 in) in diameter, are called clay. Clay plays a big role in soil fertility and structure (Box 9.1). The other categories of soil particles are silt (0.05-0.002 mm; 0.002-0.00008 in), and sand (2.0-0.05 mm; 0.08-0.002 in). Gravel and cobbles are larger than 2.0 mm (0.08 in) and are not considered in determining texture. Loam is the name given to a soil containing sand, silt, and clay (0-40% clay, and 10-80% each sand and silt). Most soils are mixtures, and are referred to as sandy clay, silt loam, sandy clay loam, and so on. In general, sandy soils are those with 70% to 80% or more sand, and clayey soils have more than 40% clay.

Box 9.1
Clay

Clay is the most important mineral portion of the soil for plant nutrition (section 9.5). It is formed in the soil from dissolved minerals resulting from the breakdown of bedrock. Clays are primarily crystalline sheet-like structures made up of oxygen (70-85%) with aluminum (Al) and silica (Si). Most clays also contain some iron (Fe), zinc (Zn), magnesium (Mg), and/or potassium (K). Most clays have a negative charge and attract positively charged cations which plant roots can remove and use as nutrients (Box 9.5 in section 9.5).

The type of clay in a clayey soil can be partially identified by the extent to which the soil shrinks and cracks when drying. Swelling, sticky clays such as montmorillonite expand when wet, and shrink and crack during drying. They have a high capacity to store nutrients, and are the common clays in the more arid drylands. Bentonite, a type of montmorillinite, is used in the bottoms of ponds because it seals them as it swells.

Kaolinite expands and shrinks very little with wetting and drying, and is the clay used for ceramic pottery. Sesquioxide clays are common in the older soils of the subhumid drylands, where intensive weathering has led to the leaching out of all Si, Fe, Mg, and Al. These soils are extremely fragile. They do not shrink and swell and are infertile and subject to irreversible hardening when the soil horizon is exposed to air through cultivation.10 These layers are sometimes referred to as plinthite (Box 9.4 in section 9.4). Both kaolinite and sesquioxides have little ability to store nutrients in the soil and their presence indicates a need to improve the soil by adding organic matter.

Loams are considered the best soils for gardens because they share the properties of sand, silt, and clay. Sandy soils are low in nutrients and have a poor water-holding capacity. Clayey soils have poor drainage and are hard when dry, and sticky when wet, making them difficult to work. Both sandy and clayey soils can be improved by adding organic matter (section 9.6). It would be too much work to change the texture of the soil in most fields because of their large size, and lower productivity/area of land compared with gardens. Changing the texture of the soil in a garden, however, is practical and worthwhile, especially if suitable materials are nearby, such as sand from a streambed.

For most purposes, an adequate idea of soil texture can be obtained by simply looking at the soil and rubbing a sample between the thumb and finger.” The hard particles of sand can be distinctly felt, and if the sample is dry the crunching sound it makes can be heard when held close to the ear. Silt cannot be felt as individual particles but gives a smooth or soapy feeling. Clay is hard when dry, and slippery and sticky when wet. If a more precise idea is needed, the tests in Box 9.2 can be used.

Soil structure is the way in which sand, silt, and clay are grouped together in the soil to form aggregates or clusters of soil particles. Humus (section 9.6) is made up of material from the breakdown of organic matter and clay, two of the most important sources of cementing materials that help create aggregates. A good soil structure for gardens is one with many aggregates that create many spaces in the soil for the movement of the water and air needed for healthy roots and plants.

Large quantities of sodium, sometimes found in arid soils, lead to a loss of soil structure by causing negatively charged soil particles to repel each other, breaking apart aggregates. Clays and small organic molecules then plug soil pores (section 12.6.2). Because of this, salt (NaCl) is sometimes used to seal rainwater catchment areas or the bottoms of ponds.

9.3.2 Soil Porosity and Permeability

Soil porosity is a measure of the amount of spaces or pores in the soil that can be filled with either air or water. Permeability is a measure of the ease with which water and air move through the soil pores (section 10.3). A combination of texture, structure, and organic matter content determine the porosity and permeability of soils. For a good balance between small pores that retain a lot of water, and large pores that allow easy movement of air and water, medium- textured loam soils with good structure and plenty of organic matter are best.14 While clayey soils have higher porosity, and so can hold more water than sandy soils, they also have lower permeability because the pores are so small. Soils with a small pore size (less than 0.03 mm; 0.0012 in), characteristic of clay soils, retain water by attraction forces and if most pores in the soil are of this size waterlogging can occur. Large pore size, and thus poor water-holding capacity, is a problem only in very sandy soils. Both situations can be improved by adding organic matter to the garden soil.

Soil porosity can also be improved by animals such as earthworms or termites living in the soil. In northern Burkina Faso farmers make depressions in the soil which collect plant debris blown by the wind, and to which they may add manure.15 The organic material in the depressions attracts termites that make tunnels in the soil. The termites are usually gone by the time the farmers plant their crops and the tunnels increase water infiltration.

Cultivating the soil, that is, hoeing or plowing it, increases exposure to air, speeding the breakdown and loss of organic matter. This reduces soil structure, porosity, and fertility, as well as encouraging wind and water erosion, and eventually leading to lower garden production. However, clayey soils need to be cultivated to loosen the soil and to mix much needed organic matter into the root zone. Whether or not cultivation in the garden is needed or advisable depends on local conditions. The more the garden soil is cultivated, the more organic matter needs to be added, and the more attention must be given to controlling erosion (section 9.7).

9.3.3 Soil Color

The color of a soil is an indication of its mineral, humus, and water content.16 Soil color can help in choosing a garden site and in understanding what soil improvements are needed.

· Reddish soil: location is generally dry with seasonal wetting, darker red means higher iron content.

· Grey, blue, or green soil: location is poorly drained and if waterlogged for long periods of time, spots (or mottles) of blues, greys, and greens often result, indicating lack of oxygen.

· Dark brown soil: within a small geographical area, darker brown often means higher organic matter content, although darker colors do not always mean a fertile soil.

· White soil: lime or salts are present, pH is high.

Box 9.2
Tests for Soil Texture

THE RING TEST For the ring test a small sample of moistened soil is shaped and its qualities observed.12 This test should be done the same way each time so that samples from different sites in the garden and from different gardens can be compared. The sample should be from the root zone(15-45 cm; 6-18 in), and gravel and cobbles should first be removed by screening. If available, window screen or mosquito netting (which often have openings with a diagonal measuring about 2 mm [0.08 in]) work well for screening the sample. If these are not available the most obvious stones and gravel can be removed by hand.

Slowly add water to about 1 tablespoon of soil, enough to form a ball approximately 2.5 cm (1 in) in diameter. Try to roll the ball into a cylinder about 15 cm (6 in) long (Figure 9.1). Sand cannot be shaped into a ball; loamy sand will form a ball, but cannot be rolled into a cylinder. Loam can be rolled into a cylinder; with a clay loam the cylinder can be formed into a ring, though it breaks easily. Clay can be formed into a ring without cracks, which holds together with handling. This test is used frequently by soil surveyors in West Africa and if done carefully gives a good idea of how the soil will behave under cultivation.13


Figure 9.1 The Ring Test (Sand)


Figure 9.1 The Ring Test (Loamy sand)


Figure 9.1 The Ring Test (Loam)


Figure 9.1 The Ring Test (Clay loam)


Figure 9.1 The Ring Test (Clay)


Figure 9.2 The Settling Out Test

THE SETTLING OUT TEST Another way of finding out about the texture of garden soils is by taking a sample of soil (as with the ring test) and shaking it up well in a straight-sided, clear glass bottle filled with water (Figure 9.2). To help break up soil particles, several pinches (about 20 cc) of table salt (sodium chloride) and an equal amount of baking soda (sodium bicarbonate) can be added. After shaking, the sand settles out in about 1 minute into a layer extending just beyond the point where the particles become invisible to the eye. The silt will settle out in 1-2 hours, and then the clay begins to settle. Because clay particles are so small it may take several days for them to settle out and leave fairly clear water, although it would take months for all the clay to settle out. The relative widths of the different layers gives an idea of the composition of the soil texture.

9.3.4 Soil Temperature

Soil temperature affects seed germination and root growth, and is the result of many factors:

· Direction of slope: determines exposure to the sun. In the northern hemisphere north-facing slopes receive less sun than south-facing ones; in the southern hemisphere the opposite is true (section 8.3).

· Depth: with increasing depth below the soil surface, the less the soil temperature changes daily and seasonally, and the more changes in soil temperature lag behind changes in air temperature.

· Color: dark-colored soils absorb more solar radiation (heat) than light soils.

· Water content: water in the soil moderates temperature; large amounts of heat are used to evaporate water, keeping soil cool on hot days, and, large amounts of heat are given off when water freezes, keeping soil warm on cold nights.

· Surface cover: covering the soil surface with a mulch slows heating and cooling both seasonally and daily, and can be used to regulate soil temperature according to the time of year and crop (sections 5.7.2 and 10.8).

When cold soil slows seed germination or plant growth the following steps can help: planting on slopes exposed to the sun, encouraging deep root growth; planting in dark-colored containers or covering the soil with dark colored stones or other mulch to absorb heat during the day; mulching during cold nights to prevent heat loss; and watering regularly. In many mountainous or hilly areas with temperate climates, gardeners locate their gardens mid-slope, avoiding both valleys where the heavier, cold air settles, and high elevations where winds can blow warm air away.

When hot soil slows seed germination or plant growth the following can help: planting on slopes shaded from the sun; encouraging deep root growth; planting in light-colored containers or covering the soil with light-colored mulch; and watering regularly.

9.4 Soil profile and depth

A verticle section through the soil displaying its layers is called a soil profile. Soils are divided by soil scientists into three layers or horizons within a profile, referred to as A, B, and C (Figure 9.3). The A horizon is sometimes referred to as the topsoil and contains most of the organic matter in the profile, which often gives it a darker color. However, in arid areas there may be very little organic matter, and the topsoil can be a light color. The depth of the topsoil is important because this is where most of the roots of annual plants grow. Shallow topsoils have to be deepened by removing or improving of the underlying subsoil, or by making mounds or raised beds of improved soil.

At the point in the profile where the color, texture, or composition of the soil changes, the B horizon begins. The B horizon, commonly referred to as subsoil, accumulates material washed out of the A horizon, such as clay, salts, and iron. Any change in soil porosity between the A and B horizons will slow or even stop movement of water downward and tends to increase the water content of soil above it (section 10.3.2). In alkaline soils of arid and semiarid areas a white, nonporous layer composed mainly of calcium carbonate and known as caliche, can form in this horizon, and if it is too close to the surface it may cause problems with root growth and drainage (Box 9.3). In semiarid and subhumid areas with acidic soils a rock-like layer called ironstone can form (Box 9.4).

The C horizon is formed by decomposition of the parent material or bedrock over which it lies. It contains no organic matter and very few mineral nutrients available to plants.

In the garden, good quality soil should be deep enough to allow plenty of room for root growth, and good drainage of any excess water. For most annual vegetables 45 cm (18 in) deep is probably adequate (section 9.8). Many trees will need deeper, well-drained rooting areas to prevent stunting or waterlogging. Gravel or rocky layers like caliche and ironstone impede root growth and drainage and should be removed from the root zone and replaced with good soil. After digging a planting hole the drainage can be checked by filling it with about 15 cm (6 in) of water. If the water drains out overnight, drainage is probably adequate. If not, the hole may have to be dug deeper to reach a better draining part of the horizon, or a new planting site found. Salt buildup, waterlogging, or nutrient deficiencies in the B horizon affect plant growth, and are problems that must be solved if deep-rooted plants, especially large perennials, are to be grown (sections 9.5, 12.6.1, 12.6.2).

9.5 Soils and plant nutrients

All living cells on earth, whether in animals, plants, or microbes, contain about the same elements in approximately the same proportions. Sixteen elements are essential nutrients for plants. The six macronutrients, required in large amounts, are carbon (C), hydrogen (H), and oxygen (0) which are all obtained from air and water, and nitrogen (N), phosphorus (P), and potassium (K) which are absorbed from the soil. The secondary nutrients calcium (Ca), magnesium (Mg), and sulfur (S) are required in smaller amounts. The micronutrients chlorine (Cl, copper (Cu), boron (B), iron (Fe), manganese (Mn), molybdenum (Mb), and zinc (Zn) are required in very small amounts. Secondary nutrients and micronutrients are obtained primarly by absorption through the roots from the soil (Box 9.5). With the exception of boron, people also require these same elements (section 2.7). In addition, people need sodium (Na), chromium (Ch), selenium (Se) and iodine (I). Plants need an adequate supply of water, air, sunshine, and soil nutrients to be healthy and to produce a good harvest. When any one of these, including a specific soil nutrient, is in short supply, plant growth slows or even stops.


Figure 9.3 A Soil Profile in an Area with Aridisols

The soil nutrient supply available to the plant depends on:

· The ability of the crop species, the crop variety, and the individual plant to obtain nutrients in relation to other plants with which it is growing.

· The chemical composition of nutrients and their relative amounts.

· Soil conditions such as the temperature, moisture, pH, porosity, and cation exchange capacity (CEC) (Box 9.5).


Box 9.3
Caliche

Caliche, or a calcic horizon, is common in arid and semiarid regions.17 It is a hard, whitish alkaline layer resembling limestone and can be many meters thick. Caliche is composed mostly of calcium carbonate (CaCO3), but magnesium carbonate (MgCO3) is usually also present. It is the same composition as the “lime” that is often applied to acid soil to raise the pH. Caliche usually occurs in the B horizon, but in very arid areas, or where there has been severe erosion, it may extend to the surface.

The sources of CaCO3 are rainwater, wind-blown dust, and soil layers above the caliche. Carbonate from these sources in the upper soil is dissolved by carbonic acid (H2CO3) formed by a combination of water with carbon dioxide (CO2) from plant roots and microbial activity, and moved into the lower layers by rainwater. When evapotranspiration is much greater than rainfall, CaCO3 becomes more concentrated and forms solids. A decrease in CO2, concentration below the root zone is also a factor. As the caliche layer builds, it may form a barrier to water penetration and cause further buildup toward the surface.

Caliche layers in the root zone should be removed in order for most garden crops to do well. However, some trees such as olive can tolerate shallow, alkaline soils, and so grow sucessfully where deep caliche layers are near the surface. To remove very hard caliche it may have to be chipped away with a pick or hoe. Caliche can sometimes be softened by soaking with water. Dry-season garden beds can be started during the rainy season and excavated as they are moistened with rain.

Box 9.4
Plinthite and Ironstone

Soil horizons rich in iron and aluminium oxides, usually leached from upper slopes, are known as plinthite. These soils tend to have low levels of plant nutrients, harden irreversibly when exposed to repeated cycles of wetting and drying, and form nodules or rock-like layers called ironstone or laterite. This process is speeded up when at or near the surface, and therefore often seems caused by excessive grazing or cultivation leading to erosion.18

The climate under which these soil layers are found may have little or nothing to do with the climate when they were formed. Ironstone and laterite are found in some areas that today are subhumid, semiarid, and even arid, including the Sahel, parts of India, and Western Australia. One soil scientist estimates that there are as much as 250 million ha of soil in semiarid West Africa with laterite 5-25 cm (2-10 in) below the surface.19

Like caliche, ironstone gravel or solid layers limit root growth and cause waterlogging by slowing drainage. Horizons with ironstone gravel have very low water-holding capacity (section 10.3.1) as do sandy, gravelly, or rocky soils in general. Soils with ironstone horizons should be avoided as garden sites unless the overlying soil is deep enough to support plant growth and the slope and topographic position allow drainage. Ironstone outcrops tend to occur in the middle of slopes where there is the most erosion (section 9.7.1). If ironstone is present, a better garden site may often be found in a nearby low-lying area that is less eroded, and where there is also likely to be more fertile soil, and better access to water, whether from wells or runoff.

Box 9.5
Nutrient Uptake by Plant Roots20

Most soil nutrients are in the form of electrically charged particles called ions. Positively charged cations, for example ammonium (NH4+), calcium (Ca++), potassium (K+), ferrous and ferric iron (Fe++ and Fe+++), and zinc (Zn++) are attracted to negatively charged soil particles, primarily clay and humus. Negatively charged anions, for example nitrate (NO3-), phosphates (H2PO4- and HPO4-), and sulfate (SO4-), are attracted to positively charged soil particles. The cation exchange capacity (CEC) is a measure of the amount of exchangable cations per 100 gm of soil. It is determined by the a-mount of clay, and of humus from the breakdown of organic matter. Montmorillonite and vermicullite are the clays with the highest CEC, illite has less, and kaolinite and sesquioxides have a very low CEC.

Plants absorb most of the water and minerals they need through the root hairs that grow on younger roots. As water increases to near field capacity, nutrient uptake increases because transport of nutrients through the soil by flow and diffusion increases, as does root growth and transpiration. Most root hairs are only about 0.01 mm (0.0004 in) in diameter and a few mm (0.1 in) long, and are actually extensions of single epidermal cells (section 5.2). The root hair cell walls are coated with a sticky substance that allows them to cling to soil particles, increasing their ability for absorption.

Nutrients are absorbed from the soil in a process that uses energy from plant food created by photosynthesis. Once nutrients and water have been absorbed, they move throughout the whole plant, carried by the flow of water or sap in the vascular system (sections 5.2 and 10.3.4).

Recently, another factor affecting nutrient uptake has been recognized. Symbiotic relationships called mycorrhizae can occur between certain soil fungi and the roots of flowering plants including garden fruits and vegetables. Mycorrhizae make nutrients, especially phosphorus, more available to the plant (section 5.2.1), and help the plant resist some fungal diseases. Absence of the mycorrhizae results in stunting and poor plant growth.21

Nutrient deficiencies often show up as changes in leaf color, including chlorosis, a change from darker green to lighter green and yellow due to a loss of chlorophyll. Care must be taken to distinguish these signs from those caused by pests and diseases (section 13.4.2).

Growing food involves managing plant communities to increase production of fruits, nuts, roots, or leaves that people want. When these products are harvested nutrients are removed from the garden, upsetting the natural cycling of nutrients, and making management of soil nutrients necessary. Figure 9.4 shows the nutrients needed by plants cycling through humans, animals, plants, microorganisms, soil, and the air. Within the soil bacteria, fungi and animals like earthworms and millipedes are constantly recycling organic matter.

Gardeners’ understanding of this cycle is shown when they replace nutrients by fallowing, rotating crops, growing cover crops, and returning plant, kitchen, human, and animal wastes to the garden, either directly or after composting. Gardens planted on or around recent animal pens, trash heaps, or latrines take advantage of the enriched soil there. Deep-rooted plants such as many fruit and nut trees can absorb nutrients at deep soil levels and make them available to shallow-rooted plants when their leaves drop to the soil surface and decompose. Nitrogen supplies can be increased by growing nitrogen-fixing leguminous crops (Box 9.7 in section 9.5.2). An adequate supply of nutrients in most gardens can be maintained by adding plenty of organic matter to the soil, and by interplanting and rotating crops.

Persistent problems for which other causes cannot be found may indicate a problem with soil nutrients, and a soil test may be appropriate. Very inexpensive kits that test for pH, nitrogen, phosphorus, and potassium may sometimes be available and are relatively easy to use. In addition, local agriculture departments in some areas of the world provide soil testing to farmers and gardeners.

9.5.1 Soil pH and Plant Nutrition

The pH of the soil is a measure of how acid or alkaline it is. pH is important to the gardener because it has a strong effect on the availability of nutrients to both crops and soil microorganisms (Figure 9.5). In most soils the pH is between 3.5 and 10. Garden soils should generally have a pH between 6 and 7.5. A neutral soil has a pH of 7, an acid soil below 7, and an alkaline or basic soil above 7. On the pH scale an increase of 1 means that there is a tenfold increase in alkalinity, and a decrease of 1 means a tenfold increase in acidity (Box 9.6). For example, a soil with a pH of 7 is 10 times as alkaline as a soil with a pH of 6,100 times as alkaline as a soil with a pH of 5 and so on. A pH of 6 to 7 in garden soil makes many nutrients easily available, but a pH up to 8 is still fine for the majority of crops.

Soils in arid areas tend to be neutral to alkaline (pH 7 or higher) especially in the topsoil, because of the lack of rain to wash out basic cations and release acidic anions from the clay.22 The more arid the region, the higher the pH. In areas with less than 600 mm (24 in) of annual rainfall pH is often as high as 8-8.5. Topography also affects pH, and the middle of slopes tend to be better drained and therefore more leached and more acidic than adjacent bottomlands or valleys.


Figure 9.4 Nutrient Cycles in the Garden

Basic or alkaline soils have a pH greater than 7 and often contain relatively large amounts of exchangeable (available to plants) calcium and magnesium, with lesser amounts of potassium and sodium. As soils become more alkaline the availability of some nutrients decreases (Figure 9.5). Gardeners can reduce the pH of alkaline dryland soils by the addition of organic matter. Sometimes elemental sulfur is also added. Sulfur lowers pH by first being oxidized to sulfate by bacteria, and eventually becoming sulfuric acid. To increase bacterial action, the surface area of the sulfur should be maximized by applying it as a powder.

Acid or acidic soils have a pH of less than 7 and also occur in drylands, for example, when parent material is an acidic rock like granite, or when a former wet climate leached the soils. In much of the West African savanna underlying granitic bedrock and many years of weathering have resulted in acidic soils.

When the pH falls below 6, some nutrients become less available (Figure 9.5). The pH of acidic soils can be increased through the addition of alkaline substances like lime (CaCO3) or ashes. Ashes are an indigenous soil amendment in many parts of the world. Their value to the farmer and gardener is shown by the fact that, like animal manure, they are applied to individual plants, for example in West Africa,23 Mexico, and Central America.24 Ashes from home fires also reach gardens and fields through trash heaps and compost piles (Box 9.10 in section 9.6.2). While burning crop residue and weeds leads to a loss of most of the nitrogen and sulfur, other nutrients, especially potassium, become more concentrated, easier to turn into the soil, and wash more quickly into the root zone than bulky plant residues.

Box 9.6
pH

The pH scale is a log scale, with pH expressed as the negative log of hydrogen ion (H+) concentration in the soil. When water molecules come apart (HOH (H+ + OH-) both cations (H+) and anions (OH-) are at a concentration of 10-7 moles (molecular weights) per liter, therefore a pH of 7 is neutral. A soil with a pH of 8 has one-tenth as much H+ and is 10 times as basic (alkaline) as a neutral soil; a soil with a pH of 6 has 10 times as much H+ and is 10 times more acidic than a neutral soil; a soil with a pH of 5 has 100 times as much H+; and so on.

9.5.2 Nitrogen

Nitrogen (N) is one of the most important nutrients required by both plants and animals. Even though the air we breath is 78% nitrogen, we cannot obtain nitrogen directly from the air. We depend on microorganisms in the soil to make nitrogen in the air available for plants, and then we eat plants (or animals that eat plants) to obtain the nitrogen we need (Figure 9.6). Plants must have nitrogen to produce amino acids, which are then used to make plant proteins. When eaten by humans, the protein in plants is broken down into amino acids and resynthesized into human protein (section 2.5). Nitrogen is also an important component of DNA.


Figure 9.5 Soil pH and Nutrient Availability

A key symptom of nitrogen deficiency in plants is whole leaf chlorosis, especially in older leaves (section 13.4.2). Deficient plants may also show slow growth and drying out along the edges of leaves.

Nitrogen is made available to plants by microorganisms in two ways: by fixing nitrogen from the air, and by releasing nitrogen by breaking down organic matter. Some microorganisms fix nitrogen in the air into ammonium (NH4+) and nitrate (NO3-) which can be taken up by plant roots. Some of these are bacteria that grow in nodules on the roots of legumes (Box 9.7). Bacteria quickly convert most NH4+ to NO3-, which is very soluble and quickly leached from the soil. The majority of nitrogen in the soil is stored in living and dead organic matter. Therefore, plants depend for nitrogen on the continual decomposition of dead microorganisms, and of all sorts of other organic material such as dead plant roots and soil organisms to make nitrogen available. Gardeners increase this supply of soil nitrogen by adding animal manure, green manure (fresh green plant matter), or compost, and by planting legumes. Chemical fertilizers are discussed in Box 9.8.

Box 9.7
Nitrogen Fixation

A critical link in our food web are the microorganisms that convert nitrogen in the air into forms that can be used by plants. Symbiotic bacteria and actinomycetes (another type of bacteria) in root nodules, and free-living bacteria and cyanobacteria (formerly known as blue-green algae) in the soil, all fix nitrogen.

Many garden crops in the legume family, such as cowpea, tepary bean, fava bean, African locust bean, and carob, fix nitrogen in root nodules inhabited by rhizobia, bacteria of the genus Rhizobium. Such crops can supply nitrogen to the soil and so to future crops that do not have rhizobia, thus reducing the need for nitrogen fertilizers in the garden. The bacteria enter the root hairs when the plants are young, causing the legume root to form tumor-like growths known as nodules. These nodules are different than the knots formed by root knot nematodes (section 13.3.2), because the nodules can be rubbed off without breaking the root apart. The legume protects the rhizobia and supplies it with food for energy, while the rhizobia fixes atmospheric nitrogen into forms that the legume can use. This is a symbiosis, a relationship in which both organisms benefit.

Specific Rhizobium species induce nodules in certain species of legumes. Local legume varieties are more likely to become colonized by local rhizobia than imported legumes. Many commercially available legume seeds will need to be inoculated with the appropriate variety of rhizobia that colonize them, since the rhizobia may not be present in local soils.

Leguminous garden crops can supply nitrogen to other crops by rotating the legumes with non-legumes, or composting leaves, stems, pods, and other parts for later use in the garden (for example carob pods or peanut leaves). In addition, legumes can be interplanted with other crops as in the familiar example from dryland Mexico of corn, with a high nitrogen requirement, and beans. However, most nitrogen is probably contributed by legumes in the season after they die, when their leaves and roots decompose in the soil. It is a good idea to leave the roots of annual legumes along with their rhizobia in the soil where they can serve as a source of bacteria for later plantings as well as provide nitrogen.

Rhizobial growth and metabolism is affected by the carbon to nitrogen ratio (C:N) in the plant in which it is living, and the amount of phosphorus, calcium, magnesium, molybdenum, and boron in the soil. The number of nodules is not a good indicator of nitrogen fixation. The bacteria may even become parasitic and remove nitrogen from the plant. An easy test for gardeners is to cut some of the nodules open. If they are reddish they contain leg-hemoglobin and are fixing nitrogen; if they are hard and greenish inside they are not fixing nitrogen and may actually be removing it from the plant.25


Figure 9.6 The Nitrogen Cycle

9.5.3 Phosphorus and Potassium

Phosphorus (P) is a component of DNA and proteins in plants and animals. Bones and teeth are mostly compounds of phosphorus. It is especially important for young plants which absorb it rapidly, and for fruit and seed production. Phosphorus deficiency causes slow growth and stunting and can lead to a reduction in normal opening of the stomata and thus to an increase in leaf temperatures and overheating (section 5.4). A reddish purple color in the leaves may be another indicator, although this color is also caused by other factors in some plants.

Soluble phosphate (H2PO4-) reacts rapidly with soil ions to form insoluble compounds inaccessible to plants. Therefore most phosphorus used by plants comes from organic matter broken down in the soil by mycorrhizae (Box 9.5 in section 9.5). At a carbon: organic phosphorus ratio of 200:1 or less, phosphorus is readily released, but when the ratio is 300:1 or greater the phosphorus is immobilized in the bodies of microorganisms. As is also true with nitrogen (section 9.6.2), too much high carbon material in garden soils or compost makes these important plant nutrients unavailable, reducing production.

Potassium (K) is very abundant in plants and important in many plant processes including cell division, carbohydrate formation, movement of sugars, some enzyme actions, and disease resistance in some plants. Uptake of potassium is especially high during early plant growth. The potassium-containing soil minerals are micas and feldspars which dissolve slowly. These sources supply half of the potassium available to plants, while decomposing organic matter supplies the other half. A pH of 6.0 to 6.5 is best for potassium availability.

Box 9.8
Commercial Chemical Fertilizers

“Chemical” is a word popularly used to describe synthetic fertilizers and pesticides that are commercially manufactured. We will use it in this way because it has become accepted, even though everything on the earth is made of chemicals. Chemical fertilizers contain high concentrations of a limited variety of nutrients in simple chemical compounds, compared with organic matter which has lower concentrations of a wider variety of nutrients in much more complex chemical compounds.

Chemical fertilizers are relatively expensive and their supply is beyond the control of the gardener. In addition, their manufacture is energy intensive and often harmful to the environment. Fertilizers available to project officials, frequently at subsidized prices, may not be available to gardeners when the project is turned over to them, or ended. Therefore, using chemical fertilizers at the beginning of a project because they are easier to obtain and apply, and provide quicker results than organic fertilizers, may jeopardize the project’s long-term success.

Chemical fertilizers can also have a bad effect on soil quality. The high concentrations of nutrients in chemical fertilizers encourage rapid plant growth, using up the organic matter in the soil. If the organic matter is not replenished the soil structure will be destroyed.

Gardeners who recycle garden and kitchen refuse, use nitrogen-fixing legumes, and have trees in their gardens do not need to use chemical fertilizers. We give a brief overview of how chemical fertilizers work because they are often promoted as part of garden projects, although we advise strongly against the use of these and other manufactured agro-chemicals in the garden.

Grades or nutrient content of fertilizers are usually given as percentage by weight of their elemental nitrogen (N), phosphorus pentoxide (P2O5), and potassium oxide (K2O) content, in that order (Figure 9.7). However, fertilizers are commonly manufactured to include other nutrients such as sulfur, and the effect of a given macronutrient fertilizer may actually be a response to another component of the fertilizer which is the limiting nutrient in the soil. For example, in northern Ghana, peanut yields increased in response to the sulfur in single super-phosphate fertilizer and not the phosphorus.26

Because the nutrient content is so high compared with organic fertilizers, chemical fertilizers can create an excess of a particular nutrient, upsetting the balance of nutrients in the soil. This makes it impossible for plants to take up the needed amount of another nutrient; this condition is referred to as an induced deficiency.27 To determine deficiencies, before chemical fertilizers are applied, a soil test should be done and then interpreted on the basis of the actual responses of specific crops already growing there.

Nitrogen in chemical fertilizer is supplied either as the ammonium cation (NH4+) or the nitrate anion (NH3-). Urea (NH2CONH2) is rapidly converted to ammonia after application to the soil. Manufactured urea is a solid, water-soluble fertilizer. Nitrate fertilizers do not lower pH, but ammonia-containing fertilizers do because the ammonium cation replaces other cations like Ca and Mg, which are then leached out. For this reason ammonia should not be used as a source of nitrogen in gardens with acid soil. Ammonia will be held in solution as NH4+ for a short time, especially in soil with high clay and organic matter. Nitrogen fertilizer, especially nitrate, is easily washed into the soil, but is also easily leached out of the root zone, and so multiple applications are often recommended.

Powdered phosphate-containing rocks can be applied to supply phosphorus, but they are not very soluble. The finer they are and the more they are mixed with soil, the more quickly they will release phosphorus. The most common water-soluble phosphate fertilizers, single superphosphate and triple superphosphate, are manufactured by mixing rock phosphate with an acid. After application the phosphate in these fertilizers is quickly fixed into insoluble forms unavailable to plants. Therefore, it is usually recommended that these fertilizers be applied in concentrated areas near the seed or growing plant. Phosphate is often applied as side dressing, that is, close to plant roots such as along the side of a row, so plants can use it quickly.

Most potassium fertilizers are water-soluble, and are produced by mining large deposits of soluble potassium salts. Potassium chloride, or muriate of potash, is a common potassium fertilizer.

The oxides, hydroxides, and phosphates of iron, zinc, copper, and manganese are insoluble but the sulfates are soluble and can be sprayed on leaves where they are absorbed through the stomata. Chelates, metal ions bonded to complex organic molecules, are also used. Chelates, which also occur naturally in organic matter, resist fixation in the soil but are still available to plants.


Figure 9.7 Commercial Fertilizer Labels

9.5.4 Other Nutrients

Sulfur, calcium, and magnesium are minor nutrients since they are needed in smaller amounts than nitrogen, potassim, and phosphorus. Chlorine, copper, boron, iron, manganese, molybdenum, and zinc are micronutrients, and are needed by plants in very small quantities.

Symptoms of sulfur deficiency include chlorosis and retarded growth. Soil organic matter is a major source of sulfur. Most soils contain adequate calcium, especially alkaline soils. Calcium is needed in large quantities for cell division, and therefore a deficiency affects the rapidly growing root and leaf tips, causing browning, rotting, and deforming. An example is leaf tip burn in lettuce and cabbage. Blossom end rot of tomatoes and peppers is caused by a calcium deficiency, and is often caused by dry soil, which makes the calcium in the soil unavailable to the plants.

Iron and zinc deficiencies, showing up as chlorosis between the veins of new leaves, are common in alkaline soils or when there are high levels of caliche (calcium carbonate, CaCO3).

9.6 Organic matter

Soil is full of living organisms. A small handful (1-5 gm; 0.05-0.15 oz) of most garden soils contains over 1 million bacterial cells, hundreds to thousands of fungi, hundreds of nematodes, and large numbers of other microorganisms (Figure 9.8). Larger organisms living in the soil include insects, millipedes, earthworms, and mammals like moles, mice, and rabbits. Roots of growing plants are another living part of soils. When these organisms die, microorganisms decompose them, releasing their nutrients for further plant growth. Harvesting garden produce removes nutrients which must be returned to the garden. The most convenient source of nutrients, and the best for the garden, is additional organic matter, the remains of plants and animals of all sizes. The addition of fresh plant remains (green manure), animal manure, and compost to the soil is a widespread indigenous practice of farmers and gardeners in drylands.


Figure 9.8 The Living Soil

In an intensively cropped dryland garden organic matter decomposes rapidly because of high temperatures, high moisture, strong structure with plenty of soil air, pH between 6 and 8, and an abundance of nutrients. In addition to decompostion, organic matter in dryland gardens is lost through harvesting of plant parts, and cultivation of the soil, exposing it to oxygen in the air which increases the loss of organic matter through oxidation. This is why adding organic matter to the garden soil is probably the most important part of dryland soil management. Table 9.1 shows the approximate nutrient content of some different types of dryland organic matter. Box 9.9 lists some of the advantages of adding organic matter to dryland gardens.

Availability of organic matter may be the greatest limitation on its use in dryland gardens. Organic matter content of as little as 3-8% of the soil weight or higher improves plant growth.28 When first making garden beds or planting holes, 25-50% of the soil volume can be compost. (See section 8.4.2 for discussion of why holes for transplants may be an exception.) Organic matter is much lighter than the mineral components of soil, therefore, a volume of soil would weigh much more than an equal volume of organic matter.

Humus consists of the small particles resulting from the breakdown of plants and animals. Humus continues to be decomposed by microorganisms that use the nutrients to multiply; as they die, their bodies are decomposed by other living microorganisms. As humus and microorganisms break down they make nitrogen and other plant nutrients in forms that are available to plants while releasing H2O and CO2 In effect, the soil humus and living microorganisms act as a nutrient storage system. Humus also improves soil structure by acting as a bond to hold together soil particles in aggregates (section 9.3.1), creating space for air and water in the soil, and increasing infiltration rates.

Gardeners often use different kinds of organic matter in their gardens. For example, in Zimbabwe manure from cows and goats, compost, and soil from termite hills is used in irrigated valley gardens, along with some chemical fertilizer.31 Because of the high use of organic matter the quality of the soil was maintained, whereas it would not have been if only chemical fertilizer was used. Tests found that the organic matter content in gardens (3.5%) was almost the same as in non-cultivated areas outside of the gardens (4%), showing that gardeners were replacing the organic matter being lost through cultivation and harvesting.32

9.6.1 Animal Manures

Gardeners and farmers are usually well aware of the value of animal manure (dung or feces, and urine) for productive crops. Most often manure is from domestic animals such as cows, horses, goats, sheep, chickens, ducks, and pigs. However, bats, wild birds, or other wild animals may also be a source. In savanna West Africa the fields that are closest to the house and are most intensively cultivated contain the rainy-season gardens. Here precious animal manure is applied at the end of the dry season to improve the soil. The Yatenga Mossi of northern Burkina Faso use the manure from penned animals in their gardens.33 It is the richest manure because in addition to dung, it contains urine which is higher in nitrogen and potassium than dung. In southeastern Burkina Faso, Mossi gardeners, especially when growing some of their produce for market, purchase manure from Fulani herders.34

Table 9.1 Approximate Nutrient Content of Some Dryland Organic Mattera

Source

Nitrogen (N)
(% dry weight)

Phosphorus(P)
(% dry weight)

Potassium (K)
(% dry weight)

Poultry droppings

2.1

1.1

0.37

Brewery wasteb

3.2

0.6

0.37

Farmyard manure

1.7

0.4

0.38

Cocoa husks

1.1

0.24

4.4

Rice straw

0.5

0.16

1.3

Cowpea husks (pods)

1.4

0.14

2.0

Yam peelings

0.2

0.13

2.7

Sugercane trash

0.08

0.06

0.4

Groundnut husks

0.3

0.08

0.6

Maize waste (cobs)

0.7

0.07

0.5

Plantain peelings

0.8

0.07

2.2

Orange peelings

0.75

0.04

0.30

Maize stem

1.1

0.07

1.20

Melon waste

0.6

0.05

0.2

Cassava peelings

0.65

0.04

1.6

a From Titiloye, et al. 1985.
b "spent grain”

Box 9.9
Advantages of Organic Matter

Organic matter from many different local sources is a high-quality, low-cost resource for maintaining dryland garden soil fertility (Figure 9.9). It provides the following benefits to garden soils as it decomposes to humus:29

· Is the source of 90-95 % of soil nitrogen, including that which is cycled through microorganisms.

· When it makes up more than 2% of soil, it can be the major source of available phosphorus and sulfur.

· Is a major source of the cements necessary for aggregate formation to create strong soil structure (section 9.3.1) with a higher proportion of larger pores, which improves water-holding capacity and water and air movement.

· May furnish 30-70% of the negatively charged sites that hold nutrient cations plants can use (Box 9.5 in section 9.5). This electrical property also gives organic matter the ability to act as a buffering agent, moderating the tendency to change pH when acid or alkaline substances are added to the soil.

· Acts as a chelate, that is, it forms compounds with metal nutrients (usually iron, zinc, copper, or manganese) increasing their solubility and availability to plants.

· Supplies carbon for energy to many soil microorganisms that perform beneficial functions such as nitrogen fixation.

· In cities where garden soils contain lead from exhaust fumes of vehicles or lead-based paints, soils containing 25% or more organic matter significantly reduce the uptake of the poisonous lead by garden crops.30

· Acts as a mulch on the soil surface (section 10.8.1).


Figure 9.9 High-Quality, Low-Cost Methods of Maintaining and Improving Soil fertility

Manure can lose many of its nutrients if not properly handled when fresh. Left in piles, nutrients in dung may be washed away and nitrogen lost as ammonia gas. Animal pens should be covered with dry organic material such as straw from cut grasses or other plants. This provides carbon and air for microorganisms which can use the nitrogen in urine and dung for growth, thus storing this nutrient in their bodies.

Weed seeds or cuttings in manure can sometimes grow in the garden. Before applying large amounts of suspect manure, it can be tested on a small area. If lots of weeds grow where the manure was applied but not elsewhere, it should be composted to kill the seeds. This is less of a problem with dung from cows, water buffalos, goats, sheep, and other animals that chew a cud because they have a two-stomach digestive system that kills most seeds. However, horses, donkeys, and birds have less thorough digestive systems that some seeds can pass through unharmed.

Manure from humans can also be used for gardens, however, great care must be taken to avoid spreading disease. The best way to do this is to make use of human manure without actually handling or transporting it, for example by letting tree roots extract nutrients from old latrines. In parts of West Africa, latrine sites are moved each year and the second year after being abandoned the old sites are filled in and bananas are planted on them.35

9.6.2 Composting

Compost is a soil amendment made from decomposed organic matter. It can be made solely from plant remains or from a combination of plants and manure. Composting fresh plant remains and animal wastes is often more convenient than adding them directly to the garden, and helps gardeners get the most soil improvement from organic matter. Making compost piles or pits prevents the organic matter from drying up or blowing away, two ways in which nutrients are often lost. If applied directly to the garden, high-carbon organic matter can tie up nitrogen during decomposition, and high nitrogen organic matter can produce considerable heat which can burn the roots of garden crops.

Like other aspects of dryland garden management, understanding the basic principles of composting allows local gardeners or projects to develop or improve techniques best suited to their needs.

Moisture is needed for the decomposition process in composting. However, moisture requirements can be kept low if the compost pile is covered with earth or woven mats to provide shade and retain moisture when the weather is hot and dry.

The size of the compost pile should be small enough and the organic matter coarse enough that the pile does not become packed down or compressed, eliminating oxygen. This leads to smelly, anaerobic (without oxygen) decay, as happens when only fresh green weeds are used. On the other hand, organic matter particles should be small enough and the pile large enough that the pile is able to hold heat and moisture, and to provide optimum surface area for microbial growth (Figure 9.10). Corn, amaranth, okra, sunflower, and other stalks are best chopped with a machete into pieces less than 15 cm (6 in) long.

Almost any organic material can be composted, including crop or food-processing residues (e.g., malt left from making beer, seed pods, grain chaff), weeds, and animal bedding with manure and urine. Leaves and stalks of younger plants contain more readily available water-soluble nutrients in their sugars and amino acids, but as the plant ages these become less available woody materials like cellulose and lignin, the most abundant compounds in cell walls. Salted foods or plants that tolerate salt such as tamarisk, which may have high salt concentrations in their leaves (section 5.6), should not be used in the compost pile. This is especially true if the soil or water used for irrigation is already salty. Where soils are alkaline, ashes or other materials that would raise the pH should be kept out of the compost. Where soils are acid ashes and other alkaline materials can be good soil amendments, but should only be mixed into finished compost or directly into the soil. Adding ashes while building a compost pile, and thus raising its alkalinity, interferes with the decomposition of the organic matter (Box 9.10).

When the compost pile is first built the carbon to nitrogen ratio (C:N) should be about 20:1 to 30:1 by weight, which is the ratio at which microorganisms use these nutrients. If it is less than 20, there will not be enough C in relationship to N, and microorganisms will use protein for energy and give off ammonia gas (NH3) which has a uniquely pungent, sharp smell. This smell is an indication that valuable nitrogen is being lost, and high carbon materials should be added to the pile. If the C:N is greater than 30 the composting process will slow down since there is not enough nitrogen for the microorganisms, and new ones cannot grow until old ones die and release the nitrogen in their protein. If the pile does not heat up and start shrinking, more high-nitrogen material (to lower the C:N) should be added. After some experience observing and working with compost piles it becomes easy to estimate the proportions of different types of local material best for making compost. When the composting process is over the final C:N will be about 10:1 to 12:1, as carbon dioxide (CO2) is lost through the metabolism of sugar for energy. Table 9.2 gives the approximate C:N ratios of some common dryland sources of organic matter, and can be used as a rough guide to combining organic matter with differing C:N ratios in a new compost pile, trying for an overall ratio between 20:1 to 30:1. Box 9.10 gives some tips for a fast compost pile


Figure 9.10 The Effect of Particle Size on Composting (1)


Figure 9.10 The Effect of Particle Size on Composting (2)

The compost pile contains millions of bacteria, fungi, and other microorganisms which feed on the nutrients in the organic matter. The fungi become dominant toward the end of the composting process and may form a grayish-white powder on the outer 10-15 cm (4-6 in) of the pile. Large numbers of these fungi occur in the soil, manure, and other compost ingredients. For this reason there is no need to inoculate the compost pile with commercial inoculante or other material.

A method called trench composting or trench bed gardening is used in some areas of southern Africa and the Philippines (Figure 9.11).36 Alternating layers of organic matter and soil are used to fill up a trench and make a slightly raised bed that is then planted. As the organic matter decomposes it releases nutrients for use by the growing plants. This is a method worth experimenting with although care must be taken that the heat of decomposition does not burn the roots of garden plants.

9.7 Preventing soil erosion

Erosion is the movement of soil from one place to another by wind or water. Erosion usually leads to a loss of productivity because the finer clay and organic matter which contains most of the fertility are lost. In addition, clay and organic matter are the major source of binding for soil aggregates, and so soil structure is weakened. When erosion is so severe that the subsoil is exposed there is a further reduction in structure.

Table 9.2 Examples of Approximate C:N Ratios of Dryland Organic Mattera

Source

Percent organic carbon (C)

Percent total nitrogen (N)

C:N ratio

Organic Matter for Compost




Alfalfa, young

40

3

13:1

Poultry droppings

27.9

2.0

14:1

Brewery wasteb

44.6

3.7

15:1

Farmyard manure

33.9

1.5

22:1

Cocoa husks

33.1

0.9

36:1

Rice straw

41.7

0.5

76:1

Yam peelings

51.0

0.3

196:1

Sugercane trash

38.0

0.1

318:1

Soil Microbes




Soil bacteria

50

10.0

5:1

Soil actinomycetes

50

8.5

6:1

Soil fungi

50

5.0

10:1

Soil humus

50

4.5

11:1

a Based on Titiloye, et al. 1985, and Donahue, et al. 1983:146.
b “spent grain”


Figure 9.11 Trench Composting (After DCFRN #9-7)

Box 9.10
For a Fast Compost Pile

Compost piles can be either fast or slow, that is take a short or long time to finish composting. For fastest piles moisture, oxygen, temperature, and pH should be kept near the optimum levels described below.37 However, for slow composting controlling these factors is not as critical, because a longer time in the compost pile compensates for lower moisture content and temperature. Slow composting, using two or three piles at various stages of decomposition, is easiest and often most practical for gardeners.

MOISTURE Between 50 and 60% moisture is best for optimal microbial growth. This is the point at which water can be squeezed out of the composting material by hand, but will not drip or drain without squeezing. The compost should have a glistening appearance after being wet down. If the pile is too wet the air will be forced out, leading to an increased population of anaerobic, denitrifying bacteria which lose nitrogen by releasing it into the air. In the dry season covering the pile with a layer of clayey earth or woven mats, or putting the compost into a pit dug up to 60 cm (24 in) deep, help to conserve moisture. During the rainy season, however, compost piles in pits can become waterlogged.

AIR Oxygen is also required by the microbes, so the pile should not be too big, preventing the center of the pile from “breathing.” A good size is roughly a 2.5 m × 2.5 m (8 ft × 8 ft) base and a height of 1.5 m (5 ft). Ammonia and other bad smells are a sign of too little air. Air circulation can be increased by piling branches and other loose, bulky material underneath the pile. Building the pile around poles which are then removed when the pile is finished provides air to the center of the pile (Figure 9.12).


Figure 9.12 Creating Air Space in a Compost Pile

TEMPERATURE Heat is released by the microorganisms in the compost pile as they metabolize sugar. If the pile is built correctly it heats up within 2 to 3 days to about 55°C (131°F), then peaks at 60-70°C (140-158°F). The temperature can be tested by digging into the center of the pile, or by sticking a wooden or metal pole into the center and leaving it there for 15 minutes. If it is hot to the touch when it is removed, the temperature is high enough.

pH The compost pile usually begins as slightly acidic, then becomes very acidic and ends up somewhat alkaline (pH 7.5-8.5). Ashes should not be used when making a compost pile as they will make the pile too alkaline at the start for the microorganisms required to decompose the organic material.

There is a positive side to limited erosion when it moves and concentrates small soil particles in sites which can then be cultivated. The low-lying areas at the bottoms of slopes and along streams and rivers often have clayey soils rich in mineral nutrients and organic material brought to these sites by rainwater from the slopes above, or by stream flow (section 11.6). The soil deposited by flowing water is called alluvium.

However, if erosion continues unchecked, infertile subsoils, rocks, and gravel can be washed down and cover the fertile soil. If streambeds or riverbeds become filled with eroded soil the level of water will be raised and consequently so will the water table, which may cause drainage and waterlogging problems in gardens next to the stream. Gardeners in valleys and other low-lying areas must therefore be concerned about erosion not only in their gardens, but on the slopes above the gardens and in the watershed of the stream that brings them water.

9.7.1 Decreasing Runoff

When rain is falling at a faster rate than the ability of a soil to absorb it, water begins running off the soil surface. In technical terms, when rainfall intensity (mm/hr) exceeds the infiltration rate (mm/hr) (the rate at which water on the soil surface enters and continues to move deeper into the soil), then runoff begins (section 11.5). Runoff water flows over and erodes the soil in two ways: a) sheet flow moves as a thin sheet of water removing a thin layer of soil, in a process called sheet erosion, b) stream flow cuts into the soil surface, removing soil in larger amounts from the sides, bottom and head of small channels (rills) or large channels (gullies), called respectively rill erosion and gully erosion (Figure 9.13).


Figure 9.13 Sheet, Rill, and Gully Erosion

Anything that decreases the amount or speed of water flowing across the soil will help to limit soil erosion. Vegetation and dead plant material on the surface also help to slow water flow and increase infiltration.

Slope is the most important factor causing runoff. Slope is the change in height of the soil surface over a given distance, and is often expressed as a percentage (Figure 11.6 and Box 11.4 in section 11.5.3). The larger or steeper the slope the more quickly water will run off the soil surface, and the more danger there is of erosion. Slope affects soils and growing conditions in many important ways (Figure 9.14). Upland soils are usually well drained, while low-lying ones tend to be poorly drained. The middle sections of slopes are most subject to erosion. Rainy-season gardens do best where there is good drainage, and dry-season gardens benefit from being located in low-lying areas where water is held in the soil, and where they can also collect rainwater. Land with steep slopes should not be cultivated if at all possible, and erosion on intermediate slopes can be decreased by terraces or contour bunds.


Figure 9.14 Some Typical Effects of Slope in Drylands

Although slopes are not the ideal location for gardens, there are many situations where they are the best place available. Cultivating on slopes greatly increases the risk of soil erosion by rainwater runoff, and the longer or steeper the slope the more danger of erosion. Contour bunds or terraces are often constructed to decrease the slope, slow runoff, increase infiltration, and capture soil eroded from areas further up the slope. Trees, bushes, and grass growing in strips between groups of bunds or terraces further protect the soil. Contour bunds or terraces can also harvest rainwater. Details of calculating runoff and catchment to garden area ratio (CGAR) are found in section 11.5.

CONTOUR BUNDS Contour bunds are piles of rocks, soil, or organic matter made in long rows following the contours of the land (Figure 9.15).38 A contour is a line along the ground which is at a constant elevation. Although contours are often constructed to decrease soil erosion, they can also capture and hold rainwater for gardens. In areas where peak rainfall is not heavy or where distance between bunds is small, earthen bunds will retain the most water. However, earthen bunds do not allow excess water to flow through them, so where rainfall and runoff are larger, provision should be made for peak rainfall runoff which can cause much damage. Spillways in the earthen bunds can be built out of rock so that any overflow will not wash away the whole bund (Figure 9.16). Bunds that include rock or dead vegetation such as tree branches are another good solution because they allow excess water to pass through.


Figure 9.15 Contour Bunds in Northeast Ghana

If bunds are not constructed accurately on the contours, runoff will tend to flow along the length of the bund, causing soil erosion and damage to the garden (Figure 9.17). One way to correct contour bunds that do not follow the contours is to remove them and start over, but this is a lot of work. An easier solution is to construct short bunds, perpendicular to the existing contour bunds, to stop and hold any water flowing parallel to the contour bunds, thus preventing it from gaining the speed and volume which cause erosion. When the contour bunds are close together the perpendicular bunds can connect them, forming basins.

In theory, information should be collected on runoff potential of the soil and rainfall pattern so that hydrological calculations can be made to design the size, shape, and placement of the contour bunds. In practice such data are often difficult to obtain, persons with appropriate skills are not available, and the time and other resources required may be more useful elsewhere. However, as with many other gardening techniques, using gardeners’ knowledge and skills, beginning small, and encouraging experimentation and adaptation can be a successful alternative to dependence on outside technical expertise. For example, in Burkina Faso farmers developed successful contour bund systems by starting small, observing the effects of runoff during the rainy season, and then adding more bunds between existing ones when necessary.39 Box 9.11 describes methods for determining contours.


Figure 9.16 A Stone Spillway in an Earthen Bund


Figure 9.17 Contour Bunds

Box 9.11
Determining Contours

A variety of methods can be used to determine contours, and gardeners can choose the best one for them. One method used successfully by farmers in the Yatenga area of Burkina Faso employs the water tube level.40 This requires a length of transparent plastic tubing. If long pieces are difficult to obtain, a section about 2.0 m (6.5 ft) long can be fastened to each end of a long piece of nontransparent garden hose. The tube is first stretched out on the ground and then filled with water by pouring or syphoning. The tube ends are fastened to two straight sticks, each at least 2.0 m (6.5 ft) long, standing next to each other. Some water may need to be added to bring the water level to within 0.5 m (20 in) of the top of both ends of the tube, and this point is marked on both sticks (Figure 11.8 with Box 11.4). Then one stick is held still while the second one is moved across the slope and then up and down until the water level on both sticks is at the original mark. This point on the slope is marked and the first stick is then moved to a third point while the second stick remains stationary, and so on (Figure 9.18). If water spills or the tube stretches (as it can when it gets hot) then water will have to be added and the level marked again on the sticks.

Another method for measuring contours is using an A-frame level (Figure 9.19). Two poles about 1 m (3.3 ft) long and a shorter middle bar are lashed together into an A-shape and a weighted string is hung from the center of the top of the A. To calibrate this level the base of each pole is marked, for example, by driving a small stake. The place where the string crosses the middle bar is then marked. Then the A-frame is turned 180°, with each stake taking the place of the other stake. Once again the place where the string crosses the middle bar is marked. The midway point between these two marks on the middle bar is the level point. The A-frame can then be used to mark a contour line along a slope by placing the two legs so that the string hangs on the level mark (Figure 9.20).


Figure 9.18 Using a Tube Level to Measure Contours


Figure 9.19 Using an A-Frame Level


Figure 9.20 Measuring Contours with an A-Frame Level

TERRACES Terraces are shelves of land built on slopes to provide level areas for gardens or other uses and are part of indigenous soil management systems in many dryland areas. Like contour bunds, terraces are also built on contours and reduce runoff and erosion, but they are used on steeper slopes (3-30%) than contour bunds. Elaborate terraced systems of agricultural production date back over 2,000 years in dryland South America and Mexico, where most present terrace systems are continuations of ancient ones.41 During the first century after the European invasion of this region (by 1600), the indigenous population was reduced from 50-100 million people to 5 million people, mainly from epidemic diseases introduced by the Europeans. This reduction in population along with violent social and cultural destruction, led to many terrace systems being abandoned, resulting in increased erosion.

Terraces have also been used for centuries in many other drylands such as those of the Arabian Peninsula and southwestern North America. In the Mediterranean, olives, figs, and grapes are often planted in small stone terraces on slopes that would be too rocky to support many other garden plants. Hopi Native Americans of Arizona, USA, have built up extensive terraces below mesa tops for spring-fed irrigated gardens where they grow chilis, onions, tomatoes, corn, herbs, and other crops42 (Figure 9.21).

Construction of terraces involves building a retaining wall and leveling soil on the slope above this wall (Figure 9.22). Additional soil will often be needed to fill in the terrace. The steeper the slope and the more shallow the topsoil the closer together the terraces should be (Figure 9.23).


Figure 9.21 Hopi Terraced Gardens


Figure 9.22 Leveling the Soil in Terraces

9.7.2 Decreasing Raindrop Impact

The force of raindrops hitting the surface of the soil can cause as much erosion as runoff. Raindrops break up and detach soil aggregates producing smaller-sized particles which are easier to wash or blow away. Raindrops move soil particles as they splash, and on a slope will move soil downhill, even without runoff. In addition, the impact of raindrops compacts a thin layer of soil on the surface which slows infiltration and increases runoff. Infiltration is also reduced as rainwater carries the detached soil particles into the soil, clogging soil pores.

Protecting the soil surface from the force of raindrops helps decrease erosion. This effect can be seen dramatically where a rock on bare soil exposed to the rain has protected the soil under it from erosion so that eventually it is perched on a pedestal surrounded by an eroded surface (Figure 9.24). The leaves of closely planted garden crops will absorb the energy of the raindrops, slowing their impact. This helps protect the soil from erosion because water that drips from leaves has much less energy than when it first hits the plants.

9.7.3 Increasing Soil Resistance to Erosion

Anything that increases soil permeability or porosity will help decrease erosion (section 9.3.2). A loam texture is a good compromise between sandy soils, with high infiltration rates but low water-holding capacity, and clayey soils, with high water-holding capacity but low infiltration rates. Loams are also ideal soils in other ways (section 9.3.1).


Figure 9.23 The Relationship Between Degree of Slope and Terrace Size

Vegetation not only protects the soil from raindrop impact, but roots help keep soil in place. An example of the ability of indigenous techniques to slow erosion is the living fences of willows (Salix spp.) and cotton-woods (Populus spp.) planted by farmers in the Sonoran Desert of northern Mexico.43 The fences are planted in the winter in riverbeds adjacent to fields “o protect them from stream erosion during the summer rainy season. Not only do these trees protect fields in most situations, but they also slow the flow rate of the water upstream. As it slows, the alluvium carried in the water is dropped, forming a deposit of this rich sediment behind the fences. Over the years, farmers have used this soil to enlarge their fields. Unlike outsiders who consider willows and cottonwoods bad because they transpire so much water, local farmers have developed a successful technique for using those trees for soil management and so see them as beneficial.

9.7.4 Reducing Wind Erosion

Wind is a major cause of erosion, especially in drylands where surface vegetation is sparse. It carries fine soil particles away, bounces heavier particles of sand along the surface and into plants, and adds to the erosive power of raindrops by increasing their speed at impact.

A small reduction in wind speed results in a proportionately much larger reduction in erosion. For example, a 13% reduction in wind speed results in approximately a 50% reduction in erosion.44 Wind erosion of soil and abrasion of plants with windblown soil particles can be reduced with windbreaks. Windbreaks can improve yields by protecting a plant, a garden bed, or one or more entire gardens (section 10.8.3). They should be placed perpendicular to the prevailing direction of the strongest wind. If strong winds come from different directions at different seasons of the year, then windbreaks may be needed in more than one direction.

Windbreaks should have from 20-50% porosity, that is allow 20-50% of the wind to pass through them. If porosity is low, the windbreak will offer too much resistance and there is a good chance it will be blown over. The lower the porosity, the greater the reduction in erosion closest to the windbreak, but the shorter the distance over which this effect will be felt. Also, with lower porosity the speed and erosiveness of the wind as it passes around the ends of windbreaks will be increased, and these areas should be protected, for example, with perennial crops or stone mulch.

The effect of a windbreak is described in terms of distances equal to its height (H). Tall windbreaks of several rows of trees can produce a 50% reduction from 16H leeward (the direction the wind is blowing toward, that is, downwind from the windbreak) to 2H windward (the direction the wind is coming from, that is, upwind from the windbreak).


Figure 9.24 Protecting Soil from Raindrops

Large windbreaks can protect the whole garden, a group of gardens, or adjacent farmland. Within the garden, smaller windbreaks are appropriate to protect single beds or even plants (Figure 10.12 in section 10.8.3). They can be made from living plants, mats, or stalks of bamboo, sorghum, or other grass, or walls of stone or mud brick. In Egypt, farmers and gardeners place lines of stalks in the ground to protect newly emerged seedlings. The Hopi in southwestern North America do the same, and also use empty tin cans to protect individual plants, and pile flat slabs of sandstone around the base of fruit trees to keep the wind from blowing the soil away from the roots. Wind-breaks can also provide shade or serve as fences, protecting the garden from animals (section 13.3.3).

9.8 Building garden beds

Garden beds separated by walkways help contain and concentrate resources for plant growth. Using beds also makes it possible to reach and work on the garden without compacting the soil in the growing area by walking on it. Two types of garden beds are sunken and raised. Although raised beds have been promoted widely, they are not always the best design for dryland conditions.

9.8.1 Sunken Beds

Sunken beds are deep pockets of improved soil, and have a long tradition in drylands. In southwest North America, Zuni Native Americans traditionally used very small rectangular beds bordered with clay berms for growing crops such as melons, herbs, chilis and onions (see Figure 12.1 in section 12.3.2). These gardens, near the main villages and surrounded with juniper stick fences, were watered by hand from larger containers using small dippers.45 The nearby Hopi people also use sunken beds in their terraced gardens.46 Until the Aswan dam was built, Egyptians used basin beds for thousands of years along the Nile river to capture the annual floodwaters.

In drylands and during dry seasons, sinking the beds is better than raising them for several reasons:

· They are easier to water efficiently by flood irrigation.

· The berms give the moist soil and young seedlings and transplants some protection from drying winds and sun.

· Young plants can easily be protected by laying palm fronds or other material across the beds (Figure 9.25).

· When wet season rains are intense, the garden is not eroded and water is not wasted, since it is captured by the beds.

In drylands with heavy rainfall in the wet season, it may be better to switch to raised beds during this period. However, in drier areas this will not be necessary because not enough rain falls to cause waterlogging, even in the wet season. The choice of bed design will also depend to some extent on the crops grown and the soil and drainage in the garden. Figure 9.26 outlines the steps for making a sunken garden bed.


Figure 9.25 Sunken Beds Protect Young Seedlings

9.8.2 Raised Beds

Raised beds are common in humid areas of the world. They have also become part of the “alternative” gardening tradition in temperate industrial regions of North America and Europe and as such have been promoted by some development workers from those areas as part of an alternative to industrial gardens.47 Raised beds are most commonly used in humid regions where they can improve drainage in areas where the soil may be seasonally flooded or waterlogged. Like sunken beds they also function to increase the depth of improved soil. However, under hot, dry conditions raised beds are difficult to water deeply. Raised beds also expose a lot of surface area which heats up soil temperatures, and from which moisture evaporates, leading to salt buildup in the growing area (Figure 9.27). Figure 9.28 outlines the steps for making a raised garden bed.


Figure 9.26 Constructing a Sunken Garden Bed (1)


Figure 9.26 Constructing a Sunken Garden Bed (2)


Figure 9.27 Evaporation and Salt Buildup in a Raised Bed in Drylands


Figure 9.28 Constructing a Raised Garden Bed (1)


Figure 9.28 Constructing a Raised Garden Bed (2)

9.9 Resources

Blakie (1985) discusses the political and social basis of soil erosion and its control on both local and global levels. Good general introductions to applied soil science, both based on Africa, can be found in Ahn (1970) and Dupriez and De Leener (1983: Lessons 33-37,40-43). We have used Donahue, et al. (1983), as a basic reference on Western soil science. A clear, simple description of how village people can build contour bunds and check-dams to reduce erosion is given in Introduction to Soil and Water Conservation Practices (1985), produced by World Neighbors and based on work in Indonesia.

References

1 Blakie 1985.

2 Chatelin 1979.

3 Ahn 1970:202-213.

4 Wilken 1987:28-36.

5 Johnson 1974.

6 Morgan 1974.

7 USDA 1975.

8 Donahue, et al. 1983:529.

9 USDA 1975:155.

10 Ahn 1970:101.

11 Ahn 1970:18.

12 Ahn 1970:19-21; Leonard 1980:11.

13 Ahn 1970:25.

14 Donahue, et al. 1983:62.

15 Pacey and Cullis 1986:160-161.

16 Donahue, et al. 1983:64-65.

17 Birkeland 1984:138-146; Dregne 1976:168-173.

18 Dupriez and De Leener 1983:93.

19 Lal 1987:1070.

20 Donahue, et al. 1983:90ff, 211-213,267; Raven, et al. 1981:560,563, 568ff.

21 Agrios 1988:507-509.

22 Ahn 1970:94; Donahue, et al. 1983:107-108; Dregne 1976:186.

23 Lagemann 1977:38.

24 Wilken 1987:57.

25 Purseglove 1974:200.

26 Ahn 1970:265.

27 Ahn 1970:149-150.

28 Donahue, et al. 1983:155.

29 Donahue, et al. 1983:148-149; CFA 1980:127-128.

30 Bassuk 1986.

31 Bell, et al. 1987:50-52.

32 Bell, et al. 1987:79.

33 Hammond 1966:35/42.

34 Delgado 1979:73-75.

35 Lagemann 1977:39.

36 DCFRN #9-7.

37 UDS 1983.

38 World Neighbors 1985.

39 Wright 1984.

40 Adapted from Wright 1984, cited in Pacey and Cullis 1986: 171; see also Chleq and Dupriez 1984:43-46.

41 Denevan 1980:222-229.

42 Soleri 1989.

43 Nabhan and Sheridan 1977.

44 Troeh, et al. 1980:410-411.

45 Ladd 1979:497-498.

46 Soleri 1989.

47 Cleveland and Soleri 1987.


Figure 10.1 The Water Cycle

10. Water, soils, and plants

Water is essential for all life, but is often a scarce resource in drylands. Water carries plant nutrients from the roots upward, and food from the leaves downward. It is also a medium for chemical reactions, cools the plant by evaporating from the leaves, and plays an important role in photosynthesis. Water as a liquid and a gas is constantly being cycled among plants and animals, soil, air, and bodies of water such as streams, lakes, rivers, and oceans (Figure 10.1).1

The goal of water management in the garden is to provide adequate supplies of water to plant roots at a reasonable investment of time, money, and other resources, without creating salinity or waterlogging problems. Another important goal of garden water management is to ensure that all members of the community have access to good quality water, and that water use today does not jeopardize the quantity or quality of water in the future.

Many indigenous techniques for watering, cropping patterns, mulches, shades, and windbreaks appear to meet these goals but they are not well documented. They are based on the same principles of water, soil, and plant relations that Western science is based on. Knowledge of these basic principles will help readers understand indigenous practices and suggest improvements where needed.

10.1 Summary

This chapter discusses the movement of water in soils, and the relationship of soil water and garden yield as the basis for specific techniques to improve water management.

The storage and movement of water in the soil depends on the texture and structure of the soil and the amount of organic matter present. Water is removed from the root zone by gravity, evaporation, and plant roots. Successful garden harvests depend on maintaining an adequate amount of water in the root zone at a reasonable cost. This is done by applying water to the garden when the plants need it; by conserving water through preventing excess loss from evapotranspiration using mulches, windbreaks, and appropriate cropping patterns; and, by preventing excess drainage loss by controlling the amount of water applied.

10.2 Dryland garden water management

Efficient use of water for gardens means using the smallest amount of water to produce the largest amount of harvest in ways that do not harm the environment, and that promote equitable local control of the source and distribution of water. Conserving scarce water resources by reducing unnecessary losses is a very important step in reaching this goal. Runoff, deep percolation beyond the root zone, evaporation, and excess transpiration are the four ways water is lost within the garden (Figure 10.2). Management strategies to prevent these losses are:

· Minimizing runoff and maximizing infiltration of rain or irrigation water into the garden soil by improving soil structure and by using terraces and vertical mulch.

· Minimizing loss of water below the root zone by not overwatering, and by increasing the soil’s water-holding ability with organic matter.

· Minimizing evaporation and excess transpiration in the garden by mulching, close spacing of plants, multiple plant levels, shading, windbreaks, careful selection of planting times, and use of heat-tolerant and drought-adapted crops.

Ways to conserve water before it reaches the garden are discussed in Chapters 11 and 12 and include:

· Improving and maintaining the quality of water used in the garden by minimizing salts, poisons, and organisms that cause plant and human diseases.

· Making use of local water resources such as rainfall, streams, floodwater, and shallow groundwater aquifers for the garden.

· Minimizing the loss of irrigation water from storage in reservoirs, tanks, pots, or other containers by covering or shading the surface to reduce evaporation, and by stopping any leaks in the container.

· Minimizing the loss of water while raising it from wells or rivers, and while bringing it to the garden in hoses, buckets, irrigation canals, or rainwater catchment plots by reducing the number of leaks or low spots and the amount of time the water is exposed to evaporation and to infiltration before reaching the garden.


Figure 10.2 How Water is Lost in a Dryland Garden

10.3 Water, soils, and plants

Plants obtain the water they need to grow through their roots. Many characteristics of the soil affect the availability of water to plants, and in turn help to determine how much water the garden will need.2

10.3.1 Water Storage in the Soil

Water is stored in the soil pores, the spaces between particles of soil (section 9.3.2). Soil scientists have developed a system for describing water in the soil in relation to plant growth. If water is added to a soil until all the pores in the soil are filled and there is no room for any more, it becomes saturated (Figure 10.3). If a saturated soil is allowed to stand with no additional water being added, the free water or gravitational water will drain down and out of the root zone. Since most soils drain gravitational water from the root zone rapidly, it is generally not available to plants. When the point is reached where no more water drains out by gravity, the soil is at field capacity (FC).

Fine, clayey soils can hold more water than coarse, sandy soils. This is because clayey soils have many small pores that add up to much more total pore space than the fewer but larger pores of sandy soils. Clayey soils also hold water more tightly than sandy soils because the smaller pores provide more soil surface area per volume of water (Box 10.1).


Figure 10.3 Soil Water and Plant Growth (After Donahue, et al. 1983:171; and Doneen and Wescot 1984:6) (1)


Figure 10.3 Soil Water and Plant Growth (After Donahue, et al. 1983:171; and Doneen and Wescot 1984:6) (2)

When the water in the soil is reduced to the point where the plant cannot absorb it fast enough to grow or even to stay alive, soil water is at the permanent wilting point (PWP). The water held in the soil between field capacity and the permanent wilting point is water that can be absorbed fast enough by plants to grow and produce. This water is called plant available soil water (AW):

AW = FC - PWP.

Most garden crops should be watered when 50-75% of plant available water has been depleted (section 10.7). Even though a plant will be able to survive on the remaining 25-50%, it may develop a water deficit. This means that water will be lost through transpiration faster than it can be absorbed, reducing production in many crops (section 10.4). However, an experiment with tomatoes, sweet peppers, and cantaloupe melons in Tucson, Arizona, USA, showed that some crops or crop varieties may be able to survive without growing for several weeks when soil water is well below the wilting point, and then recover and produce a harvest when water content rises above the wilting point.4

Too much water can also harm or even kill plants. If a soil remains saturated because water is being added faster than it can drain, or because there is a high water table, the soil becomes waterlogged. Most plants cannot grow in waterlogged soils because there is no air to provide oxygen, since it has been squeezed out by the water (Figure 10.3)

Box 10.1
How Water is Held in the Soil

Water in the soil at or below field capacity is held by electromagnetic forces. One of these forces is cohesion, the tendency of molecules of a substance to stick to each other, as when water molecules are attracted to each other (section 5.2). Adhesion is the attraction of molecules of different substances to each other, as in the attraction between water and soil molecules. A combination of cohesive and adhesive forces holds layers of water molecules on the surface of soil particles.

The water closest to the surface of the soil particles is held most tightly, so as water is taken up by the plant roots, the remaining water becomes more and more difficult to absorb. The energy needed to remove water from the soil, called the soil water potential, is most commonly measured in units called bars. The greater the negative value the more tightly the water is held, and the more energy is required to remove it. For example, the water in garden soil with a water potential of -0.5 bars is more easily available to plants than water in soil with a water potential of -0.7 bars. Field capacity is often considered to be -0.3 bars and permanent wilting point -15 bars.3 In practice, however, the permanent wilting point depends on the crop and variety, and on the soil’s properties including texture, structure, and organic matter.

10.3.2 Water Movement in the Soil

The way water infiltrates and moves within the soil influences plant growth.5 The rate of infiltration is influenced by soil permeability (section 9.3.2) and slope (section 9.7.1). Holes in the soil surface and channels beneath the surface of the soil made by plant roots, earthworms, termites, moles, and other animals increase infiltration. Cultivating the surface improves infiltration, but it also increases evaporation in the cultivated layer (section 10.3.3). Because of smaller pore size, clayey soils have slower infiltration rates than sandy soils, which can mean higher runoff and evaporation from the soil surface. Adding organic matter and using vertical mulches both increase the infiltration rate of clayey soils.

The rate and pattern of water movement below the surface is not obvious from simply observing the pattern of wetting on the surface. It depends on soil texture, structure, depth, and the organic matter content. Most water movement in the soil after a good rain or irrigation is due to the downward force of gravity. However, because of capillary action, some movement takes place in all directions toward areas with less water. Capillary action is the movement of water through very small spaces (capillary spaces) from wetter areas to drier areas. In clayey soils with many fine pore spaces, water moves further horizontally by capillary action than in sandy soils with much larger particles and fewer and larger pores. Therefore, an equal amount of water wets a larger surface area of clayey soils than of sandy soils (Figure 10.4 a, b).

Soils with distinct layers of different textures in the root zone, for example, of sand or of clay in a loam soil (Figure 10.4 c), hold more water than uniform soils because the layers slow the downward movement of water, increasing the field capacity. An abrupt change from sandy to clayey texture will slow the movement of water, as will a transition from clayey to sandy texture (Figure 10.4 d, e). A dense layer of caliche, ironstone, or rock will practically stop water flow, causing the soil above it to become saturated (Figure 10.4 f).

A separate layer of organic matter will also slow the downward movement of water. However, when mixed in with the soil in the root zone, organic matter speeds downward movement (Figure 10.4 g). Organic matter improves soil structure as it breaks down to form humus, creating soil aggregates and larger pore spaces that water can penetrate faster and deeper. Organic surface mulches protect the soil surface from compaction by raindrops, and vertical mulches, which are open to the surface, allow water to penetrate quickly to the root zone (Figure 10.4 h; sections 10.8.1 and 10.8.2).

10.3.3 Evaporation

Evaporation is the change of water from a liquid to a gas form. When water evaporates from the soil into the air, plants can no longer use it. In drylands, evaporation from the upper 10 cm (4 in) of wet soil is very rapid because temperatures are high and capillary action moves water upward quickly in this layer. Below this level, soil water is protected from evaporation by the layer of soil above it which acts like a surface mulch (section 10.8.1). When soil is at field capacity, most of the water 10 cm (4 in) below the surface will be removed from the soil by plant roots, not evaporation, capillary action, or gravity.6

Shading crops from the sun, protecting them from the wind, and mulching the soil surface are all ways to reduce evaporation. Vertical mulches speed infiltration to the root zone (section 10.8.2). Maintaining a high organic matter content which promotes good soil structure (section 9.3.1), and efficient irrigation methods (section 12.2) also reduce evaporation.

Sometimes cultivation to break up the soil surface, which interrupts capillary action and so reduces evaporation, is recommended.7 This method may be useful when it is difficult to find mulching materials, and when plants are young and do not cover the soil surface. In this situation cultivation can also help to discourage weed growth, and may avoid problems with pests hiding in the mulch and eating young plants. However, cultivation only slows down evaporation in the soil below the cultivated layer, while increasing evaporation in the cultivated layer itself. This technique requires continual work since cultivation has to be repeated after each irrigation or heavy rain. In addition, if cultivation is used over a wide area it can lead to soil erosion by wind and water.8 For these reasons we recommend using mulch rather than cultivation, whenever possible, for reducing evaporation in gardens.

10.3.4 Water Uptake and Transport by Plants

Plants need a much greater proportion of water than animals of the same size and weight. This is because most water is recycled internally in animals, whereas in plants water is continually being lost through transpiration when stomata are open to obtain the CO2 required for photosynthesis (sections 5.3 and 5.4). Water and dissolved nutrients move from the roots to the stems and leaves through the vascular system as described in section 5.2. When this water is lost through transpiration it must be replaced with more water absorbed from the soil by the roots in a continual cycle.

The rate of transpiration from garden crops is affected by many factors, such as the garden microclimate and the types of crops being grown. Transpiration is commonly measured in combination with evaporation, which is referred to as evapotranspiration (ET). Maximum evapotranspiration (ETm) is the rate of evapotranspiration that occurs when the crop requirements for water are fully met. Garden water management tries to maintain ETm by keeping water in the root zone at no less than 25-50% of field capacity.

A major goal of dryland garden management is to minimize excess evapotranspiration due to stressful environmental conditions such as high temperature, low humidity, wind, or poor infiltration. Shading plants and protecting them from drying winds, along with the measures to decrease evaporation from the soil given in the previous section, will help to reduce excess evapotranspiration.

The relationship between transpiration, soil water, and garden yield is discussed in the next section, along with more suggestions for increasing garden production.


Figure 10.4 Water Movement in the Soil (After Doneen and Wescot 1984:22-27; and Gardener 1979)

10.4 Soil water and garden yield

A plant experiences water stress or water deficit when actual evapotranspiration (ETa) falls below ETm, that is, its requirements for water are not met (section 5.5). This is often the result of insufficient water in the soil, but may also be caused by disease or pest damage which hinders plants’ ability to take water from the soil and use it.

When a crop experiences water deficit, the yield, or the amount that is harvested, is often reduced. The amount of reduction depends on the stage of the life cycle in which the water stress occurs, the crop and crop variety, and the amount of water deficit.

As crops develop through vegetative, flowering, yield formation (fruit, seed, tuber), and ripening stages, their sensitivity to drought changes. In general, crops are most sensitive to drought during flowering, followed by yield formation, with early vegetative growth and ripening being the least sensitive9 (Figure 10.5). Therefore it is most important that garden crops have enough water during the flowering and fruiting stages.

The specific crop and variety also affect the reduction in yield as drought increases. The more drought-resistant a variety is the less its yield will be reduced by drought, and the more stable its production will be over time in a marginal environment.10 Figure 10.6 shows that for more drought-resistant plants, drought results in a relatively smaller yield reduction (B) than with less drought-resistant (drought-sensitive) plants (A). New varieties bred specifically for high production may be more sensitive to drought than locally adapted, indigenous varieties. They may have higher yields (M) than drought-resistant indigenous varieties when there is no drought, but also have a greater reduction in yield with drought (D1) (compare A with B in Figure 10.6). As drought worsens (D^), yield is reduced below that of indigenous varieties (N). Thus/where drought is likely because of lack of rain or irrigation water during the garden growing season, drought adaptation is an important criterion for choosing crops (section 5.5). This means that growing a high-yielding new variety unadapted to drought is more risky for gardeners in areas subject to drought.


Figure 10.5 Water Requirements, Yield, and Crop Life Cycle (After Doorenbos, et al. 1979:38)

Thus, there are three main approaches to increasing garden yield by minimizing drought:

· Changing planting times to improve the fit between soil water availability and crop demand.

· Using more drought-adapted crops, crop varieties, and crop mixtures (section 5.5).

· Increasing available soil water by applying more water; by using mulch, shade, and windbreaks to decrease water lost to evapotranspiration; and by decreasing water lost to runoff and excess deep percolation (sections 10.3 and 10.8).

Trying any of these approaches to increase yield often means experimenting with new methods, and taking a certain amount of risk. Increased investments of labor and other resources such as organic matter, shading materials, or water may also be required. Each gardener will have to decide if these increased investments are worthwhile. The greater yields that may result from such increased investments must be considered in light of other benefits that could be obtained by investing the time and resources in different ways.


Figure 10.6 Yield Response to Water (After Doorenboos, et al. 1979:38)

10.5 How much water?

Gardeners learn to judge when and how much to water their gardens by observing variation in plant growth and yields in relation to different watering methods. Most successful gardeners never make numerical calculations about water requirements. (See Box 10.2 for ideas about estimating water requirements for large projects.) To estimate water requirements for individual household gardens it is probably best to observe the amount of water being used by other gardeners or farmers in the area and use this as a basis for experimenting.

There is very little information on the water requirements of dryland mixed gardens. In one case study, an annual average of 48 liters/m2/week (1.2 gal/ft2/week) was applied to two urban gardens in the Sonoran Desert of North America.11 The gardens were mixtures of annual vegetables, producing food throughout the year. The amount of irrigation water needed differed greatly during the year depending on temperature, rainfall, and humidity. For example, even with a moderate amount of shading and mulching the amount of irrigation water applied in one garden during the hot, dry season (April-June) (averaged for 2 years) was 67 liters/m2/week (1.7 gal/ft2/week), with rainfall supplying another 3.6 liters/m2/week (0.09 gal/ft2/week), or only about 5% of the total (Figure 10.7). In contrast, during the cold, wet season (January-March), only 7.7 liters/m2/week (0.19 gal/ft2/week) of irrigation water was applied, with rainfall supplying another 8.6 liters/m2/week (0.2 gal/ft2/week), or about 53% of the total.

The objective in watering is to wet the soil where the roots are growing and a short distance beyond (Figure 10.8 and section 5.2.1). Too little water can restrict root growth, increase salt in the root zone, and a larger proportion of water will be lost to surface evaporation. Too much water means a waste of resources and can cause waterlogging (section 12.6.1). When there is a problem with salty soil or water, more water will be needed to wash any accumulated salts below the root zone (section 12.6.2).


Figure 10.7 Seasonal Variation in Water Use in a Desert Garden (After Cleveland and Soleri n.d.a)


Figure 10.8 Wetting the Soil Beyond the Root Zone

A sharp stick can be pushed more easily into wet than dry soil, and so can be used to check depth of watering. Depth of watering can also be determined by digging down through the root zone in several places after watering and checking soil moisture (section 10.7).

Box 10.2
Estimating Water Requirements for Large Garden Projects

For large-scale, monoculture production it is possible to make many sophisticated measurements which are then used to estimate watering frequency and the amount of water needed. For household gardens such measurements are usually unnecessary as well as impractical because of the equipment, experience, time, and expense involved, and because of the much greater complexity of mixed gardens compared with large fields planted with only one crop. However, in dry lands where water is often scarce it may be possible to improve the design or actual water management of larger garden projects and anticipate their approximate water needs. This can be done with an understanding of how garden water requirements can be estimated using simple calculations.

When designing a garden project involving a large number of gardens, or in situations where there is no local information on water consumption, as may be the case in refugee camps or new urban squatter settlements, estimates of the amount and frequency of watering can be useful. Making monthly estimates of water requirements involves knowing the water-holding capacity of the soil, the approximate rooting depth of the garden crops, the water requirement of the crops due to evapotranspiration, and how much water will be lost through drainage below the root zone.12 Variation between seasons, or even within seasons, is often very large due to changes in rainfall, temperature, and type and growth stage of garden crops. After initial estimates have been made, the next step is to adjust them by measuring the amount of water applied, and monitoring its effects in the garden.

The garden should be watered when about 50-75% of the plant available water in the principal root zone has been used up, that is, when the soil is at 25-50% of field capacity.13 We will use 50% as a conservative estimate. If the soil water is allowed to drop below this level, reduction in yield may result, since many plants will not be able to maintain the transpiration rates required for maximum production.

The amount of water needed at the time when the crop would otherwise begin suffering from drought can be calculated using the following formula:

W = (pr)(AW)(d)/Ea,

where

W = amount of water needed to bring the soil in the root zone to field capacity (liters/m2 or mm of depth; see Chapter 17 to convert English measures to metric);

pr = the proportion of plant available soil water which a particular crop can actually use without ETa becoming less than ETm, that is without experiencing water deficit-estimated at 0.5;

AW = plant available water (mm/m), the difference between field capacity and permanent wilting point (section 10.3.1);

d = root depth of the crop (m);

Ea = water application efficiency, the ratio of the water in the root zone to the total amount of water applied (section 12.2).

Rooting depths of crops are usually given for soil conditions which do not limit root growth. Potential rooting depth is not as important as depth of maximum water absorption by roots, which usually occurs at shallower levels. For example, for shallow-rooted garden crops such as onions and spinach whose potential rooting depth is up to 0.6 m (2 ft), depth of maximum water absorption is only ID-15 cm (4-6 in); crops with medium potential rooting depth (up to 1.2 m or 4 ft) such as chilis, beans, and squash may draw most of their water from the area 25-40 cm (10-16 in) below the surface; and for deep-rooted crops such as grapes, olives, almonds, and watermelons with a potential rooting depth of 2.0 m (7 ft), the depth of maximum water absorption may be 1.2-1.4 m (4-6 ft).14 If root growth or water penetration is limited by caliche, ironstone, rock, or other layers in the soil, then rooting depth will be reduced. Root growth also responds to the amount of water applied to the garden. Small applications of water that wet only the top 5-8 cm (2-3 in) of the soil will limit root growth to this depth, and can cause a buildup of salt in the soil.

As an example, let us consider an urban neighborhood group that wants to start a community garden on vacant land. The group must estimate the amount of water needed in order to negotiate with the city government to obtain an adequate supply of piped water. Let us assume that the garden soil is sandy loam with plant available water storage capacity of about 120 mm/m, or a deficit of approximately 60 mm/m when 50% of the plant available water in the root zone has been used up. The garden is laid out in 1001.5 m × 1.5 m garden beds with each participating household cultivating one or more beds and irrigating with a hose. They plan to grow annual vegetables for which a rooting depth of 0.45 m (18 in) is quite adequate. To irrigate the beds to field capacity when 50% of plant available water in the root zone has been depleted, and assuming a water application efficiency of 80% (allowing 10% of the irrigation water to leach salts below the root zone, and another 10% to be lost to evaporation before infiltation), the amount of water needed would be:

W = (pr)(AW)(d)/Ea = (0.5)(120mm/m)(0.45 m)/0.80 = 34 mm of irrigation water.

Since 1 mm of water on 1 m2 = 1 liter, and the area of the beds = 1.5 m × 1.5 m × 100 = 225 m2, the gardeners will apply (34 liters/m2)(225 m2) = 7,650 liters of water.

If the soil were a clay loam with plant available water capacity of 200 mm/m, then they would need:

W = (0.5)(200 mm/m)(0.45)/0.80 =56 mm
of irrigation water,
= (56 liters/m2)(225 m2) = 12,656 liters of water.

However, they would not need to water as often, since there would be more water in the root zone:

200 mm/m -120 mm/m = 80 mm/m, or
(80mm/m)/(120mm/m) = 67% more.

To calculate the total water requirements for the growing season, the gardeners would also need to know how often this amount of water would have to be applied. (Box 10.3 in section 10.7 discusses calculating frequency of irrigation.)

10.6 Measuring water applied to the garden

If the amount of water needed by the garden is to be calculated, some method of measuring the water is required. Even if the water applied to gardens does not need to be calculated in advance, measuring that water may be an important way to estimate water needed when garden programs are expanded to other households, communities or seasons, or for estimating costs where gardeners will have to pay for water.

If water is being applied in containers, such as buckets or calabashes, either by hand or with a shaduf (section 12.7.1) or other mechanism, the volume of the container can be determined by pouring water from it into another container of known volume. Once the volume of the watering container is known, the number of containers of water used in the garden is counted and multiplied by this volume to find the total amount applied.

When water is delivered from a pipe or hose, the time taken for water to fill a container of known volume can be determined by using a watch that shows seconds. The number of seconds required to fill the container is then divided into the total time taken to irrigate. It is best to measure the rate of flow several times and take an average, especially if the water pressure is subject to change. For example, if the valve is opened to maximum flow and it takes 1.75,1.50, and 2.00 minutes to fill a 20-liter (5.3-gal) container, then the average is (1.75 + 1.50 + 2.00)/3 = 1.75 minutes. If it takes 25 minutes to water the garden then the total water used is approximately (25/1.75) × 20 = 286 liters (75.6 gal).

Water applied by a sprinkler can be measured the same way as rainfall (section 11.4.2), preferably by averaging the water collected at several places within the garden.

There are many indigenous methods for measuring irrigation water delivered in a canal. In Yemen, irrigation water taken from a cistern is measured in a number of ways: by the movement of shadows cast by fixed indicators, such as trees, along the cistern’s edge; by a stick or stone submerged in the water which is gradually exposed as the water level drops; or by a “water clock,” a copper bowl with a hole in the bottom which is filled with water - the time it takes to drain out is the unit of measurement used.15 These are all good ideas for reliable, inexpensive measuring devises. For any system of measurement to work it is essential that all those using it understand it and agree to its legitimacy.

Water delivered in a canal can also be measured with devices called weirs which require precision construction, calibration, and measuring.16 Less accurate, but also inexpensive and easier, is the float method, in which the area of cross section of the canal (A in m2) is multiplied by the velocity of water (V in m/sec) to obtain flow (Q in liters/sec):17

Q = (A)(V)

This requires measurement of the cross section of the canal at several places over a relatively straight 20-30 m (65-100 ft) length (L) to obtain an average (Figure 10.9).

If the average width of a 25-m (82-ft) section of a semicircular canal is 1 m, and the average depth is 0.5 m (1.64 ft), then the radius (r) is 0.5 m (1.64 ft), and the cross section (A) is found by using the formula for the area of a circle divided in half:

A = [p(r2)]/2=[(3.14)(0.52)]/2= 0.39m2.

A watch that shows seconds is then used to measure the time (t) it takes for an object to float between a string stretched across either end of the measured section of canal. A bottle half-filled with water or soil will be carried along below the surface, and a stick can be placed in the top to make it easy to follow. This should be done several times to obtain an average. The result is multiplied by 0.8 to adjust for the fact that surface flow is faster than average flow. If the bottle took an average of 40 seconds to move the 25 m (L) then the velocity (V) of water in the canal is:

V = (L/t)(0.8) = (25 m/40 sec)(0.8) = 0.5 m/sec.

Flow (Q) is then

Q = (A)(V) = (0.39 m2)(0.5 m/sec) = 0.2 m3/sec, or 200 liters/sec.

10.7 When to water

For individual gardens, the time to water can be judged by observing both plants and soil.18 Perhaps the most obvious sign is wilting. However, a few plants such as cucurbits can wilt during hot afternoons before they need to be watered. If they recover quickly as the temperature drops in the evening, then they probably do not need watering, but may benefit from shading. Wilting even when the soil is wet is probably caused by disease or pest damage, for example, beetle larvae feeding on roots, or fungi blocking the movement of water in the plant’s vascular system (Chapter 13). Plants should be watered before or when they begin to show signs of water stress, since prolonged wilting can cause a permanent reduction in yields.

Other signs that plants may need watering are leaves that feel very warm during midday, leaves turning darker green, and a lack of new growth. Dieback of growing tips is often a sign of severe water stress.

Table 10.1 Soil Texture and Water Deficita

The amount of water in mm/m is the depth of water (equal to liters/m2) that would have to be applied to bring the soil to field capacity. A permanent wilting point of -15 bars is assumed. See section 9.3.1 for determining soil texture.

Texture

Field capacity (0% deficiency)

Time to water
(50-75% deficiency)

Permanent wilting point
(100% deficiency)

Sand (coarse)

No free water when squeezed, will not form ball, but wet outline left on hand

Appears dry, will not form ball with pressure, but still some clumping (40-65 mm/m)

Dry, loose, single grains flow through fingers (85 mm/m)

Sandy loam (moderately coarse)

No free water when squeezed, makes weak ball, outline left on hand

Appears dry, will not form a ball, sticks together slightly (65-100 mm/m)

Dry, loose, flows through fingers (125 mm/m)

Loam (medium)

No free water when squeezed, can form cylinder, wet outline left on hand

Crumbly but makes weak to good ball when squeezed (85-125 mm/m)

Powdery, dry, small clods easily broken into powder (170 mm/m)

Clay loam (fine)

No free water when squeezed, can form ring, wet outline left on hand

Pliable, forms ball but not cylinder (100-160 mm/m)

Hard, cracked (200 mm/m)

a Based on Doneen and Wescot 1984:9,11; Merriam, et al. 1980:759; and Stegman, et al. 1981:798.


Figure 10.9 Measuring Water Flow in a Canal

Garden soil can also be read for signs that it is time to water (Table 10.1). Most crops should be watered when 50-75% of the plant available water has been used up. The soil in the root zone where most of the roots are growing should be sampled by digging down in several places. The descriptions in Table 10.1 are a guide for making rough estimations of soil water deficit based on handling of soil samples. Experimenting with crops and crop mixes, time of planting, time of day water is applied, mulching, and shading, can increase the length of time between waterings, reducing water use in the garden. In Box 10.3 we discuss a method that can be used to calculate irrigation frequency for large garden projects.

10.8 Mulches, shades, and windbreaks

Reducing evapotranspiration in the garden reduces the gardener’s time and the amount of water that must be invested. Increasing infiltration and storage of water in the root zone, decreasing temperatures, and increasing humidity are the basic methods for minimizing evapotranspiration. This is done by using mulches, windbreaks, shades, and cropping patterns. Irrigation methods that decrease water use are discussed in Chapter 12.

Mulches are materials applied to the garden soil to modify temperature, air movement, water infiltration, and weed growth. There are two basic kinds, surface mulches and vertical mulches. Mulches made of organic materials improve soil structure and fertility as they are broken down by soil microorganisms. Woody mulches with a high C:N may tie up soil nitrogen temporarily (section 9.5.2).

Box 10.3
Calculating Irrigation Frequency for Large Garden Projects

Box 10.2 described how to calculate the amount of water needed for a single watering of a large garden project. While details are beyond the scope of this book, we describe here the basic principles of how to estimate the frequency of watering, in order to obtain a complete estimate of water requirements.

The frequency of irrigation required to maintain maximum yield can be calculated as:19

i = W/ETm,

where

i = irrigation interval in days;

W = amount of water needed to bring soil in the root zone to field capacity when 50% of the plant available water is depleted;

ETm = maximum evapotranspiration per day of the garden crop or crop mixture growing under optimal conditions including adequate water.

ETm can be estimated by observing local gardens, or calculated by

ETm = (kc)(ETo),

where

ETo = theoretical evapotranspiration;

kc = crop coefficient for the garden crop or crop mixture.

ETo is a standard measure of the effect of climate on evapotranspiration, and has different values under different climatic conditions. It is “the rate of evapotranspiration from an extensive surface of 8-15 cm (3-6 in) tall, green grass cover of uniform height, actively growing, completely shading the ground and not short of water.”20 This is calculated by one of several methods using climatic data for the specific location including temperature, humidity, wind, and sunshine. The pan method uses actual measurements of evaporation from a pan of water which are then converted to ETo. Values of ETo and ETm are usually expressed in mm/day and calculated for 10 or 30 day periods. The crop coefficient (kc) varies with the crop, stage of growth, and environmental conditions. Values of kc or graphs from which they can be read are available, to be used with specific calculation methods.21

To calculate irrigation frequency for the example in Box 10.2 we assume that ETm has been estimated at 4 mm/day. For the sandy loam soil that needs 34 mm of water to bring soil in the root zone to field capacity when 50% of plant available water has been used, watering frequency is

i = W/ETm = (34 mm)/(4 mm/day) = 8.5 days, that is, every 8-9 days.

For the clay loam soil with a higher water-holding capacity, and W = 56 watering frequency is

i = (56 mm)/(4 mm/day) = every 14 days.

10.8.1 Surface Mulches

Surface mulches cover the soil surface, shading and cooling it, and so helping to reduce evaporation (Figure 10.10). Mulch protects the soil from the impact of raindrops, which compact the soil surface, thus decreasing infiltration and increasing runoff. A thick surface mulch also discourages growth of unwanted weeds which compete with crops for water (section 8.6). Plant debris such as leaves, straw, and weeds can be used as a mulch, as can sand or stones. Plants like purslane, mat bean, melons, and squash provide a living mulch by spreading out horizontally to cover the soil surface. Since much of the water held in the mulch itself after irrigating will be lost to evaporation, watering under the mulch or combining surface mulch and root zone irrigation - for example, by using a vertical mulch - are good strategies.

In one experiment in semiarid India, a grass mulch on three different annual pulses significantly improved production.22 The crops received 160 and 230 mm (5 and 7 in) of rainfall in each of two 3-month growing seasons. The mulch reduced the soil temperature at a depth of 10 cm (4 in) resulting in increased root length and weight. It increased height and weight of the plant at the active growing stage, and resulted in less weed growth, and an increase in total plant yield, grain yield, and water-use efficiency in comparison with controls which had no mulch. Of the different rates of mulch application tested (3, 6, 9, and 12 T/ha), the most effective application rate was 9 T/ha. On a scale more appropriate for gardens this is equal to 0.9 kg/m2 (0.2 lb/ft2).

Special care should be taken when mulching around newly planted seeds (section 6.5.2). Sometimes mulches are hiding places for garden pests such as cutworms or sow bugs which can quickly destroy young seedlings (section 13.4). If the seedlings are being damaged, mulch should be removed from the planting furrow or area immediately around the seedlings.

Stones and rocks can also be used for surface mulching. In the process of cooling during the night, the moisture that evaporates from the soil condenses on the lower surface of the stones and rocks instead of being lost to the air. Because of the difficulty of planting and cultivating areas mulched with stone or rock, and the fact that they can reflect sunlight and hold and radiate heat, we suggest that these mulches only be used on perennial crops, especially trees. Lower-lying, more tender garden plants can suffer from the heat absorbed and radiated by the rocks. With annual crops gravel and sand can be used as a mulch. In arid northwest China, a 5-16 cm (2-6 in) thick gravel, pebble, and sand mulch has been used on melons and other annual vegetable crops for the past 300 years.23


Figure 10.10 Surface Mulch

10.8.2 Vertical Mulches

Vertical mulches provide water a pathway to the root zone, and can reduce the area of soil surface that is wet. Narrow trenches in garden beds can be filled with stalks like those of millet, maize, sunflower, Jerusalem artichoke, or amaranth. The air spaces created by these stalks conduct the water quickly down to the plant root zone where it is less likely to evaporate than when it is spread on the soil surface and infiltrates slowly (Figure 10.11). The principle is the same as root zone irrigation (section 12.4). In addition, these organic mulches will improve soil water-holding capacity as they decompose.

The beneficial effect of vertical mulch has been demonstrated in a laboratory experiment where 5.5 cm (2 in) of water was applied to a furrow, and the same amount poured into a vertical mulch of barley straw 7 cm (3 in) wide and 15 cm (6 in) deep.24 After 17 days of evaporation, 42% of the water applied in the furrow remained in the soil, while 72% of that applied in the vertical mulch remained. When water was applied rapidly, so that it overflowed the vertical mulch and wet the surrounding soil surface, only 60% of the water remained after 17 days of evaporation. Subsequent field experiments with sorghum showed that a similar mulch increased yields 20-40% over plots that were identical but with no mulch.25


Figure 10.11 Vertical Mulch

Experiments with vertical mulches in gently sloping Vertisols in semiarid India have shown that sorghum stubble packed in trenches 15 cm (6 in) wide improved water infiltration and produced larger yields of grain and straw of an “improved variety” of sorghum, especially in years of drought.26 Vertical mulch in trenches 30 cm (12 in) deep and 2 m (6.5 ft) apart gave 25 times more grain yield and 2 times more straw yield than the areas without vertical mulches in a very dry year. The beneficial effects appeared to last 4 years, probably a result of improved soil structure as the straw partially decomposed after the second year. While sorghum stalks are used to feed cattle in this area, the bottom third of the stalks is not well liked by the cattle. The experimenters therefore felt that using the stalks for vertical mulching would not be a great loss for the cattle, especially if the mulching increased total straw production. However, alternative uses of the bottom third of the stalks, such as for fuel, were not considered.

10.8.3 Windbreaks, Shades, and Cropping Patterns

Wind blows away the protective layer of moist air near the surface of leaves and soil, and direct sunlight raises leaf and soil temperatures. Thus wind and sun can greatly increase rates of evapotranspiration in the garden leading to greater water requirements. This means that more water must be applied to the garden or yields will be reduced. Windbreaks, shades, and cropping patterns that protect the garden from harsh winds and sun can reduce water requirements and increase yields (Figure 10.12). Young seedlings or new transplants often require extra protection from sun and wind, especially in the hot, dry season. (For more on windbreaks see section 9.7.4, and on orientation toward the sun, section 8.3.)


Figure 10.12 Windbreaks Protect Garden Plants from Wind and Sun (1)


Figure 10.12 Windbreaks Protect Garden Plants from Wind and Sun (2)


Figure 10.12 Windbreaks Protect Garden Plants from Wind and Sun (3)


Figure 10.12 Windbreaks Protect Garden Plants from Wind and Sun (4)


Figure 10.12 Windbreaks Protect Garden Plants from Wind and Sun (5)

Mixed cropping in the garden, and dense planting of crops with different aboveground heights, creates shade for the soil and for shorter plants (section 8.5.1). In addition, different rooting depths help to make the best use of water in the soil.27 In cold winter areas, trees such as peach, pomegranate, or fig that lose their leaves to let in the winter sunlight, but leaf out in the summer to provide shade and wind protection for other garden crops, are especially good (Figure 8.8 in section 8.3).

Branches, palm fronds, stalks, and other plant parts can be pushed into the soil or tied to frames of branches or bamboo. On the Mediterranean island of Pantelleria, Sicily, Italy, gardeners remove large pads from a prickly pear cactus and prop these up to shade their tomato plants.28 Mats or cloth can also be tied to frames, and leaves, bark, straw, or old baskets can be used to shade small plants. All of these materials should allow some wind to pass through, and be anchored securely in the ground, or they will be blown over.

Houses, walls, trellises, and fences, especially tall, living fences can give protection from the wind and sun (section 8.8).

10.9 Resources

Technical details of crop, soil, and water relationships in production are presented in Doorenbos and Kassam (1979) and calculations of crop water requirements are shown in Doorenbos and Pruitt (1977). Ahn (1970:127-147) gives an excellent, easy-to-read description of soil and water relations with reference to West Africa.

References

1 See also Dupriez and De Leener 1983:55-58.

2 See Ahn 1970:127-147.

3 Donahue, et al. 1983:166-170.

4 Delgado 1984:57-58.

5 Donahue, et al. 1983:185-191; Doneen and Wescot 1984:21-27; Gardner 1979.

6 See Ahn 1970:133,137-139; Doneen and Wescot 1984: 27-28; Henderson 1979.

7 E.g., Dupriez and De Leener 1983:107.

8 Troeh, et al. 1980:475.

9 Doorenbos and Pruitt 1977:61-63; Dupriez and De Leener 1983:134-140.

10 Flinn and Garrity 1986.

11 Cleveland and Soleri n.d.a; Cleveland, et al. 1985.

12 Stern 1979:69-74; Doorenbos and Pruitt 1977:68-82.

13 Halderman 1977.

14 Doneen and Wescot 1984:33-35.

15 Varisco 1983.

16 Merriam, et al. 1980:749-758; Stern 1979:141-147.

17 Stern 1979:141.

18 Halderman 1977.

19 See Doorenbos and Kassam 1979:26-27.

20 Doorenbos and Pruitt 1977:1.

21 E.g., Doorenbos and Pruitt 1977:38-41.

22 Gupta and Gupta 1983.

23 Ming and Yun-wei 1986.

24 Fairbourn and Gardner 1972.

25 Fairbourn and Gardner 1974.

26 Rama Mohan Rao, et al. 1977.

27 See Dupriez and De Leener 1983:115-123.

28 Gait and Gait 1978.

11. Sources of water for the garden

Water for gardens can come from many different places: rainfall, rivers, streams, wells, and even water left from washing, bathing, or spilled at the pump (Figure 11.1). The cost of water, the amount available, its quality, and whether gardeners can depend on a steady supply, are often determined by the source of water.

11.1 Summary

The most important sources of water in drylands are:

· Rain, either falling directly on the garden or harvested from adjacent areas.

· Surface water from rivers, streams, or lakes, which may first be stored in reservoirs behind small dams, and can be transported to the garden through irrigation canals or by natural or managed flooding.

· Groundwater which may be obtained through shallow, hand-dug wells, or deeper wells and bore holes in the case of a community water supply.

· Piped water, originally from various sources, which is especially important in cities.

· Recycled water from household washing and food preparation, or from water spilled at a pump or stand pipe.

For later use during dry periods, water can be stored in the soil, behind berms or dams, or in cisterns, pots, and jars.


Figure 11.1 Using Water Spilled at the Pump

Box 11.1
Measuring Water Salinity

The level of water salinity is usually expressed in terms of electrical conductivity of the water (ECw,). Conductivity is measured by special instruments in units of millimhos/centimeter (mmhos/cm), or the more recently adopted standard of Siemens/meter (S/m) or deciSiemens/meter (dS/m), where

1 S/m = 10 dS/m = 1,000 mmhos/m = 10 mmhos/cm

The better water conducts electricity the saltier it is, and the more production will be reduced. Water with a conductivity of 0.7 dS/m or less is good for all crops, 0.7 to 3.0 dS/m is harmful to sensitive crops, and greater than 3.0 dS/m is harmful to most crops.3

Salt content can also be expressed in mg of dissolved solids/liter of water. Salinity can be determined in these terms if there is access to a scale that is accurate to 100 mg (0.1 gm), perhaps in a laboratory at a secondary school, university, or agricultural experiment station. A liter of water is completely evaporated in a pot protected from dust. This may be done slowly by leaving the pot to stand in the sun, or more quickly by boiling it over a heat source. The remaining solids are then weighed. The total dissolved solids (mg/liter) indicate saltiness: 0 to 500, good for all crops; 500 to 2,000, harmful to sensitive crops; and over 2,000, harmful to most crops.4

Over time the amount of salts leaving the root zone through drainage should equal the amount brought in through watering and rainfall. To prevent buildup of salts it is very important that there be enough extra water applied to the garden at regular intervals to wash out the salt, and adequate drainage to carry that water away from the root zone.

If natural drainage does not exist, then drainage should be provided (section 12.6.1). The saltier the irrigation water, the larger the amount of water that must drain below the root zone. The amount of water needed to leach or wash down salts carried in by irrigation water is expressed as the leaching requirement (LR), and can be calculated if the salinity of the water is known (section 12.6.2).

Ion toxicity problems often accompany salinity problems. Ions are absorbed by plants along with water, they accumulate in the leaves as a result of transpiration, and can poison plants. Chloride is the most common cause of ion toxicity. It shows up as drying (“burning”) of leaves beginning at the tips and moving along the edges. Leaves may drop prematurely. Leaf burn due to sodium toxicity begins at the edge of older leaves and moves between the veins toward the center.

When crops continue to have problems after soil salinity has been reduced, then it may be worth having a water sample checked in a laboratory. If tests show that ions such as sodium, chloride, boron, and selenium are present in harmful levels in water used in the garden,5 an alternative source of water should be found.

11.2 Water quality for plants

All water, even rainwater, contains some impurities. Impurities may include salts, toxic ions, manufactured chemicals such as pesticides, disease-causing microorganisms, and small animals such as nematodes.1

Salty or saline water is a common problem in drylands (Box 11.1). Surface and groundwater contain naturally occurring salts which can build up in the soil to levels that reduce garden yields. Salty water is more difficult for plants to absorb, and causes symptoms similar to those produced by water stress; leaf edges often turn brown, white or yellow.2 Salty soils make the problem worse). However, sometimes salt-tolerant crops can be grown successfully (section 5.6).

11.3 Water quality for people

If water is used only for the garden and not for human or animal consumption, the danger of disease is reduced, but not eliminated. In Zimbabwe, for example, wells located in gardens are used for irrigating and clothes washing but not for drinking water.6 Wells for drinking are located outside the gardens and close to houses. There are a number of diseases that can be spread through contact of the skin with water, or by biting insects that breed in water. Making a great effort to improve household nutrition and income through increasing garden production is not worthwhile if these benefits are undone by an increase in infectious disease.

While methods to improve water quality may seem relatively simple to the outsider, there are many political, economic, social, and cultural reasons why people may find it difficult to change established patterns of behavior that affect their health. This is true even when they understand the negative effects of these behaviors on their health. This difficulty is as common in industrialized countries where the level of formal education is very high, as it is in Third World countries where the level of formal education is very low. For example, in the industrialized countries of North America and Europe the role of tobacco and alcohol in causing sickness, accidents, and death are well known. Yet well-educated people in those countries spend billions of dollars on these products every year, encouraged by manufacturers’ advertisements. Tobacco producers in the United States even receive government subsidies.

In Box 11.2 we briefly review prevention of some of the most important infectious diseases that can be spread by contact with water in the garden. It may be very difficult to prevent children from drinking water meant only for irrigating, especially during the dry season when sources are limited, but every effort should be made to do so. Discussion of the diseases spread by drinking contaminated water can be found in community health-care books.7

Box 11.2
Human Disease and Water Quality 8

Hookworm disease is caused by several species of microscopic hookworms which hatch in damp soil and enter the human body through bare feet. They then attach to the walls of the intestine and can cause loss of blood leading to weakness and severe anemia. Hookworm eggs leave the body in feces.

Strongyloidiasis is another disease spread through human feces. The worm enters moist soil and feeds there before penetrating the skin of another person. Strongyloides and hookworms are nematodes, a group of small roundworms, some members of which also infect garden crops (section 13.3.2). Wearing shoes and defecating well away from the garden and other places where people walk help to prevent the spread of these disease-causing nematodes.

Leptospirosis is a disease with a variety of symptoms lasting up to 3 weeks. Fatality is low but increases with age. Domestic animals like goats spread the disease-causing organism in their urine. From water or moist soil it enters the skin, especially the mucous membranes (moist areas such as the nose, mouth, or eyes), or where there is a cut or scrape. People should avoid walking barefoot where animals urinate or defecate.

Schistosomiasis (also known as bilharzia) is one of the most prevalent diseases spread by the use of irrigation water, especially slowly moving water in canals, lakes, and rivers. The disease is caused by tiny blood flukes, worms whose eggs are present in the urine and feces of infected people. These worms live part of their lives in a certain kind of small, freshwater snail, then leave the snail’s body and enter directly through the skin of a person drinking, washing with, or standing in the water. Inside the body the worms cause bleeding of the bladder or intestine, leading to loss of blood in urine or feces. The best method of control is sanitation systems that eliminate urination and defecation in or near canals, ponds, lakes, or other sources of garden irrigation water. Building access areas so that gardeners do not have to stand in the water to irrigate, and removing from canals and other water sources the vegetation snails need for food are also important.

Mosquitoes can breed in even tiny amounts of water, such as in the ends of bamboo poles or pieces of a broken pot. Malaria is one of the most devastating diseases of the drylands and is spread by mosquitoes. Malaria organisms live in the blood and other organs where they can cause serious illness and death. A large number of other diseases are also spread by mosquitoes, including yellow fever and other virus diseases, as well as filariasis (elephantiasis), caused by a nematode. During the rainy season when mosquitoes are breeding, the incidence of these diseases increases dramatically. Tin cans, calabashes, pots, or other containers used for watering the garden should be emptied of water after use and stored upside-down, to prevent collection of rainwater. Water storage containers should be covered to prevent mosquitoes from laying eggs. Areas around wells, shadufs (section 12.7.1), and other water-lifting devices should be drained to prevent puddles from forming. One way to do this is to direct this water into a garden bed as shown in Figure 11.1.

11.4 Rain

Seasonal patterns as well as amounts of rainfall vary greatly in the drylands, and help to determine what kinds of gardens are grown. If rain is adequate, gardens can be grown in the rainy season with little or no additional water. This is especially true when the rainy season is also the cool season, as in North Africa, the Middle East, and the west coast of South America. Rainfall is also the source of water for streams, rivers, and lakes and for many underground aquifers which can be tapped with wells. The need to understand rainfall patterns is greatest when gardeners depend on rain falling directly on the garden, or harvested from nearby catchments.

Gardeners and farmers in drylands have methods of deciding exactly when to plant during the beginning of the rainy season. This is very important, since variability of rainfall is greatest at this time (and at the end of the rainy season). This is also a time when germinating seeds and young plants are very sensitive to drought, and planting materials and irrigation water are limited. Often there are rains before the true beginning of the rainy season. Planting after such rains could result in seeds or cuttings failing to grow if additional rain does not follow soon enough. On the other hand, if planting is too late, the garden crops’ maximum need for water will occur when the peak rainfall has passed.

To determine the start of the rainy season so that they can plan the planting of crops, Nigerian farmers use many different signs: astronomical (stars, moon), social (elders, festivals), weather (clouds, heat, rainfall, humidity), and other signs in their environment (bird and insect behavior, vegetation changes).9 In Nigeria’s western savannas for example, the Yoruba use the following as indicators that it is time to begin planting: leafing out of two trees, iroko (Chlorophora excelsa) and baobab, sky signs such as changes in color and cloud formations, the cessation of singing of a certain bird that sings only in the dry season, and shifts in wind direction. While these farmers, like most farmers everywhere, may not understand the physical causes of rainfall patterns, they are good observers of these patterns and their relationship to rainfall and other environmental events.

11.4.1 Rainfall Records

Field workers can easily measure and record rainfall10 and doing so increases their understanding of the relationship between water and garden management.

For planning large-scale projects, especially those involving water harvesting, records for the project site or for the closest weather station should be used, if possible. Even records for 1 year are better than nothing at all. Several publications describe regional climates and provide monthly and yearly weather data for representative weather stations. One is the World Survey of Climatology series.”

Rainfall records for a number of years can provide the average, maximum, and minimum rainfall, and number of rainy days over the period of record keeping, by the year, month, or week. They may also show the average, earliest, and latest dates for the beginning and end of the rains, or for the last and first freezing nights. For designing water-harvesting plots (section 11.5) it is also useful to know the amount of maximum daily rainfall and intensity of rainfall measured in mm/hr.

Decisions such as when to plant and what crop mixtures to grow often depend on amount and timing of rainfall. Such decisions are based on estimates of the probability of receiving a certain amount of rain during a given period. Probability means the chance that a certain event will occur based on the information available (Box 4.2). The greater the probability of adequate rainfall, the less the risk to the farmer or gardener. The traditional methods of judging the beginning of the rainy season described above are one way of calculating probabilities. Box 11.3 gives an example of another way to calculate rainfall probabilities.

When gardens depend on rainfall, there will always be a certain probability of failure due to lack of rains. The amount of failure acceptable to the gardeners will depend on the risks they take in terms of the labor, water, and other resources they invest, and on how important the produce from the garden is. Western agricultural scientists often use an 80% success rate as a standard on which to base management practices.12 This means that, in terms of water, gardening systems would be designed so that, on average, they would produce a successful harvest on rainfall alone 8 out of every 10 years.

11.4.2 Measuring Rainfall

Simple plastic or glass rain gauges can be purchased from scientific, agricultural, or garden supply houses. One of good quality may cost around US $10.00 or more. For drylands the tapered type is far better than the flat-bottomed kind, because it makes reading small amounts of rainfall much easier (Figure 11.2).

Box 11.3
Calculating Rainfall Probabilities from Records

Probabilites can be calculated from rainfall records. For example, in the desert areas of southwestern North America, the beginning of the summer rains in July is an important time for planting summer crops including corn, beans, and squash. We can calculate the probability that the coming year (or any year) will have enough rainfall for July planting. Given data on the amount of rainfall in the month of July for the past 30 years, the amounts of rainfall from most to least are first listed (Table 11.1). Using a simple formula, the probability of any amount can be estimated, as shown in the table. For example, if a successful harvest from a July planting depends on at least 45 mm of July rainfall, then it will be successful in only 68%, or less than 7 out of every 10 years on the average. This means that, on the average, during 32% of the years, planting will have to be done again in August, or supplemental irrigation will have to be provided. It does not mean that in any given 10-year period there will be 7 years of successful harvests. Probability is a calculation based on long-term, average observations; it is a useful guideline but should never be taken as an absolute predictor of growing conditions and resulting harvests.

This method assumes that there are no trends through time toward more or less rainfall. The greater number of years the calculation is based on the greater the reliability. In calculating rainfall probabilities for water-harvesting design, it has been suggested that a minimum of 10 years of records be used, eliminating the 2 wettest years if they are extreme.13

Table 11.1 Calculating Rainfall Probabilitiesa

An example of July rainfall for Tucson, Arizona, USA, 1957-1986

m

Year

Rainfall (mm)

m

Year

Rainfall (mm)

m

Year

Rainfall (mm)

1

1981

156.7

11

1970

64.3

21

1980

45.2

2

1958

132.1

12

1975

60.5

22

1973

44.2

3

1964

122.4

13

1971

55.4

23

1963

42.2

4

1974

112.8

14

1982

54.1

24

1969

38.4

5

1959

99.6

15

1965

54.1

25

1962

35.1

6

1972

88.6

16

1979

51.8

26

1957

31.8

7

1985

79.8

17

1983

50.3

27

1976

30.0

8

1984

74.2

18

1968

50.0

28

1978

19.8

9

1967

69.1

19

1986

46.2

29

1977

19.3

10

1966

65.3

20

1961

46.0

30

1960

18.5

p = m/(n + 1)
p = estimated probability of equal or greater rainfall
m = position in the series, ranked from highest to lowest rainfall
n = number of years of measurements = 30

Example #1: To find the probability of a July with over 45 mm of rainfall, find 45.2 mm in the table above and note that its position is m=21, then

p = 21/(30 + 1) = 0.68 = 68%

This means that there is a 68% chance (about 2 out of 3) of having a July with more than 45 mm of rain and a 32% chance of a July with less than 45 mm of rain.

Example #2: To find the amount of rainfall that can be expected with a 60% probability (P0.60)

m = p(n + 1)
m = 0.06(30 + 1) = 18.6

Rounding down to 18 to be conservative we look for m=18 in the table and find that for m=18 rainfall was 50 mm. Therefore, 60% of the years the rainfall in July will be 50 mm or more.

a Data from NOAA (1987), procedure based on WMO (1983).


Figure 11.2 Measuring Rainfall with Rain Gauges

Rainfall can also be quite easily and accurately measured without a rain gauge. All that is needed is a large collecting container, such as a metal or plastic bucket, and a small calibrated container, such as a measuring cup used for cooking. Rainwater collected in the large container is poured into the smaller container and the volume is measured. The volume in cm3 is then divided by the area of the collection container opening in cm2. This could also be done using in3 for volume and in2 for area, although it may be more difficult with small quantities of water.

For example, a bucket with an opening that has a diameter of 26 cm (10 in) (Figure 11.3) is set out to collect rainwater. The collected water is then poured into a measuring cup and comes up to the 6.5 oz (191 ml) mark.

A (area of bucket opening) = p r2 = (3.14) (13cm)2 = 530.7 cm2

Vol (volume of water) = (6.5 oz) (29.6 cm3 per ounce) = 192.4 cm3

P (Precipitation) = Vol/A = 192.4 cm3/530.7 cm2 = 0.36 cm = 3.6 mm of rainfall.


Figure 11.3 Measuring Rainfall with a Bucket and Measuring Cup

11.5 Harvesting rainwater for dryland gardens

Gardening with rainwater runoff can be a simple way to increase the effectiveness of natural rainfall without the expense or trouble of importing water. Water harvesting is often combined with measures to reduce soil erosion, such as terraces and contour bunds (section 9.7.1). It involves harvesting rainwater from one area (catchment) and applying it to the garden (a plot or a single tree or other plant), to add to the amount of water available from direct rainfall alone. Where there is abundant, reliable, and inexpensive irrigation water, rainwater harvesting may not be worthwhile. (Harvesting floodwater from streams, rivers, and lakes is covered in section 11.6).

Rainwater harvesting is not limited to the Third World. It is increasingly popular among industrial world farmers, for example in the western United States where the cost of groundwater for irrigation has risen due to falling groundwater tables and the growing cost of energy to pump it.

In this section we discuss local systems of water harvesting, some basic concepts of runoff, and how to measure runoff and estimate the ratio of catchment to growing area in designing runoff gardens. We emphasize microcatchments, which are much more efficient at harvesting rainwater than larger catchments.14 There is no need to transport the harvested water to growing areas, and because the water flows fairly slowly, only simple structures are needed to control it. Few if any new engineering and construction skills are needed for design and construction of small microcatchments. However, even the smallest system, if improperly designed, may be destroyed during the first rainstorm.

It may well be a better investment in all but the largest projects to encourage local gardeners to experiment rather than hiring “experts” to come up with sophisticated calculations for design.15 Even for large projects, it will be necessary to check calculations against experience in the garden. The simplified outline of runoff garden design given here along with local experience and knowledge, can provide a starting point for experimentation without the need for an expensive design process or detailed rainfall data. We emphasize that even this design process is not necessary to employ water harvesting for gardens. Experimentation beginning with small microcatchments and building on the knowledge gained may be the best choice in most situations.

11.5.1 Patterns of Water Harvesting

Rainwater harvesting gives gardeners more options in terms of the seasonal timing of gardening:

· During dry periods in the rainy season, in unusually dry years, or in areas with inadequate average seasonal rainfall, the rain that does fall can be harvested to supplement that falling on the garden.

· Gardens started at the end of the rainy season or the beginning of the dry season may be successfully grown on rainwater harvested and stored in the soil.

· Gardens grown mostly in the dry season can be irrigated with water harvested in the rainy season and stored in a container or the soil.

While many people in drylands probably practice some form of water harvesting, not much data has been gathered on existing traditions.16

In areas such as North Africa and the Middle East the rainy season occurs in the cool winter when evapotranspiration rates are low. Food can be produced there during the rainy season on much less rain than in most of the drylands of sub-Saharan Africa, India, and much of Mexico where the rainy season occurs in the summer, accompanied by high evapotranspiration rates. Small-scale water harvesting in areas of the Negev Desert, which receive only 100 mm (4 in) of rainfall during the cooler winter season, has been successful with some crops, especially drought-resistant fruit trees.17 Olive trees have been grown in southern Tunisia using this method for centuries.18 However, in summer rainfall areas, especially in the tropics, more total water is needed to compensate for losses to high evapotranspiration. Water harvesting here may only be appropriate where there is at least 300-600 mm (12-24 in) of rainfall.19

Even without any water control structures such as berms or terraces, cultivation methods are often adjusted to take advantage of rainwater which collects naturally in certain areas within a garden or field. An example is the planting depressions made by Mossi farmers in Burkina Faso described in section 9.3.2.

In Jaisalmer District in the desert area of Rajasthan, India, the technique of khadins was first employed over 500 years ago.20 Sheet and gully runoff from low hills and ridges is collected behind berms in the hot, rainy season. Much of this water infiltrates into the soil, and any remaining water is drained off for planting in November, and production during the cooler dry season. Crop roots follow the dropping water table, which by the end of the dry season is 2 m (6.5 ft) below ground level. Such large-scale water harvesting systems are widespread in India.21 Although normally planted primarily with major cash or staple crops, gardens are sometimes a part of these systems, as distinct plots in the field, or as a few vegetable crops interspersed among the main crops.

Many experiments with runoff farming have also been carried out in the Third World. In India, micro-catchments for growing jujubes were evaluated over a 5-year experimental period in which mean annual rainfall was 558 mm (22 in).22 It was found that a catchment area with a 10% slope and only 87% as big as the growing area, produced yields of jujube fruit 2.3 times that in the control with no microcatchments. Another example comes from Kenya where contour ridges 3 m (10 ft) apart and semicircular berms were compared with the traditional deep digging receiving no runoff.23 It was found that during a rainy season with 539 mm (21 in) of rainfall, the plots with ridges yielded 2.3 times as much sorghum grain, and 7.8 times as many cowpeas as the control, while those with semicircular berms yielded 3.4 and 6.5 times more, respectively, than the control.

An experiment in the drylands of the western United States measured the difference between planting sorghum on a flat soil surface and planting sorghum in 50 cm (20-in) wide furrows with 75-cm (30-in), steep (33% slope) catchments on both sides (CGAR = 3:1, see section 11.5.4).24 In a 14-week growing season with 180 mm (7 in) of rainfall, the rainwater catchment plots yielded more than twice as much sorghum.

11.5.2 Building on Local Knowledge

As with all aspects of garden design, the most important foundation for equitable improvements is what the local people already know and practice.25

An example has been recorded in northern Kenya where the Turkana traditionally grow sorghum in floodwater gardens26 (section 11.6.2). The Israeli model of microcatchments for growing trees was the inspiration for outsiders to design a project to build contour ridges 10 m (330 ft) apart for annual crops in the area. However, the rainfall was far too erratic, both in timing and location, and none of the crops produced a harvest. This is an example of inappropriate technology transfer without consideration of environmental conditions or of the Turkana’s own knowledge based on their use of seasonal stream and river flow to grow gardens (section 11.6.2). As a result, after several hundred Turkana had been paid with food to clear and contour 200 ha (494 a), the project was converted to an irrigation scheme using more reliable river water. However, working on their own, some observant Turkana were fairly successful in using water harvesting to extend a garden into a natural depression, similar to those they used traditionally.

Local knowledge and experience can provide the foundation for the evolution of a successful rainwater harvesting project. In Yatenga, northern Burkina Faso, project workers introduced the idea of microcatchments for growing trees for erosion control.27 However, based on local knowledge and needs, the project was gradually changed to focus on rainwater harvesting for food production. The revised project included construction of stone contour berms modeled on the stone berms traditionally used in the area. These allowed the water to slowly infiltrate into the soil; any excess passed through the stone berms. Contours were defined with a tube level (Box 9.11 in section 9.7.1 and Figure 11.8 in section 11.5.4) which the farmers were trained to use, and no hydrological calculations were made, for example, to determine spacing of berms. Rather, the farmers started with one or more simple, small berms, and expanded them or added spillways based on their experience from season to season.

11.5.3 Catchments and Runoff

Estimating the amount of rainwater that can be harvested from a catchment is the key factor in designing and using rainwater runoff. Runoff can be calculated by placing values from field measurements or published tables into equations. This approach may sometimes be useful in project planning. For most garden applications it is more practical to actually measure the runoff in sample garden plots. No matter which method is used, experiments by gardeners will always be necessary to check the results.

Water can be successfully harvested from roofs of thatch, tile, packed soil, or metal and directed into garden plots. This water can also be stored in containers such as large clay pots next to the house for later irrigations when there is no rain. Roadways or pathways sloping toward the garden can also be modified to harvest rainwater. Special care must be taken to ensure that catchments are not polluted with poisons that could be carried into the garden. Small areas can be shaped into catchment systems, called microcatchments, up to about 0.1 ha (1,000 m2, or 10,763 ft2) (Figure 11.4). Microcatchments direct rainfall runoff into adjacent growing areas. Microcatchments and growing areas can take many shapes, but for simplicity and efficiency they should be next to each other. In general, the best collection and growing areas have quite different characteristics (Table 11.2).

The amount of rainfall that runs off the catchment depends not only on the amount of rainfall itself, but on the characteristics of the catchment area. Some of the rainfall is intercepted by vegetation and evaporates before it reaches the ground. Some of the rain that does reach the ground infiltrates into the soil. Infiltration is increased by anything that slows the water flow - such as vegetation, organic matter, rocks, or gravel - or that increases porosity - such as cracks caused by shrinking of soils containing montmorillonite clay, coarse (sandy) texture, and looseness (lack of compaction), or holes made by animals or plant roots. Also, depressions in the soil surface capture water in puddles allowing time for the water to infiltrate.

Soil texture has a strong influence on the amount of runoff.28 Sandy soils should be avoided for water harvesting catchments because they are so permeable. Exceptions are some sandy clay loams and sandy clays, which are excellent. Soils high in silt and fine sand tend to crust easily and so make good catchment surfaces. Loam soils are not too good but can work. The clays can be excellent or very poor depending on clay type and soil structure. Clays high in montmorillonite tend to crack, creating a broken-up surface with high permeability, therefore, soils containing much montmorillonite, such as Vertisols, should be avoided. Oxisols with sesquioxide clays also tend to be highly permeable. In contrast, clay soils high in kaolinite crack very little, are very impermeable, and make good catchment areas.

The threshold rainfall is the amount that falls before water begins running off the catchment. The more rain that is trapped by vegetation and in puddles, or infiltrates into the soil, the higher the threshold rainfall will be. Runoff percent (R%), also called runoff ratio, is the percentage of rain falling on a given catchment that runs off the catchment:

R% = R/P × 100

where

R = runoff
P = precipitation.


Figure 11.4 Microcatchments

Table 11.2 Comparison of Desirable Characteristics for Catchment versus Growing Areas for Rainwater Harvesting

Characteristic

Catchment area

Growing area

Runoff

maximum

minimum

Permeability

minimum

maximum

Slope

3 to 7%

0%

Vegetation

none

dense

Soil surface

smooth, no openings, tendency to crust

rough, many openings, no crusting

Mulch

none

yes

Organic matter

none

yes

Salts

some help crusting

none

A large range of R% values can be expected even from soils of similar texture. Although estimates can be made using published values as a guide, it is better to make actual measurements of the proportion of rainfall that runs off.

Field measurements of R% can be made quite simply. If the catchment area is bordered by low dikes to prevent water from running onto or off of the plot, a container (such as a cleaned 208-liter [55-gal] steel drum) can be sunk in the low corner to collect the runoff. The amount of rainfall and runoff should be measured every day on which there is rainfall. Even a few weeks of such measurement during a rainy season will give a far better estimate than any calculations can.29 If long-term rainfall records are available, they can be used to extrapolate and gain an idea of how big a catchment must be to reliably supply the water needed by a garden.

Runoff percent is calculated by dividing the total amount of rain falling on the catchment area (P) into the volume collected in the container (R) (Figure 11.5). For example, if it rains 12 mm on a 10 m2 catchment area, this is 120 liters of water; if 45 liters are collected from the runoff plot, then the runoff percent is

R% = (R/P) × 100 = (45/120) × 100 = 38%.

Runoff percent from small, smooth, impermeable surfaces like plastic sheeting or sheet metal roofs is almost 100%. In contrast, runoff from larger, irregular, permeable, vegetated soil surfaces may be very low. Runoff from soil surfaces can be increased by a number of treatments such as asphalt or wax, but these are expensive and can be harmful to the environment.

Treatment with common salt (NaCl) or other sodium compounds disperses the soil particles to seal the surface, and has been used successfully in Arizona, USA, and other areas.30 However, the limited increase in R%, the cost of salt, the high salt content in runoff water during the first year, and the inability to use the catchment area for cultivation in the future limits the usefulness of this technique, especially for Third World gardens.


Figure 11.5 Calculating Runoff Percent

Microcatchments for harvesting rainwater for gardens are best treated only by clearing vegetation and large stones, smoothing, and perhaps compacting. A number of studies have shown that on catchments 10 m (33 ft) long or less, with slopes 7.5% or less and on sandy loam to clay loam soils, R% is about 25-30% with only clearing and smoothing. The surface of the soil can be smoothed when wet with a flat rock or shovel. When the soil is also compacted, R% can increase up to 70%.31 Treatment of these microcatchments with sodium compounds increased R% only a small amount to 30-75%.

Most of the characteristics of the catchment area that increase runoff also increase soil erosion, which is undesirable. Therefore, a balance must be reached between maximizing runoff and minimizing erosion. On collection areas longer than 10 m (33 ft), slopes greater than 7% may lead to severe erosion. (Measuring a slope is discussed in Box 11.4.) On very short collection areas, slopes may be up to 10% without causing serious erosion. A 3-7% slope is good for most micro-catchments. However, if the soil surface has been smoothed, any increase in catchment slope above 3% will do little to decrease the threshold rainfall and will increase erosion instead.32 While it is generally best to keep the runoff areas weed free, weeds such as amaranth or purslane, or drought-resistant perennials like prickly pear cactus, may sometimes provide enough food to make them tolerable, and may help to prevent erosion.

11.5.4 Estimating the Catchment to Garden Area Ratio

The catchment to garden area ratio (CGAR) is the ratio of the area of the rainwater catchment to the garden area where the water is used. If the ratio is too small then there will not be enough water for the garden, and if it is too big, there may be so much water that part of the catchment or the garden itself is washed away. The highest R% will be from the shortest slopes. This is because there is less distance for the water to flow over and infiltrate into. Also, the slope can be steeper than on longer catchments without causing soil erosion. This is illustrated by a 5-year experiment in India growing jujube trees on loamy sand, with average annual rainfall of 558 mm (22 in).33 A 5.1-m (17-ft) long catchment with a 10% slope stored 472 mm (19 in) of rainwater in a 3-m (10-ft) soil profile in the growing area, compared with 313 mm (12 in) for a 14.5-m (58-ft) catchment with a 0.5% slope, and 380 mm (15 in) for a 10.7-m (35-ft) catchment with a 5% slope.

We can use the following equation to estimate the CGAR:

CGAR = [(ETm/Ea) - P]/R, where

ETm = maximum evapotranspiration for the garden crops (section 10.3.4),
Ea = efficiency of water application (section 12.2),
P = precipitation (rainfall) (section 11.4), and
R = runoff (section 11.5.3).

ETm can most practically be estimated by talking to local gardeners and observing the amounts of water needed to grow gardens in the area.

Box 11.4
Measuring a Slope

Slope is usually measured as percent of change in vertical distance for a given horizontal distance (Figure 11.6). Thus a drop of 1 m (3.3 ft) over a horizontal distance of 3 m (10 ft) is a 33% slope (1/3 × 100). A drop of 0.5 m (1.5 ft) over a horizontal distance of 10 m (30 ft) is a 5% (0.5/10 × 100) slope. When the horizontal distance is an even number, calculations are easier. If possible the slope should be measured over the entire distance under consideration for a garden or catchment area, to eliminate the effect of irregularities.

If a line level or carpenter’s level and a long piece of string are available it is quite easy to measure the slope (Figure 11.7).34 It is important that the string be stretched tightly and that the level be placed exactly in the center of the string. When the bubble is centered between the two lines, the vertical distance or drop is then measured and divided by the horizontal distance.

The tube level described in Box 9.11 can also be used to measure slope (Figure 11.8). The vertical distance is the difference between the water levels.

When the harvested water is applied directly to the growing area, as with most microcatchments, then Ea can be assumed to equal 1.0 and will drop out of the equation. When the harvested water is led through canals, used in sprinklers, or other irrigation systems, or when the garden plot being irrigated is large, then an efficiency rate will have to be entered (section 12.2).

It is more realistic, although more complicated, to use estimates of ETm, P, and R for each week of the garden growing season so that CGAR can be calculated for the week where (ETm - P) is greatest, that is, when the difference between water needed and water available is greatest. In areas where P is high early in the growing season then it may be possible to store some of the runoff in a reservoir for use during periods when (ETm - P) is greatest, thus reducing the CGAR.

Box 11.5 gives an example of microcatchment design using the CGAR.


Figure 11.6 Slope


Figure 11.7 Using a Line Level to Measure Slope


Figure 11.8 Using a Tube Level to Measure Slope

11.6 Harvesting stream flow and floodwater

Rainwater harvesting captures water flowing in sheets or rivulets, but water flowing in streams, rivers, or lakes can also be captured for use in the garden (section 12.3).

Floodwater gardening makes use of seasonal water flows in streams and rivers to produce crops. Compared with other sources of water, floodwater is not as easy for the gardener to control, yet it often requires much less investment of time and resources. In addition, alluvial deposits of clay, silt, and organic matter, made by flowing water as it slows down, create soils that are deeper, more fertile, and have higher water-holding capacity than adjacent upland soils. Many people in drylands use some form of floodwater gardening, and understanding and building on local knowledge is important in the improvement of existing systems or the design of new ones.

At least two different methods are used for floodwater gardens: water spreading at the base of seasonal streams, and flood recession cultivation on seasonally flooded river and lake terraces.

Box 11.5
Designing a Microcatchment Garden Plot

In this example, traditional varieties of beans, corn, squash, and greens are to be grown in the summer rainy season in the Sonoran Desert of southwestern North America.

· Growing area: Sunken beds 1 m2 will be built on a site with a slope of 5%.

· Rainfall: It is decided that failure 2 years out of 10 is acceptable. Since there is some production even when available water is less than optimum (ETm), a rainfall probability of 60% is chosen (P0.6). g). Monthly rainfall data are available for the last 30 years. The corn, beans, and squash will be planted in early July and will flower and set fruit in late August. Much less water will be needed during ripening in September, which has a lower rainfall. Therefore, CGAR will be calculated based on July rainfall, which is very similar to that of August (see example #2 in Table 11.1). P0.6 for July is 50 mm.

· Runoff: Runoff percent (R%) is first calculated from trial catchments of the compacted and smoothed clayey soil and found to be 50%.

Runoff is then calculated as R = (R%)(P0.6) = (0.50)(50 mm) = 25 mm.

· Catchment to garden area ratio: Mixed gardens in the area growing a variety of different vegetables use 113.6 mm/wk or (52 weeks/12 months = 4.33 wk/month, and so 113.6 mm/wk × 4.33=) 492 mm/month during this period. CGAR = (ETm - P)/R = (492 - 50)/25 = 17.

· Catchment area: Area × CGAR = 1 m2 × 17= 17m2.

· Depth of planting basin: Since the depth of the root zone (d) is limited by a caliche layer at 0.8 m which allows only very slow drainage, and available water in the soil is 60 mm/m, storage in the root zone is: (AW)(d) = (60 mm/m)(0.8m) = 48 mm. To avoid waterlogging during intense rainstorms, we limit the depth of the planting basin to 60 mm with excess flowing around the up-slope end of the sides of the basin. The berm on all but this overflow area is raised another 60 mm to 120 mm to create a freeboard, a margin of safety protecting it from erosion due to rapidly moving overflow.

11.6.1 Water Spreading

Water spreading is a technique of diverting seasonal stream flow to flood low-lying adjacent areas for irrigating crops. It can be practiced with a fairly elaborate system of water control using dikes, berms, terraces, and even canals and storage tanks, or with a very simple system requiring little construction and using the natural pattern of water spreading and storage in the soil.

Water spreading was commonly used by natives of arid southwestern North America including the Tohono O’Odham, although only a few practice it today.35 Stream channels (known by the Spanish term arroyos) are dry most of the year, but will flood after an intense summer rainstorm. At the mouth of the arroyo where the slope changes abruptly, the waters naturally slow and spread, depositing their load of sediment and organic matter in an alluvial fan. The soil here is deeper and more fertile than in surrounding areas. To increase production the Tohono O’Odham build dikes of soil, rocks, or brush to further slow and spread the water. A number of domestic garden crops such as melon, watermelon, and squash are inter-planted with grain and dry beans. Many wild species, some used as leaf vegetables, are also encouraged.

Floodwater can also be diverted artificially from streams. In the Oaxacan valley of southern Mexico, for example, for extra water to supplement direct rainfall, many fields depend entirely on floodwater which flows along dirt roads and is diverted by 20-30 cm (8-12 in) high berms.36 Where the level of the road is up to 1 m (3.3 ft) below that of the field, small rock dams are built to raise the water to the level of the field.

Forms of water spreading are common from the northern savannas of West Africa eastward to Sudan and Somalia.37 This technique maybe especially appropriate for pastoralists in arid and semiarid regions who rely primarily on animals or trade, and for whom any big investment in irrigation may not be economical.

11.6.2 Flood Recession Gardening

Flood recession gardening is growing fruits and vegetables in areas that are flooded in most years. In northwestern Nigeria 100-500 m2 (1,076-5,380 ft2) dry-season gardens are cultivated in the flood plain of the Sokoto valley by 80% of the farmers.38 A variety of annual vegetables are grown primarily on moisture left in the soil by the flooding river, although hand-dug wells provide supplemental water.

Traditionally the Tonga of Zambia and Zimbabwe in central Africa depended primarily on sorghum and millet cultivated in regularly fallowed, rain-fed fields, and secondarily on maize, cucurbits, groundnuts, sweet potatoes, and tobacco (Nicotiana spp) cultivated in both wet and dry seasons on Zambezi River terraces.39 The potential ETm of 2,296 mm (90 in) is 3.5 times higher than the yearly rainfall of 651 mm (26 in), which falls in a single rainy season. The Tonga use soil moisture from flooding in three distinct locations as they are exposed by dropping floodwaters: kalonga, small pockets of moist tributary stream-beds; kuti, on the first river terrace; and jelele, on the riverbank itself.

In northern Kenya where the annual rainfall is less than 200 mm (8 in), Turkana pastoralists practice a combination of water spreading and floodwater cultivation.40 They grow gardens of about 1,000 m2 (10,763 ft2), consisting primarily of quick-maturing (65 days) sorghum. When the rains fail, there is no harvest. We consider these to be gardens because sorghum is a minor crop, yielding only about 128 kg per 1,000 m2 plot (282 lb/10,763 ft2), but providing a valued supplement to the main source of food from herding goats.

The Turkana plant at the beginning of the rainy season at three different sites:

· Where minor tributaries enter the Kerio, a major seasonal river, and most of the water soaks quickly into the alluvial soil.

· In depressions or former meanders of the river where there are rich soil deposits occasionally flooded by the river, and groundwater is sometimes tapped by hand-dug wells.

· In depressions subject to flooding in the delta of the Kerio River where it enters Lake Turkana, and where the water table is higher because of the lake.

No berms or any other water-control devices are used. The gardens are surrounded by thorn brush fences to protect them from livestock and the area is divided into individual women’s plots. Platforms from which young girls scare birds are also built in the gardens. Soil fertility is provided by silt brought in by the floodwaters, and by the cattle and goats that graze the plots during the rest of the year.

11.7 Groundwater and wells

Groundwater may be the only source of water for dry-season gardens in areas where streams or rivers dry up when the rains stop.

11.7.1 Groundwater

Rainwater infiltrates down into the soil until it meets an impermeable layer in the soil profile, and then saturates the layer above it (Figure 11.9). This underground water is known as groundwater. If water can flow in this saturated layer, it is known as an aquifer and its upper surface as the water table. Aquifers are usually made of sand, gravel, or porous rock. The water may flow through the aquifer until it emerges as a spring, or until it flows into rivers, lakes, or oceans.

Wells are holes dug down into the aquifer to obtain groundwater. When there is no impermeable layer between the aquifer and the surface it is called an open aquifer, which is the most common type of aquifer tapped by hand-dug wells. When the aquifer is overlain by an impermeable layer and the water is under pressure that forces it up to the surface through any openings, it is an artesian aquifer. Wells that tap artesian aquifers are artesian wells. Water flows up and out of artesian wells naturally because water enters the aquifer at a higher level creating pressure that forces the water up the well.

The water table of an open aquifer, and thus the supply of water in a well, will vary depending on rainfall and amount of rain that percolates down to recharge the aquifer. The amount of percolation can be increased by slowing runoff so that the water has enough time to infiltrate into the soil. This can be done by using terraces, contour berms, garden basins (sunken beds), or dams built across seasonal streams. Some of the same terms used to describe soil (section 9.3.2) are also used when referring to aquifers. Porosity measures the amount of water an aquifer can hold, and permeability refers to the ease with which water flows through the aquifer.

11.7.2 Locating a Well

The best method for deciding where to locate a well is to survey existing wells in the area and to talk with people who have built wells. In many drylands there are experts in indigenous methods of digging wells; they will have many valuable ideas about well location using observations of the environment. For example, vegetation that stays green into the dry season may be tapping a groundwater aquifer with its roots. Geologists, hydrologists, botanists, and others from universities, government departments, and development projects may also have information about local ground-water conditions.


Figure 11.9 Wells to Bring Groundwater to the Garden

11.7.3 Hand-Dug Wells

Hand-dug wells are also known as large-diameter wells because they are 1 m (3.3 ft) or more in diameter to allow workers room to excavate them. Many people have traditionally dug wells by hand to reach ground-water. Very-large-diameter wells that are enlarged below the surface, can also serve for collecting and storing surface runoff during the rainy season, as well as for groundwater during the dry season. These are called cistern wells and water from them should not be used for drinking. In northern Ghana for example, there are many of these ancient cistern wells consisting of a 0.4- to 2.0-m (1.3- to 6.6-ft) diameter hole through a cap of plinthite (Box 9.4) up to 2.0 m (6.6 ft) thick, with a cavern 0.6-1.5 m (2-5 ft) high and 6 m (20 ft) or more across hollowed out of the clay and shale below.41 The market town of Salaga in the north of that country is known as the city of 1,000 wells, many of which have linings of plinthite blocks and wooden lids with locks.

In sandy soils hand-dug wells over 1 m (3.3 ft) may have to be lined to prevent the sides from collapsing, while in clayey soils lining may not be required until after 5 m (16 ft). It is best to ask for advice from local people with experience. The lining can be of brick or stone, but reinforced concrete is much stronger and is preferred if the cement and the steel reinforcing rods are available and affordable. A field-tested method for constructing one type of hand-dug well has been described.42 Since the cost of lined wells increases in direct proportion to the diameter, it is best to keep the diameter to a maximum of 1.3 m (4 ft), which is enough space for two workers. These wells consist of three units (Figure 11.10):

· The shaft to the water table, which is constructed by digging down in sections of up to 5 m (16 ft), depending on the soil type. In sandy soils the sections should be much shorter for safety reasons. After each section is dug a reinforced concrete liner is poured.

· Precast reinforced concrete caissoning rings within the aquifer, which allow water to pass through and into the well.

· A well head, also of reinforced concrete, which protects the top of the shaft from crumbling and from water washing in soil, debris, and microorganisms which spread human or plant diseases. It also helps to protect people and animals from falling in.

Water tables are usually higher in the area of streams, rivers, and lakes, even in the dry season. People often dig shallow wells in these locations to water dry-season gardens, as do the Turkana in their floodwater gardens (section 11.6.2) or the people of Dar Masalit in Sudan (section 12.3.2). In northern Ghana, wells dug in dry streambeds to water gardens are severely damaged in the rainy season when flowing water collapses and fills them in; they have to be redug annually.43 One drawback of streambed wells is that the clayey alluvial soil in which they are often located is low in permeability, so water moves slowly through them and therefore the wells take a long time to fill. This can be remedied to some extent by increasing the depth, and in the case of very shallow wells, the diameter. In permanent wells the investment of making lateral extensions at the bottom of the well to increase inflow may be worthwhile.


Figure 11.10 Cross Section of a Lined, Hand-Dug Well (After Watt and Wood 3979:27)

Qanats (a Persian word) consist of a slightly inclined tunnel bringing water from an aquifer to the surface with no need of water-lifting. They are excavated by a series of vertical wells joining the tunnel sections. Qanats have been used in Iran and other areas of the Middle East and North Africa for over 2,000 years, and are still being used in some places. They are called foqjara in northwest Africa and aflaj on the Arabian Peninsula.

11.7.4 Small-Diameter Wells

Small-diameter wells are constructed using a variety of methods and without the need for underground workers.44 Thus they are much safer to construct than hand-dug wells. In addition, they have potentially greater discharge because it is easier to extend the well deep into the aquifer. They are also easier to keep clean. The major disadvantages of small-diameter wells are that specialized equipment is needed for their construction, and a pump or special small-diameter buckets are needed to raise the water. Also, because they are narrow they cannot store as much water as larger-diameter hand-dug wells, which limits their usefulness in low-permeability aquifers where water flow is very slow.

11.8 Water storage

The best place to store water is in the root zone of garden plants. There the water is readily available to the growing plants when they need it, there is no water surface exposed to evaporation, and no special storage containers are necessary. Water from intermittent streams may be stored in the alluvium deposited behind small dams.45 Such dams are very popular in Mexico where they have been used for thousands of years.46

However, water storage in the soil is not always possible because sometimes too little water will be available from rainfall, the soils will not be able to hold enough water, or drainage will be insufficient, leading to too much water in the root zone (sections 10.3.1 and 10.3.2).

There are a variety of different containers that can be used for water storage. Large, locally made ceramic vessels are often easily available. Other popular containers are the steel drums used for packaging substances such as fuel, pesticides, soap, and foods. Only those used for soap and foods are acceptable for water storage. Determining the size of the container needed depends upon the amount of water available to store and the most practical and affordable container.47 For example, for storing rainwater runoff from small roofs, start with the maximum runoff available:

(horizontal area of roof used for collecting [Figure 11.11]) × (runoff percentage) × (monthly rainfall).

The runoff available is then divided by the water requirement per m2 (or per ft2) of garden for the needed growing season to see what size garden could be maintained going into the dry season.

Storing water from the peak of the rainy season could allow extending the growing season, on a small plot, or even growing a second crop in a dry-season garden. However, storing water for gardens should be carefully considered because for many households the cost of constructing or purchasing a container will be too great to justify its use for watering a garden.


Figure 11.11 The Horizontal Area of a Roof

11.9 Resources

As mentioned in this Chapter, local experts such as well diggers, and gardeners and farmers practicing water harvesting, are the best resources for information and ideas about local water resources. For a general review of water harvesting and floodwater farming see Pacey and Cullis (1986) and UNEP (1983). Watt and Wood (1979) give detailed instructions for hand-dug wells. Chleq and Dupriez (1984: Chapters 7, 8, 9) and Koegel (1977) give a brief review of both hand-dug and small-diameter wells. Watt (1978) describes construction of steel-reinforced concrete storage tanks. Nissen-Petersen (1982) has technical information on building various size water containers, mostly larger than household size. For a discussion for field workers of the treatment of human diseases carried by water we recommend Where There is No Doctor (Werner 1977). Chapter 12 in that book describes the prevention, signs, and treatment for some of those diseases.

References

1 Ayers and Wescot 1985; CFA 1980:23ff.; Cox and Atkins 1979:300-304; Stern 1979.

2 Ayers and Wescot 1985:15-21.

3 Ayers and Wescot 1985:8.

4 Ayers and Wescot 1985:8; Stern 1979:75.

5 Ayers and Wescot 1985:77-83.

6 Bell, et al. 1987:39-40.

7 See for example, Werner 1977: Chapter 13.

8 Based on Beneson 1985; and Werner 1977.

9 Oguntoyinbo and Richards 1978:181-187.

10 Dupriez and De Leener 1983:125-133.

11 Bryson and Hare 1974 for North America; Griffiths 1972 for Africa; Schwerdtfeger 1976 for Central and South America; Takahashi and Arakawa 1981 for Southern and Western Asia.

12 Dancette and Hall 1979:110.

13 Fraiser and Myers 1983:7.

14 Shanan and Tadmor 1979:6.

15 Pacey and Cullis 1986:170-172.

16 Dupriez and De Leener 1983:100-108; Pacey and Cullis 1986:127.

17 Evenari, et al. 1982.

18 El Amami 1979.

19 Pacey and Cullis 1986:130.

20 Kolarkar, Murthy, and Singh 1983.

21 Pacey and Cullis 1986:135-141.

22 Sharma, et al. 1982.

23 Smith and Critchley 1985.

24 Fairbourn and Gardner 1974.

25 Pacey and Cullis 1986:154-155.

26 Van Doorne 1985:15-16,47,54.

27 Pacey and Cullis 1986:165-173; Thomson 1980.

28 Evett 1985b.

29 Evett 1985b.

30 Dutt 1981.

31 Evett 1983:97.

32 Evett 1983.

33 Sharma, et al. 1982.

34 Leonard 1980:21-22.

35 Nabhan 1979.

36 Kirkby 1973:40.

37 Pacey and Cullis 1986:154.

38 Adams 1986.

39 Scudder 1962,1982.

40 Morgan 1974.

41 Cleveland 1980:76.

42 Watt and Wood 1979; see also Chleq and Dupriez 1984: Chapters 7, 8,9.

43 Cleveland 1980:76.

44 Koegel 1977.

45 Chleq and Dupriez 1984:114-119.

46 UNEP 1983:23-24,127-133.

47 Pacey and Cullis 1986:59-62.

12. Irrigation and water-lifting

In drylands, rain falling directly on the garden is often inadequate, and additional sources of water must be found. Irrigation is the conveyance of this water from its source and its application within the garden. Regardless of the source of water, the principles of irrigation are the same.

Small-scale, locally controlled systems of water management are often environmentally sustainable. One reason for this is that they can respond and adapt to changes in local needs and conditions. Their replacement by large-scale, centrally controlled systems has frequently increased production, but in many cases it has also led to increasing inequity and loss of water and soil resources.1 This has happened in ancient systems as well as in modern ones. Falling water tables, soil erosion, waterlogging, salinization, ground-water pollution, and increasing poverty are common problems with large-scale water control systems. While small-scale systems are not trouble free, these problems are more common and harder to prevent in large-scale systems. As a result, there is growing interest in small-scale water management and irrigation, including household gardens.2 For example, in Zimbabwe, indigenous irrigated gardens in valleys existed before the Europeans arrived, and are today being recognized as an important national and household resource, after years of being ignored by irrigation development projects and discouraged by governments.3

In areas of irrigated agriculture, gardens often depend on water from the same irrigation system that delivers water to the fields. Gardens here may be small plots in the fields or near the house, or narrow strips along canals or roads. These gardens have been neglected and undervalued, but they can help improve household income and nutrition even though significant improvement in equity and environmental sustainability will require major changes in the values and structure of the irrigation system.4

There are many examples of indigenous irrigation systems that function successfully without imported or capital-intensive technology. Some of these systems and the principles on which they are based are discussed in this chapter. Irrigation systems for large garden projects, especially those involving heavy construction and imported designs and equipment, require consulting with experienced local people or professional engineers.5

12.1 Summary

Water can be applied to the garden by surface, subsurface or overhead irrigation. Surface irrigation is the application of water using basin, furrow, or trickle systems. Water can also be applied directly to the root zone using pots, vertical mulch, or sand and gravel columns. Overhead or “sprinkler” irrigation in gardens is usually done by hand. Salinity and waterlogging are often the result of irrigation in drylands, but can frequently be avoided in small-scale systems with proper design and management. If the water supply is at a lower level than the garden, water can be lifted by hand, or by using the power of animals, water, wind, solar energy, or fossil fuels.

12.2 Irrigation efficiency

Irrigation efficiency is a measure of how much water is actually placed in the root zone of growing crops, compared with the amount of water that is extracted from a well, stream, or other sources and put into the irrigation system. Understanding irrigation efficiency is important for saving water, a scarce resource in drylands; but first we need to discuss extraction, conveyance, and application.6 The quantity of water taken from a particular source such as a well is the amount extracted. What happens to the water during conveyance, that is, from the time of extraction until it reaches the garden, will affect the amount of water the garden receives. If all of the water extracted reaches the garden then the conveyance efficiency, that is, the amount of water applied to the garden (Id), as a percentage of the amount of water extracted, is 100%. However, losses of water by evaporation, leaking through holes and cracks in hoses and canals, and percolation into the soil of earthen canals and ditches, mean that conveyance efficiencies are usually less than 100%, often much less.

Application efficiency (Ea) is the water applied to the garden (Id) divided into that portion needed to bring the soil in the root zone to field capacity (W): (Ea = W/Id × 100). (Multiplying by 100 gives the efficiency as a percentage). Application efficiencies are less than 100% because of evaporation from the surface, runoff of excess water out of the garden, and deep percolation below the root zone.

Irrigation efficiency equals application efficiency multiplied by conveyance efficiency. An irrigation efficiency of 50% means that 50% of the water extracted from the source infiltrated to the root zone and stayed there. It also means that twice the amount of water needed to bring soil in the root zone of the garden to field capacity will have to be extracted, since 50% of it is lost in conveyance or application in the garden.

An irrigation efficiency of 100% means that all of the water extracted from the source reaches and stays in the root zone, and is equal to the amount needed to bring the soil in the root zone to field capacity (W)7. While no irrigation system can achieve 100% irrigation efficiency, good management can keep efficiency relatively high, that is over 50%. In addition to simply wasting water, low irrigation efficiency reduces production because it leaches nutrients below the root zone, and over the long run in some areas it can raise the water table into the root zone, causing waterlogging and an increase in salt content of the soil.

Shading and mulching the soil surface and adjusting the timing of irrigation reduce evaporation. Gardeners also try to increase efficiency by ensuring equal time for water infiltration in all areas of the garden being irrigated, with no water lost in runoff, or deep percolation below the root zone, except as required to leach out salt (section 12.6.2). The smaller the garden and the closer the water supply, the easier it is to achieve irrigation efficiencies well above 50%.

In large garden projects there are several steps that can be taken to make irrigation efficiency as high as possible:

a) Estimate the amount of water needed (W) to bring garden soil to field capacity (Box 10.2, section 10.5).

b) Apply water until all soil in the root zone is at field capacity; determine this by checking soil water in the root zone at various sites within each bed, furrow, or other irrigation unit of the garden (section 10.7).

c) Measure the water actually delivered (Id) to the garden (section 10.6).

d) Determine application efficiency (Ea) by calculating W/Id × 100, and adjust the amount of water and method of application accordingly.

12.3 Surface irrigation

Basin, furrow, and trickle irrigation are the methods of surface irrigation most often used in gardens. In some cases, water may first have to be transported to the garden from some distance.

12.3.1 Transporting Water to the Garden

Water can be delivered to the garden directly from a rainwater catchment area, floodwater diversion, well, stream, or standpipe, but is sometimes carried in a bucket, pot, hose, or canal. Delivery systems do not have to be elaborate or costly; they can be built from inexpensive, readily available local materials. An additional advantage of such systems is that they are easy to adjust and repair. Large-scale irrigation systems are usually accompanied by an elaborate social organization for regulating distribution to all those with rights to cultivate.

Indigenous irrigation systems using perennial and nearly perennial stream flow for food production are fairly common, for example, in hilly areas of east Africa. In Tanzania the Sonjo use water from streams and springs to irrigate heavy bottomland in the dry season, and supplement rainy-season production on sandy upland fields.8 Flumes (elevated canals) made of hollowed logs are used to bridge low spots. Beans and two to three kinds of cowpeas are interspersed with the main crops of sweet potatoes, millet, and sorghum in these plots.

Another example is found among the Taita living in the hills of Kenya.9 Water from the Mwatate River is obtained by intakes built of sticks, rocks, and soil which are located in small pools along the steep gorge through which the river flows. The intakes divert water from the river into canals that carry it to the fields. Water from the canals is also used for drinking and washing, livestock, construction, and a school brick-making project. The heaviest rains wash out the intakes, but this protects the canals and fields below from too much water flow which could wash them out also. The earthen canals are partly dug into the upper slope and partly built up from the lower slope. In some places flumes made of hollowed banana stems are used to bridge short gaps, and in other places canals are dug into the hillside to avoid tree roots or rocks.

Traditionally these canals have been used to minimize risk from variable rainfall at the end of both the short and long rainy seasons, and if rains are good in a particular season, the canal system may not be repaired and used. With gravity delivery and plentiful supply, the benefits of such a low-cost, low-risk system may indeed outweigh the advantages of maximizing conveyance efficiency. In other cases high pumping costs may justify creating delivery systems with greater conveyance efficiency.

The Taita grow their staple crops of maize and beans in most of the irrigated fields, but minor crops such as bananas and sugarcane are also grown. Farmers with fields close to the main road can produce vegetables for market, some even year-round, and many fields have been completely given over to commercial vegetable production. The system is maintained and the water distribution managed quite successfully by user groups based on kinship. This is in contrast to user group organizations created by some new irrigation schemes, without regard for existing social institutions.

12.3.2 Basin Irrigation

We discussed building raised and sunken beds in section 9.8. Sunken beds are also referred to as basin beds because the berms form a basin that holds water. Basins can also be formed on raised beds or on level ground by simply hoeing up soil into ridges to enclose areas of various sizes. Terrace basins on a slope should be smaller than those on flat ground to improve application efficiency, to minimize the amount of soil moved, and to avoid rocky or infertile horizons or layers (section 9.4). Basins can be filled by hose, with water from an adjacent irrigation ditch, or by pouring water from a container (Figure 12.1).

An example of basin irrigation using canals comes from Dar Masalit in east central Sudan, where people cultivate both rainy-season and dry-season gardens in the clay soils of dry streambeds, called wadis in Arabic.10 Groups of people usually cooperate to dig a well and fence in an area against animals, then divide up the land inside the fence. Each gardener makes about 10-50 basins 0.5-1.0 m2 (1.6-3.3 ft2) and a network of small canals to water each basin in succession. Every 3-4 days they spend the morning drawing water from the well and pouring it into canals. For young people this is mostly a pleasant way to pass time with friends, but in large gardens the work can be tiresome.

In basin irrigation the goal is to have level basins and to fill them as quickly as possible. They should be filled with enough water so that soil in all parts of the basin is wetted throughout the root zone with minimal loss of water below the root zone, except for deep irrigations to flush out salts (section 12.6.2). Efficiency is decreased by low spots that get too much water. It is also decreased when there are different types of soil within the bed, so that some areas, for example, of sandy soil, reach field capacity sooner than other areas, for example, of clayey soil. In this case much water could be lost to deep percolation in the sandy soil by the time the clayey soil reaches field capacity. In general, because of deep percolation, sandy soils have lower application efficiencies than do clayey soils.

Given the goal of efficient application, the size of the basin will be determined by a combination of crop water needs, soil texture and structure, and water quality and speed of delivery. The depth of the basin, that is, the distance between the top of the berm and the soil surface within the basin, depends on the amount of water needed, which in turn depends on soil texture, rooting depth, and the crops grown (sections 10.5 and 5.2.1). The deeper the rooting depth of the crop, the deeper the basin needs to be.

The faster the rate of infiltration, the smaller the surface area of the basin should be so that water is not lost to deep percolation in one area of the basin before other areas are wet. Porous soils with sandy texture or strong structure have higher rates of infiltration than soils with clayey texture and weak structure. Porous soils hold less water, and the depth and surface area of the basin should be smaller than in clayey soils. This is because water will sink into the soil quickly so that the basin does not need to hold as much on its surface; water will not spread evenly over a large surface area from a single point of application. Infiltration is slowed when large quantities of suspended matter in the irrigation water clog the pores in the surface layer of the soil.


Figure 12.1 Basin Irrigation of Zuni Sunken Garden Beds (After a photograph in Ladd 1979:497)

The faster water can be applied, the larger the basin; and the slower the application, the smaller the basin. If the basin is too big, or the water is applied too slowly, too much water will have infiltrated at the point of application by the time all soil is at field capacity. In this case the basins will have to be watered at several different places, or made smaller.

12.3.3 Furrow Irrigation

In furrow irrigation water is delivered to the garden through a network of furrows (Figure 12.2). For example, among the Taita living in the hills of Kenya, a farmer cuts away the side of the canal at the upper end of a field and puts a rock in the canal to divert the water; a network of earthen furrows distributes the water within the field.11 In small gardens water can be guided carefully to each plant by tiny earthen dams and furrows.

Furrow irrigation can be used where water is available at the high point of the garden, and the garden has a fairly uniform gentle slope. If the slope is greater than about 2%, then the furrows should not run straight up and down the slope, but at an angle to it, in order to maintain the furrows at a 2% slope.12 This allows the water to flow down the furrow, but prevents the water from flowing too quickly and eroding the furrows.

The goal of furrow irrigation is an even flow along the length of the furrow so that by the time water has reached the low end of the furrow, an adequate amount of water will have infiltrated the soil at the high end13 (Figure 12.3a). This will depend primarily on the rate of application, length of furrow, degree of slope, and soil texture. The length of furrow is often the easiest variable to adjust. Gardeners can experiment. Efficiency is decreased when furrows are too long, or water is applied too slowly, so that by the time the low end is adequately wetted, the high end has excess water (Figure 12.3b). If the water is applied too fast, there will be excess water at the low end of the furrow by the time the middle part is at field capacity (Figure 12.3c).

12.3.4 Trickle Irrigation

Also known as drip or localized irrigation, trickle irrigation is increasingly popular in industrialized countries because of its high irrigation efficiency. It is especially appropriate for sandy soils with low water-holding capacity. In the most commonly promoted system, water is delivered to each plant or group of several plants through emitters (small holes or valves) on a small-diameter, flexible plastic line. Water must be filtered to prevent clogging of the lines and emitters, and pressure must be regulated to ensure even distribution. For most limited-resource households in the Third World, this type of trickle irrigation is inappropriate because of the expense of purchasing the system and replacement parts, and the difficulty of maintaining it.14


Figure 12.2 Furrow Irrigation

An alternative system of trickle irrigation using plastic infusion or drip sets discarded by hospitals has been experimented with in desert areas of western Rajasthan, India, for growing cauliflower.15 The area has loamy sand soil, average annual precipitation of 360 mm (14 in) and potential ETm of 2,063 mm (81 in). Application of 315 mm (12.5 in) of water supplied 100% of ET requirements for cauliflower during the 88-day growing season with yields of 2.3 kg/m2 (0.5 lb/ft2). Results of experiments showed yields comparable with those using conventional trickle irrigation under identical circumstances, yet this system cost nothing, and any clogging was easily remedied by squeezing the suspended water bag or removing the emitter tube from the ground. (CAUTION: If discarded infusion sets are tried in the garden, all needles and syringes should first be removed and destroyed and medical workers should certify that there is no danger of spreading disease.)


Figure 12.3 The Effect of Water Application Rate on Efficiency of Furrow Irrigation

12.4 Root zone irrigation

Root zone irrigation is the delivery of water below the surface of the soil, directly to the root zone. In drylands it has the advantage of minimizing evaporation of water from wet soil surfaces and upper soil layers. One technique is vertical mulching discussed in section 10.8.2. Trenches or holes in the garden bed, which are filled with sand and topped with gravel to slow evaporation, also lead water to the root zone. If trickle irrigation emitters are buried, they too can be used to create a root zone irrigation system.

12.4.1 Pitcher Irrigation

Pitcher irrigation is watering by filling a buried, un-glazed ceramic pitcher or pot through which water slowly seeps into the root zone. Covering the top of the pitcher reduces evaporation. An extra advantage of the pitcher method is that clay pitchers are readily available because they are commonly used for storing and cooling water in many drylands. However, they still cost money and this may be prohibitive for poor households.

A problem that may occur is salt accumulation in the soil between pots, since not enough water is applied to flush the salt below the root zone. In addition, clean water must be used because any clay, silt, or organic material in the water can clog the pores of the pitcher and prevent water from seeping out. Leaving water to stand in a container for a while allows some of these particles to settle out before pouring the water into the pots. Salty water will also clog the pores.

Some informal experiments with pitcher irrigation in India used growing holes 90 cm (24 in) in diameter and 60 cm (35 in) deep filled with manured soil16 (Figure 12.4). Unglazed ceramic pitchers were placed in the center of the holes and filled with water 2-3 days before seeds were planted. Germinated seeds were planted 2-3 cm (0.8-1.2 in) from the outside of the pitcher in soil that had been wetted by adding a small amount of water directly. About 3 liters (0.8 gal) of water was added to the pitcher daily. Using one pitcher per 12.5 m2 (135 ft2) several species of cucurbits were grown with yields ranging between 0.16 kg/m2 (0.5 oz/ft2) on 19.8 mm (0.8 in) of water for pumpkin to 2.56 kg/m2 (8.4 oz/ft2) on 18.6 mm (0.7 in) of water for watermelon. Bottle gourd used only 12.3 mm (0.5 in) of water, but produced only 0.53 kg/m2 (1.7 oz/ft2).

12.4.2 Water Table Irrigation

Water table irrigation is the method used in flood recession gardening. Plant roots follow dropping water tables as the floodwaters recede during the dry season, as in Tonga cultivation along the Zambezi River in Africa, or they obtain water from a high water table during the rainy season, as in Turkana cultivation on the Kerio River delta at Lake Turkana (section 11.6.2). Gardens located below earthen dams benefit from the raised water table due to the seepage of water. In Sri Lanka, for example, this allows the cultivation of fruit trees such as plantain, which normally only grow in more humid areas.17 The Hopi Native Americans of North America plant peach and apricot trees and melons on sandy hillsides where the heavier subsoil is kept moist by subsurface flow of water from nearby seeps and springs.18

12.5 Sprinkler irrigation

Sprinkler irrigation is similar to rainfall. The easiest method is to sprinkle water from a small, hand-held container. This is appropriate for small seedlings or transplants or a very small garden (section 6.5.1). Watering cans can be used, but are not necessary, and are often expensive. Larger systems with a hose and a nozzle or a mechanical sprinkler can also be used, but require a delivery network of pipes or hoses and water under pressure produced by a pump or an elevated tank or reservoir. Sprinkler irrigation is not efficient in areas with high winds which distort the watering pattern and increase evaporation. Overall, sprinkling systems are expensive and require constant maintenance and the availability of spare parts. In many situations where piped water is available it is good drinking water, and it may be more important to reserve it for domestic use than to use it to irrigate a garden.

A major advantage of sprinkler irrigation is that irregularly sloping areas can be watered with no need for leveling. In spite of the fact that more water may be lost to evaporation from the air and plant leaves, in some situations sprinkler irrigation can have a greater efficiency than surface irrigation. In sandy soils, especially, it allows more even distribution than furrow or basin irrigation. In clayey soils with slow infiltration rates, the rate of water application for sprinkler irrigation may have to be very slow to avoid surface runoff and soil erosion.


Figure 12.4 Pitcher Irrigation (After Mondal 1974)

12.6 Irrigation problems

Waterlogging and salinity are two of the major problems that occur with irrigation, and are common in drylands everywhere.

12.6.1 Waterlogging

Waterlogging occurs when the water table rises into the root zone of garden crops. Since the roots of almost all garden crops require oxygen, waterlogging causes reduced yields and eventually death because the water forces the air out of the soil. Waterlogging also causes nitrogen deficiency shown as a yellowing of the leaves, because anaerobic bacteria which convert nitrates to ammonia multiply under these conditions. Soil becomes waterlogged when there is lack of drainage because of a naturally occuring high water table or a soil layer or horizon that is much more impermeable than the layer above it (Figure 12.5). Because more water than is needed by crops must be applied to flush out salts, this water can eventually raise the water table when there is poor drainage. Low irrigation efficiency due to leaky canals, low spots in the garden, and excessive deep percolation make the problem worse.

To provide for drainage during especially heavy rains, cuts may have to be made in berms around basins to let out excess water, or drainage ditches may have to be dug at the end of furrows. Raised beds can help keep plant roots out of the waterlogged soil but if the water table has risen substantially the garden site may have to be abandoned.

12.6.2 Salinity

Salty soils occur naturally in arid areas where not enough rain falls to wash soluble salts down and out of the root zone. Irrigation makes the situation worse, since surface water and groundwater contain more salt than rainwater. Salt tends to build up in the soil as water is continually added through irrigation. As water is used by plants and evaporates from the soil surface, the salt in that water concentrates in the soil. The high temperatures and low humidity in drylands mean that salinization often accompanies irrigation.

It is important to water to the bottom of the root zone and slightly beyond, and to reduce excess evapotranspiration so that irrigations can be spaced as far apart as possible to minimize wetting of the upper layers of the soil from where most water is lost to evaporation.

Salts are redeposited at the point where the water movement stops or where the water evaporates. White salt deposits can be seen on high spots in the garden after rain or irrigation water has evaporated. Planting at low points in the furrow or in basins rather than at the highest point of the raised bed, mound, or ridge helps plant roots avoid those salt deposits (Figure 12.6). In areas where there is poor drainage and a high water table, water with its dissolved salts may be constantly drawn up into the root zone. Waterlogging makes this problem worse. Raising the root zone by building up mounds or beds for planting may be the only alternative to try before abandoning the site.


Figure 12.5 Waterlogging Due to an Impermeable Layer in the Soil

If soils are salty then extra care must be taken not to add salty kitchen waste to the compost. Plants growing in salty soils, like the salt bush (Atriplex spp.) in the deserts of southwestern North America, accumulate salt in their leaves, and should not be used for compost.


Figure 12.6 Planting to Avoid Areas of Salt Buildup

With large amounts of sodium (Na*), the pH (section 9.5.1) can reach 10 or higher and only a few salt-tolerant crops are able to grow. Obviously these are not good soils for gardens, and where they occur, the best solution may be to bring in better soil from elsewhere, plant in containers, or find another location.

We visited a squatter settlement located in a dried-up lake bed outside of Mexico City. Faced with the extremely poor, salty soil, many women had brought in soil and manure from other areas and were growing trees, herbs, vegetables, and flowers in boxes, used tires, cans, shampoo bottles, and other containers they had found on the streets (Figure 8.2 in section 8.2.2).

Where high water tables are not a problem, leaching can be used periodically to rid the root zone of salt. Leaching is washing salt from the root zone by adding excess irrigation water, and is commonly used to prevent salt buildup. The amount of water needed will depend on the salt sensitivity of the garden crops grown and the amount of reduction in yield that can be tolerated. For many commercially grown crops these values can be obtained from published tables.19 The leaching requirement may be calculated as follows:20

LR = ECw/ECc

where

LR = leaching requirement, or fraction of applied water necessary to leach salts, beyond amount to meet ETm,

ECw = salinity of water applied to the garden (section 11.2 and Box 11.1),

ECc = salinity of the soil which allows an acceptable yield for the garden crops (section 5.6).

Calculating LR is a complex problem subject to great error as a result of slight differences in estimates used in the calculation.21 It is also important to note that there are several different methods for calculating LR in use, based on different assumptions.22 For example, the one recommended in a recent FAO publication23 differs markedly from that given above for all but a narrow range of values. Whichever calculation is used, there is no substitution for starting small, experimenting, and understanding the local situation.

12.7 Water-lifting

Water for irrigation from wells, cisterns, canals, rivers, streams, and lakes needs to be lifted whenever it occurs at a level lower than the garden.24 The choice of a water-lifting method is determined by the gardener’s needs and the power requirement. The power requirement is the product of the necessary flow rate and the lift, adjusted for efficiency losses in the lifting system. This requirement can be expressed as the volume-head product in units of m4/day (flow in m3/day × vertical lift in m). Human-powered systems can operate up to 100 m4/day, while a large animal can increase this output approximately 5 times. Most gardening requirements fall within this range. However, gardens will often be only one of many uses of water-lifting systems. Therefore, mechanical pumping systems with capabilities between 1,000 and 10,000 m4/day may be used for gardens. Detailed discussion of water-lifting devices is beyond the scope of this book, but we offer a brief review of some of the most common ones particularly appropriate for gardens.

Depending on the type of device used, the efficiency of water-lifting can vary from less than 5% to 75%.

Efficiency in this case is a measure of the amount of energy expended by the human, animal, fossil fuel, or other energy source in the process of lifting water, compared with how much water is actually lifted. If a water-lifting method is only 5% efficient, this means that 95% of the energy used in the operation did not result in water lifted to the garden. Most energy is lost due to water spilling out or leaking while being lifted, awkward and strenuous movements required by the person or animal, and water-lifting devices that are poorly designed or maintained.

12.7.1 Lifting with Human and Animal Power

Direct-lift methods are the most common for human-and animal-powered water-lifting systems. When the water only needs to be raised a small distance, a variety of hand-held containers can be used. In Oaxaca, Mexico, and other areas of southern Mexico and Central America, small-scale commercial vegetable producers irrigate small basins using a 10-14 liter (2.6-3.7 gal) clay or metal jar called a cantaro lowered into shallow wells and poured onto the garden25 (Figure 12.7). These gardeners can irrigate at a rate of 10-20 liters/min (2.6-5.3 gal/min). At a rate of 15 liters/min (4 gal/min), a gardener could irrigate 15 m2/hour (160 ft2/hr) (calculated by the authors, assuming a need for 60 mm/m2 [0.75 in/ft2]).


Figure 12.7 Water-Lifting and Watering by Hand Using a Ceramic Jar in Oaxaca, Mexico (After Wilken 1977)

The rope and bucket is one of the most available and easy-to-use methods for lifting water from deeper levels, although it is still hard work (Figure 12.8). The bucket can be made of leather, discarded rubber tire tubes, tin, or other material, and, like the rope, can be made locally.


Figure 12.8 Water-Lifting with a Bucket and Rope

Scoop irrigation is another traditional method used in Central America.26 It involves lifting water a short distance from canals with a wooden or metal scoop, plastic bowl, or gourd and splashing it over the growing area. The delivery rate is about 60 liters/min (16 gal/min). Scoops are more efficient if suspended, and can lift water up to 0.5 m (1.6 ft), at about 165-250 liters/min (66 gal/min) or 10-15 m3/hr (353-550 ft3/hr), although this is only about 25% efficient.27

The counterpoise lift, or shaduf, is a traditional method from the Middle East and East Asia consisting of a container (leather bag, metal bucket, lined basket) on one end of a pole with a counter weight on the other and a fulcrum in the middle. The counter weight lifts the water, and the operator’s weight is used to return the bucket to the water source against the counterweight. It can lift water up to 3 m (10 ft) at 30 liters/min (8 gal/min). In northwestern Nigeria, for example, farmers use shadufs with buckets to raise water from hand-dug wells to water dry-season gardens,28 and they are used extensively in Egypt to water small gardens of vegetables and fruit trees along the canals.

The pi cottah, used primarily in India, is similar to the shaduf but is operated by two people, one of whom acts as a moving counter weight to eliminate much of the strenuous work of returning the water container against a stationary counter weight.29 Although it can lift water 5-8 m (16-26 ft), its output is small, and it is used primarily to water small vegetable plots.

Bucket systems may also be adapted to animal power to increase flow, such as with the mohte, or self-emptying bucket. This traditional device employs either a tipping action or simple flap valves in the bucket or bag to discharge the water at lifts of 5-10 m (16-33 ft). The system can be arranged for the animal to walk back and forth in a straight line or in a circle, thus requiring less supervision.

The low-lift dhone, or see-sawing gutters from Bangladesh, can deliver about 300 liters/min (80 gal/min) at a 1 m (3.3 ft) lift. This device uses flap valves and can be operated by a single person shifting the weight back and forth at the fulcrum.

The Persian wheel is a direct-lift device that uses an animal like a water buffalo to turn a wheel, around which is traditionally strung a rope with earthenware pots attached to it. By increasing the length of rope, while keeping the wheel diameter fixed, Persian wheels are capable of lifts up to an effective height of about 10 m (33 ft), where discharges may range on the order of 50 liters/min (13 gal/min), down to about 2 m (7 ft), where they can approach 600 liters/min (160 gal/min). Efficiency is high, approximately 50% or more.

Variants of the Persian wheel include the zawaffa, sakia, and tablia, some of which can attain 75% efficiency but are limited to a lift of 2 m (6.6 ft) or less by the diameter of the wheel. Discharges over 2,000 liters/minute (500 gal/min) can be achieved with the devices when driven by large animals, although they are normally operated at 50% to 75% of this flow rate. They can also be fitted with belt drives for engine-driven operation for even greater flow rates, as long as fuel purchase is possible.

Pumps that displace water by pushing or pulling are also suited to human- and animal-powered systems. The Archimedean screw is an example of a displacement pump. It is a long tube with a helical screw inside which is turned by hand to raise water a short distance, such as from a canal to the garden. It was supposedly invented by Archimedes around 2,000 years ago and today is used in India and Egypt.30 The Archimedean screw is very efficient, up to 75% when working at full capacity, and about 50% overall.31 It is limited to a maximum of 1 m (3.3 ft) lift and requires two operators. Archimedean screws of traditional construction have efficiencies in the range of 30% and provide discharges from about 300 to almost 1,000 liters/min (80-260 gal/min).

Another rotary displacement water lifter is the Chinese chain and washer pump, a method thought to have been in use for 2,000 years. Steady rotary power is applied to a shaft that drives a series of disks linked on a chain and pulled through a pipe, pulling water through at the same time. The energy source can be humans, animals, or engines. Chain and washer pumps are effective up to about a 15 m (50 ft) lift and are capable of discharges up to 300 liters/minute (80 gal/minute), with an efficiency usually less than 50%.

The mechanical hand pump does not have a good record in the Third World. Compared with other hand-powered lifting methods it has been expensive, subject to breakdown, and is often poorly maintained. One of the major problems with maintenance is the lack of control that local people have over water projects using mechanical hand pumps, including a lack of training in maintenance and repair. Some of these problems can be traced to poor project design and implementation. However, sturdier pumps are now being manufactured and may be a good choice for small-diameter wells. Many such pumps are designed primarily for the low flows needed for domestic water supply. There have recently been many evaluations of the wide variety of hand pumps, and they should be consulted before a choice is made.32

12.7.2 Lifting with Other Power Sources

Mechanical pumps powered by engines obtaining energy from gasoline or diesel fuel are becoming more and more popular in the Third World. When used in hand-dug wells care must be taken not to empty the well faster than water can infiltrate into it. If this does occur it can lead to sediment from the aquifer flowing into the well and causing sides of the well or the lining to collapse. Unless there is a reliable supply of fuel and spare parts, and maintenance is available at a reasonable cost, such pumps can make the risk of gardening unacceptable for poor households.

A market garden project in Lesotho used pumps to create pressure for irrigating sloping land with sprinklers.33 Gardeners grew mostly cabbage on plots up to 500 m2 (5,400 ft2). Fuel costs (in 1985), at about US $0.50/liter ($1.67/gal), took up to 50% of potential profits. It would have been cheaper to locate gardens far enough down the slope from the dam to create pressure for sprinkling by gravity. The cost of installing a plastic line for the required distance would have been less than the cost of a pump, and would have eliminated the need for fuel. Alternatively, level beds could be created and irrigated by flooding, without the need for high pressures for sprinkling.

To avoid dependence on fossil fuels, many devices for lifting water using energy from the sun (photovoltaic cells), the wind (wind mills), or composted organic matter (biogas) have been promoted as “intermediate” technology for the Third World. However, most of these are intermediate only from the standpoint of the industrialized world, and human and animal power remain the most useful and reliable technology for most Third World rural areas,34 and anywhere else that households wish to be able to buy and maintain the pumps themselves.

12.8 Resources

A brief review of small-scale irrigation techniques, though not generally applicable to garden-scale production, can be found in Stern (1979: Chapters 5, 6, and 7). Jensen (1980) contains chapters on technical aspects of larger-scale irrigation that may be consulted by those planning large irrigated garden projects. Kennedy and Rogers (1985) discuss basic principles of a wide variety of human- and animal-powered water-lifting devices, while Hofkes (1983) gives a brief review of a variety of water pumps, and Chleq and Dupriez (1984: Chapter 10) review water-lifting with reference to dryland West Africa.

Fraenkel (1986) gives a comprehensive discussion of water-lifting for irrigation and a basis for comparing human, animal, fossil fuel, and renewable energy sources in a handbook form. Arlosoroff, et al. (1987), is one of a series of World Bank publications documenting the progress of an extensive United Nations project conducting laboratory and field tests of a wide variety of hand pumps to assess their technical and economic performance in different Third World settings.

References

1 Denevan 1980; Lawton and Wilke 1979; Manners 1980:51.

2 Stern 1979:19-23.

3 Bell, et al. 1987:4,41.

4 Chambers 1988.

5 See Jensen 1980; Stern 1979: Chapters 12,14.

6 Doorenbos and Pruitt 1977:79-81.

7 Doneen and Wescot 1984:54-59.

8 Gray 1963:36-37,55.

9 Fleuret 1985.

10 Tully 1988:128-130.

11 Fleuret 1985.

12 Stern 1979:43.

13 Stern 1979:45.

14 Doneen and Wescot 1984:52.

15 Kolarkar, Singh, and Lahiri 1983.

16 Mondal 1974.

17 Leach 1961:17-18.

18 Soleri and Cleveland 1989.

19 See e.g., Ayers and Wescot 1985:31-35.

20 Donahue, et al. 1983:380.

21 Stegman, et al. 1980:801.

22 Smith and Hancock 1986.

23 Ayers and Wescot 1985:24.

24 See Stern 1979: Chapter 15; Watt and Wood 1979: Chapter 22.

25 Wilken 1977.

26 Wilken 1977.

27 Kennedy and Rogers 1985.

28 Adams 1986.

29 Kennedy and Rogers 1985:18, 20.

30 Stem 1979:116.

31 Kennedy and Rogers 1985:15-16.

32 E.g., Arlosoroff, et al. 1987; Kennedy and Rogers 1985.

33 Evett 1985a.

34 Kennedy and Rogers 1985:1.

13. Pest and disease management

Gardens in drylands can be oases of green vegetation that attract insects, worms, birds, rodents, and other wild animals, and provide good growing conditions for weeds, fungi, viruses, and bacteria. Most of these do not affect garden production, and many improve it. Those that reduce production are called pests or pathogens. However, most problems in the garden are not just due to a pest or pathogen, but to temperatures that are too high or too low, too little sunlight, poor soil quality, too much or too little water, planting inappropriate crops, or planting at the wrong time of year.

A pest is any insect or other animal that has a negative effect on the garden. A parasite is an organism that lives on or in a host organism without benefiting it, and parasites that cause disease in their host are pathogens. Healthy plants will resist pests and pathogens much better than those that are unhealthy. In this book we emphasize reducing damage from pests and pathogens by enhancing natural controls through garden management, not by using commercial pesticides which kill both pests and beneficial organisms. (We use the word “pesticides” as a general term to include insecticides, fungicides, nematicides, herbicides, etc.)

In addition to general principles, we give examples of a few pests and pathogens common in dryland gardens and ways to manage them. This is not an exhaustive description of all dryland pests and diseases, rather it is meant to inspire creative thinking and experimentation by both gardeners and project workers when responding to pest and disease problems in local gardens.

13.1 Summary

The most important way to prevent pests and diseases from causing problems in the garden is good plant, soil, and water management as described in the preceding chapters of Part II: appropriate crop varieties; pest- and pathogen-free propagation materials; good soil texture, structure, and pH; adequate nutrients and moisture in the soil; and control of temperature, sunshine, and wind. A key ingredient is diversity; in the genetic makeup of crop varieties, and in the mixture of crops and crop varieties which encourage variety in the insects and microorganisms in the garden.

When pest and disease damage does decrease production enough to cause concern, there are diagnoses and responses that can limit the damage. The first step is careful observation. Insect damage includes chewing, cutting, and sucking of leaves, stems, and roots; transmitting disease; and eating or parasitizing beneficial insects. Infectious plant diseases are caused by fungi, bacteria, nematodes, protozoa, and viruses, which can attack any part of the plant. These small organisms cannot usually be seen with the naked eye, and so diagnosing disease relies even more on the plant’s symptoms than is true with pests.

Looking at the garden in terms of yearly cycles, and comparing it to other gardens, fields, and natural plant communities in the area can help to diagnose a problem and find appropriate solutions. If the problem is severe and difficult to control, considering other crops or even garden sites may be an option.

In tables and accompanying figures in this chapter we summarize some examples of observation and management of pests and pathogens, including those that cause wilt, abnormal growth, and leaf and fruit problems. The goal of diagnosis is to understand the situation well enough to choose the most appropriate management strategy.

13.2 An ecological approach

Agriculture, including gardens, always involves the need for increased management and resources in re- turn for increased production. The selection of plants for increased yield and better taste, for example, means reducing resistance to pests. While gardeners try keeping losses to pests at a minimum, some pest or disease damage is a natural part of a healthy garden, so it is important not to overreact to a few chewed, spotted, or dead leaves. Understanding the basic principles of ecological pest and disease management is more important than trying to remember all of the specific diseases and pests and the actions that can be taken to manage them. This is because many problems do not require specific identification and the same pest or pathogen acts differently and requires a different response depending on the crop, location, weather, time of year, gardener’s time and expectations, and other factors. The goal of management is not having a perfect harvest from every crop in the garden, but having the largest harvest for the smallest amount of work and resources invested.

Most insects and microorganisms either cause no damage, or are beneficial to crop production. In nature many pests and pathogens do not grow to large enough numbers to cause significant damage because they are kept in check by natural controls. In general, as their numbers increase, so does the rate at which they die from lack of food, predators, disease, the weather or other causes.1 We emphasize managing the garden to provide enough water and nutrients for healthy crops, and encouraging natural controls on pests and pathogens so that they do not cause enough damage to lower yields appreciably. In some cases pests and pathogens can be kept away from the crop or garden by adjusting planting times, or by making physical barriers, for example, fences to keep goats out of the garden, or ant barriers on trunks and stems. Another precaution is preventing the introduction of soil nematodes when transplanting seedlings into the garden.

Sometimes pest or pathogen populations do get large enough to cause extensive damage to many crops in the garden, or to one or two very important crops, such as a stand of sweet corn that is almost ripe, a prize mango tree, or a hedge of pomegranates. This is when specific management techniques are used to eliminate specific pests and pathogens that are threatening the garden. This approach is a short-term one compared with ongoing garden management and exclusion of pests or pathogens.

Pest and pathogen populations are kept under control naturally or through garden management in one of the following ways:

· The crop plant itself (e.g., timing of its life cycle, thorns, unpleasant chemicals).
· The physical environment (e.g., temperatures, moisture, separation in space or time, barriers).
· Other organisms (e.g., birds, snakes, spiders, insects, nematodes, fungi, bacteria, and viruses).
· The application of chemicals (e.g., botanical or manufactured chemicals on plants and soil).

13.2.1 Pest and Disease Management by the Crop Plant

Crop plants have evolved along with pests and pathogens, and have genetically adapted to living with them. Healthy crops can usually resist the buildup of pests or pathogens, and can tolerate even heavy damage when it occurs. Most plants produce far more leaves and flowers than required so that partial defoliation by insects or attack of buds, blossoms, or young fruit may not decrease yield significantly.2 In a mixed garden if one or two crops do succumb, others can grow to take their place.

As with adaptation to drought (section 5.5), we can think of adaptation to pests and pathogens in terms of escape or resistance. Plants escape damage through life cycles in which growth and reproduction occur when there is little threat from pests or pathogens.

Plants may resist damage by avoiding it. Chemicals in the leaves may slow down eating by pests or even kill the pests. Some plants have chemical odors that repel insects, while others lack the color or odor that attracts certain pests. Plants can make it physically difficult for animals to eat them. Thorns may repel rabbits; a thick cutical prevents caterpillars from chewing; and silica (sand) particles that occur naturally in the leaves of many grasses like maize, sorghum, and rice, wear down the mouth parts of insects that chew the leaves.

Resistant plants that are eaten or are diseased may also tolerate attacks of pests or pathogens and suffer little loss in yield. Root or tuber crops, for example, may have many leaves removed or have extensive disease in their aboveground parts with little loss in harvest.3 Aphid attacks on beans at the time fruit is setting may increase yields by diverting energy away from vegetative growth to seed formation.4 Increased yield in cereals frequently results from tillering after the initial shoot of the seedling is eaten by a shootfly, stem borer, or cutworm.5 When chewing removes the main growing tip in young plants of leaf crops like amaranth or basil, it can lead to bushier plants and increased leaf production, just as pruning would.

One of the most important ways of reducing pest and disease problems is to use varieties that produce good harvests without much care. We advocate using indigenous varieties for a number of reasons (section 14.2). Indigenous crops are adapted to the local soils and climate and therefore are likely to be more resistant to local pests and diseases than commercial varieties. Gardeners are familiar with their indigenous varieties: they know how and when to plant them, how they grow, what pest and disease problems those varieties have, and how they like to eat the harvest. All of this means less risk for the gardeners, an important consideration for low-resource households. Another advantage of using local seed and other planting material is that it eliminates worry about introducing exotic pests or pathogens that could be harmful to local varieties. Some of the varieties may only be known and grown in the immediate area, and talking to local gardeners is the best way to learn about them.

Obviously, knowing his crops and garden environment is a big advantage for the gardener. For example, some pests and diseases may be a problem in just one area of the garden and for a limited amount of time. Many crops can be planted and harvested during a range of times, and adjusting these times can help to escape pests and pathogens. Often if a crop fails miserably because the soil or air temperature was too hot or too cold, or because a pest or disease was especially abundant, it will prosper if planted at a different time or in a different location.

13.2.2 Environmental and Mechanical Management of Pests and Diseases

Managing soil, water, temperatures, and other aspects of the garden environment is critical for keeping pest and pathogen populations low. Creating a garden environment through soil and water management that encourages healthy plant growth is the best pest and disease control strategy, because healthy plants are better able to withstand attack. Anything in the plant’s environment that decreases vigor makes it more susceptible to pests and pathogens. The plants most frequently attacked are very young or old; those that have too much or too little water or shade; and those growing in soil that is too low in organic matter or nutrients, or too high or too low in pH. However, some conditions that would otherwise be good for the plant may also encourage pests and disease. For example, mulch - which is so beneficial under hot, dry conditions - may harbor insects, fungi, and bacteria that can harm plants, especially young ones. This helps illustrate why absolute rules about management are not useful, because every situation is unique, requiring observation and experience to adjust management strategies.

Another way to manage the garden environment is with mechanical control, that is, physically protecting crops with trap crops (plants that pests will feed or live on instead of the garden crops) and barriers, and by picking off, washing off, or crushing pests. Compared with fields, gardens are smaller, intensively managed production systems. Because of this some management methods such as mechanical control, which are not always appropriate for fields, can be very effective in gardens. Picking pests such as beetles, caterpillars, and grasshoppers off garden crops by hand (Figure 13.1), or crushing them as with aphids, is practical and very effective especially if done before the populations get very large. Success depends on understanding the life cycle and habits of the pest. For example, watching for aphids and destroying them as soon as they are seen prevents growth in aphid population to the point that they start being born with wings. Early in an infestation most aphids are wingless, but as stress increases due to rising population density or disappearance of food sources, more and more of them are winged (Figure 13.2). Avoiding this winged stage keeps the aphids from spreading to new areas where their population would grow rapidly once again.


Figure 13.1 Hand-Picking is a Good Way to Control Garden Pests

Small insects like mites, aphids, and thrips, can be washed from leaves with plain water or crushed with fingers. Removing diseased and insect-infested plants and burying them in the ground or compost pile also helps. Other means of mechanical control include making a barrier that prevents the pest from getting to the plant, such as collars against cutworms (Figure 13.9 in section 13.3.1). In northern Thailand, grass, raffia, paper, or plastic bags are placed over fruit like guava, citrus, pomegranate, and jack fruit to protect them from fruit flies.6 The lasora tree (Cordia myxa), is a drought-resistant tree grown for its fruit and used as a living garden fence in arid northwestern India. Researchers at the Central Arid Zones Research Institute in Jodhpur, India, told us that the large, tough lasora leaves are used by local farmers to cover ripening pomegranates, protecting the fruit from pests.

Sometimes wild plants, weeds, or other crops can be secondary hosts for pests or disease (section 8.6.2). Solanaceous weeds are important alternative hosts for tomato, eggplant, and potato pests, and all important sweet potato pests feed on the widespread morning glory.7 Careful observation and experimentation is the only way to find out if a weed or other plant is attracting pests that cause damage to crops, or is trapping those pests, thus keeping them from harming the crops. Once pests have settled on trap plants these plants can be destroyed by burning or burying them before the pests move to the crop plants.

If pest and disease problems increase with continued planting of the same crop or closely related crops in the same location, planting other crops in rotation, especially those from other botanical families, often helps. A clean fallow, one with no weeds, also helps to break the life cycles of pests and diseases like soil nematodes and some soil-borne pathogens by separating them in time from their host plants. However, many pests easily move such long distances that fallowing is not effective.8

13.2.3 Pest and Disease Management Using Other Organisms

Biological control is the action of parasites, predators, and pathogens in keeping another organism’s population density lower than it would otherwise be.9 Gardeners use biological control when they encourage organisms that help decrease pest or pathogen numbers on their crops. For example, encouraging predators such as spiders, toads, guinea fowl, geese, or chickens in the garden can help control insect pests (Figure 13.3).

Dryland gardeners have for a long time understood the ecology of the garden and have distinguished between similar animals whose habits are very different. Medieval date growers on the Arabian Peninsula were among the first practitioners of biological control. They transported predatory ants seasonally from nearby mountains to their oases to control another species of ants that attacked the date palms.10


Figure 13.2 Aphids (1)


Figure 13.2 Aphids (2)


Figure 13.2 Aphids (3)


Figure 13.3 Chickens and Other Birds can Help Control Some Garden Pests

Most cases of successful modern biological control of insect pests involve bringing predators or parasites from the pest’s area of origin.11 Most insects now bred commercially for biological control are nonspecific pests or pathogens; that is, they attack more than one prey or host. Examples are lacewings, ladybird beetles, and Trichogramma wasps. Importation or large-scale breeding of predators of common garden pests are not practical or affordable for most dryland gardeners, who practice biological control through garden management for healthy plants and by maintaining a diverse garden environment.

Biological control is defined not only by the use of organisms, but also by the ways in which they are used to cope with pest and pathogen problems. Biological controls are meant to control pest or pathogen populations, keeping them at acceptable levels but not eliminating them.

There is concern among some researchers who believe that biological controls are currently being developed and used in the same way as chemical pesticides, that is, to completely kill off the target pest or pathogen, not to control the population.12 Used in this way the “biological control” provides a strong selective pressure for genetic resistence among the targeted pest or pathogen, and eventually the controlling organism is no longer effective and other methods must quickly be found. This appears to be the case with Bacillus thuringiensis (Box 13.1).

13.2.4 Pest and Disease Management with Chemicals

Chemicals are nonliving substances, some of which are essential components of living cells. Certain quantities of some chemicals can repel, make sick, or kill pests and pathogens. As mentioned in section 13.2, the chemical composition of some crop plants deters pests or pathogens from damaging those plants. Botanical chemicals occur naturally in plant products, and some of them, for example, neem and citrus oils, garlic juice, and chili powders, are used to protect other crops (Box 13.2). Some botanical chemicals like nicotine from tobacco are very poisonous to people and animals and should be used with great care.

Synthetic chemical pesticides are manufactured chemical compounds, such as dichlorodiphenyltri-chloroethane (DDT), malathion, or captan usually specifically created for pest or pathogen control. Synthetic chemical compounds manufactured for other purposes, like kerosene or soap (Box 13.2) are also used. Naturally occurring chemicals such as sulfur and bordeaux (copper sulphate and lime) have been used as fungicides; copper compounds have also been used to control bacteria.


Figure 13.4 Ingredients for Safe Homemade Pesticides (Soap)


Figure 13.4 Ingredients for Safe Homemade Pesticides (Neem)


Figure 13.4 Ingredients for Safe Homemade Pesticides (Chili)


Figure 13.4 Ingredients for Safe Homemade Pesticides (Garlic)

Box 13.1
Biological Control Using Insect Pathogens

Since pathogens of plants and insects will not harm people or other large animals directly, some can be used for pest control. Probably the most widely known pathogen for control of pests is Bacillus thuringiensis (Bt), a bacterium that produces a toxin that kills certain kinds of caterpillars and fly larvae, but is not poisonous to other animals.13 It is not an important control in nature, but it is cultured commercially in large-scale fermentation vats and sprayed onto crops, just as are synthetic insecticides, and so also kills many beneficial caterpillars (larvae of Lepidoptera). The Bt gene that manufactures the toxin that kills the caterpillars is also being genetically transferred into crop plants, with the likely result that with such wide use insect resistance to it will increase, thus decreasing its effectiveness.14

Although little information is available on this subject, gardeners can experiment with making their own pest spray. Some researchers claim that it is possible to collect diseased pests and use them to make a spray that will spread the disease pathogen to other, similar pests.15 When applied to plants or areas in the garden where that pest is a problem the spray infects and eventually kills those pests.

Caterpillars with viral infections commonly hang by their back “legs” from stems or branches.16 This behavior can be used to identify specimens to use in making the spray. In one example,17 diseased cabbage looper caterpillars (Trichopulsia ni Hubner) were identified this way. Eight to 10 of these caterpillars were ground up and diluted with water to make enough spray to treat 0.5 ha (1.2 a) of crops. Three to 4 days after spraying, the caterpillars on those crops died. There are claims of positive results using this method of biological control with several other caterpillars including fall armyworms (Spodoptera litura) and corn earworms (Heliothis zea). The one thing that is certain is that this is an area needing much more research by gardeners and others interested in the use of insect pathogens.

The discovery of DDT during World War II ushered in the modern era of synthetic organic pesticides and their use has been growing ever since. Presently more than 4.5 million metric tons of pesticides are used in world agriculture each year, 30% of this use is in the Third World.23

Integrated pest management (IPM) evolved in response to increasing awareness of the environmental and health costs of synthetic pesticide use. Originally IPM was meant to reduce the use of synthetic pesticides by integrating them with other control methods.

In this way pesticide use could be limited to specific, critical times such as just prior to pest reproduction, instead of being continually applied as the sole form of control. However, IPM is frequently used by the pesticide industry to preserve or even intensify the commercialization of pest control.24

Although naturally occurring and botanical pesticides can be harmful, synthetic ones are especially dangerous because these pesticides are available in large amounts. Because they are created specifically to kill pests or diseases, synthetic pesticides contain high concentrations of poisons, which are toxic to many living things like beneficial insects and other animals, including people.

Box 13.2
Safe Homemade Pesticides

Some safe ingredients said to be useful for controlling garden pests are listed in this box (Figure 13.4). The “sprays” described can all be applied using bunches of grass or small twigs to splash and paint the mixtures on plants. While very little research has been done to determine if and how these ingredients work, many gardeners (including us) feel that they often do work. The best approach is to experiment, trying them first on small areas to determine if they have any positive or negative effects.

CHILI EXTRACT18 USE: Spray to repel insects or slow their feeding. RECIPE: Whole hot (spicy) chili peppers are ground, including the seeds which are a concentrated source of capsaicin, the fiery-hot active ingredient in chili sprays. The ground chilis are left to soak overnight in water. Soap can also be added to this mixture. The concentration of this spray is best determined by testing; if too weak it will be ineffective, if too strong it can burn leaves. Strength will depend on how spicy the chilis are. Care should be taken as this spray can burn the skin and eyes. REPORTED TO CONTROL: Aphids, caterpillars, beetles, and other insect pests.

NEEM SEED EXTRACT19 USE: Spray to repel insects or slow their feeding; kills pests when they eat it. RECIPE: Seeds of fruits fallen from the tree are cleaned, dried, and stored in a dry, ventilated place. When needed, seed hulls are removed, and seeds are finely ground and hung in a cloth sack in a container of water, using between 25-50 gm seed/liter of water (3-7 oz seed/gal of water). The ground seeds are soaked overnight in the water before using. This mixture should be made fresh for each use as it can lose its effectiveness over time and with exposure to sunlight. REPORTED TO CONTROL: Caterpillars, beetles, grasshoppers, and other garden pests.

SOAP20 USE: Spray to repel insects. RECIPE: 30 cm3 (1 fl oz) of soap mixed into 5 liters (1.3 gal) of water. Some soaps contain harsh additives so it is best to test the mixture on a small area first to make sure the plants will not be harmed. REPORTED TO CONTROL: Piercing and sucking pests like aphids and thrips.

GARLIC EXTRACT21 USE: Spray to repel insects. RECIPE: 100 gm (3.5 oz) crushed or grated garlic soaked 24 hours in 2 teaspoons of oil, then 0.5 liters (17 fl oz) water and 10 gm (0.4 oz) of soap are added. When ready to use add about 20 liters (5 gal) of water to this mixture. REPORTED TO CONTROL: Aphids and some caterpillars and beetles.

BAIT FOR FRUIT FLY TRAPS22 We list here three recipes for fruit fly bait (Figure 13.11 in section 13.3.1) [CAUTION: Human and animal urine should be used with care because they can spread disease, see Box 11.2 in section 11.3]:

· 1 liter (34 fl oz) water, 150 ml (5 fl oz) urine, 100 gm (3.5 oz) sugar, 1.5 small spoons (teaspoons) vanilla, 10 gm (0.4 oz) pyrethrum (Box 13.3 describes pyrethrum and the synthetic pesticides that have been modeled on it).

· 1 small spoon pyrethrum, 300 ml (10 fl oz) honey, 1 small spoon vanilla, 300 ml (10 fl oz) fruit pulp (melon, etc.), 10 liters (2.6 gal) water.

· Peel or pulp of 1 orange, 100 ml (3 fl oz) urine, 0.5 liters (17 fl oz) water. Mix and let stand overnight then dilute with 15 liters (4 gal) water before use.

Synthetic pesticides usually kill both pest and predator, leading to a more severe problem. For example, some early olive orchards in southern California, USA, suffering from black scale insects were treated with a kerosene spray.25 Comparisons between sprayed and unsprayed orchards found that sprayed orchards continued to have high populations of black scale, and no benefical predators, while unsprayed ones had less black scale and much greater numbers of the black scale predatory beetle, Rhizobius ventralis. When infestations of scale insects in orchards are fought with DDT and dieldrin, chalcid and braconid wasps that parasitize scales are destroyed, and population outbreaks of scale result.26 Another problem with synthetic chemical controls is that some can take a very long time to break down, remaining toxic for many years. As a result they spread through the food chain, increasing their damage to living organisms and the environment.

The persistance of toxic synthetic chemicals in the environment and throughout the food chain is one reason why we do not recommend the use of synthetic chemical controls in the garden. In addition, these pesticides are often expensive and hard to obtain for poor households; they undermine self-reliance, and lead to increased pest problems. However, because synthetic pesticides are being heavily promoted in the Third World, often in small-scale garden projects, it is important to have some knowledge of them (Box 13.3).

Pesticides are promoted worldwide by companies through advertising whose purpose is to increase sales. These advertisements contain information that is even less objective than the information provided by labels or extension agents.31 Yet, even pesticide labels often fail to give information necessary for safety.

Unfortunately, social responsibility for safety often succumbs to the profit motive, and pesticides are presented as the answer to all problems. At the national level it is usually the poor who pay the highest costs of pesticide use and receive the smallest benefits.32 The inability to read and understand labels, lack of awareness of the dangers, no access to medical care, and poor safety equipment make poor households and agricultural workers especially vulnerable to the dangers of synthetic pesticides. In fact, inadequate protection from those chemicals is a serious problem even for farm workers in rich countries like the United States.

The danger of pesticide poisoning is probably worst in the Third World. For example, the government of Kwara State in West Central Nigeria has for the last 10 years mounted a strenuous campaign to increase synthetic pesticide use among farmers, and it is claimed that many traditional pest control methods have been displaced by the use of synthetic chemicals.33 The chemicals are subsidized by the government and sold at only one-third of their cost. While the state had formerly provided overalls, boots, gloves, and soap to those using dieldrin, DDT, and other very dangerous pesticides, this has been eliminated for financial reasons, and there has been an increase in serious health problems. We give some emergency first aid suggestions for pesticide poisoning in Box 13.4.

Box 13.3
Synthetic Pesticides27

Following is a list of pesticides by broad chemical categories with some examples of each (some of their trade names are given in brackets). There are many more synthetic pesticides than those listed here, and new ones are being constantly introduced to the marketplace and the environment.

Contact poisons are absorbed through the body surface, stomach poisons have to be ingested by the insect or other animal, and systemic poisons are first absorbed into the plant tissues before being eaten by the pest or affecting the pathogen.

ORGANOCHLORINES (e.g., aldrin [Aldrex], BHC Lindane, Chlordane, DDT, dieldrin [Dieldrex], endrin [Hexadrin], heptahlor [Drinox], endosulfan [Cyclodan], toxaphene). Organochlorines are used primarily as insecticides, but some are also used as herbicides (weed killers) and fungicides (fungus killers). They are contact and stomach poisons affecting a wide range of animals. Organochlorines are soluble in water and fat, and persist in the environment. They are therefore effective pesticides because they can be dissolved in water to make a spray which is then absorbed through insects’ exoskeletons, and remains active for a long time. These same characteristics make Organochlorines extremely dangerous since they accumulate in the body fat of vertebrates (birds, amphibians, reptiles, and mammals, including humans). For example, even years after exposure, a lactating woman can have Organochlorines in her breast milk, which are then passed on to the nursing child.28 Immediate effects of poisoning in humans include skin rash, dizziness, and excitability, while long-term dangers include cancer and damage to the liver, brain, kidney, and reproductive system.

ORGANOPHOSPHATES (e.g., azodrin, diazinon [Dianon] [Neacide], malathion, methyparathion, parathion [Fusferno], tepp [Vapotone]). The first organo-phosphate compounds were developed during the Second World War by a German team researching nerve gases, and are some of the most toxic substances known to mammals and birds.29 Used as contact and systemic insecticides and acaricides (materials toxic to mites), they have replaced organochlorines for many uses because they break down relatively quickly. Organophosphates interfere with nerve transmission, and early symptoms of poisoning in humans include headache, dizziness, and flu-like symptoms. Residues of these chemicals may be more dangerous than the original compound especially when temperatures are high, when there has been no recent rainfall, and when there is a large amount of dust in the air and on plants.30

N-METHYL CARBAMATES (e.g., aldicarb [Temik], carbaryl [Sevin], carbofuran [Furadan], methiocarb, propoxur [Baygon]). N-methyl carbamates are used as insecticides and nematicides (nematode poisons). Symptoms of poisoning are similar to those of organophosphates, but these pesticides are not as dangerous, except for aldicarb, which is extremely toxic.

DITHIOCARBAMATES (e.g., thiram, ziram, ferbam, vapam, maneb, zineb). Dithiocarbamates are used as fungicides. Exposure to large quantities of these chemicals causes skin rash and respiratory problems in humans, and birth defects and cancer in other mammals.

NITROPHENOLS (or substituted phenols, e.g., dinoseb, DNOC [Sinox], pentachlorophenol [Dowicide]). Nitrophenols are used primarily as herbicides and fungicides, but also as insecticides and acaricides. These chemicals are easily absorbed through the skin. Large exposures cause headache and overheating, and so they are very dangerous in hot weather.

DIPYRIDYLS (e.g., diquat, morfamquat, paraquat). Dipyridyls are used as herbicides. Contact with skin or breathing spray can cause inflammation of eyes, nose, mouth, and throat; nosebleeds; and coughing.

CHLOROPHENOXYS (e.g., 2,4-D, 2,4-DB, 2,4,5-T, dichlorprop, erbon, falone, MCPA, MCPB, MCPP, silvex). When fresh these herbicides are easily absorbed into the skin, although when dry they break down very quickly in the environment. Symptoms of poisoning in people are skin rash and eye, nose, and throat irritation. TCDD is a contaminant of 2,4,5-T which appears to cause liver and kidney damage, cancer, and birth defects in animals. While chlorphenoxys are quickly eliminated from the body, TCDD is not. Agent Orange, a combination of 2,4-D and 2,4,5-T, was widely used during the Vietnam War and led to many lawsuits against the US government by members of the US military who claim to suffer serious long-term health problems due to exposure to it.

PTHALAMIDES (e.g., captan, captafol, folpet). These fungicides are very irritating to the human skin and respiratory track, and cause birth defects in animals.

PYRETHROIDS (e.g., bioallethrin [D-Trans], cypermethrin [Cymbush, Ripcord], permethrin [Ambush, Kalfil], pyrethrins [Pyrethrum], resmethrin [Chryson, Synthrin]). Pyrethrins were originally extracted from pyrethrum flowers (Chrysanthemum cinerariaefolium) and are powerful contact insecticides that rapidly break down in sunlight. A number of synthetic pyrethrins have been developed that have higher toxicity and last longer. Prolonged skin contact can result in a rash; headaches and sickness can be caused by inhaling the dust or spray.

Box 13.4
Emergency First Aid for Pesticide Poisoning

Knowing what the pesticide is and having its chemical formula are very important for medical workers to decide the best treatment. Often however, this information is not provided by the manufacturer, distributor, or salesperson. In addition, trained medical workers are not always available. David Werner gives the following emergency treatment for pesticide poisoning in his book Where There is No Doctor.34

First remove all clothing that might be contaminated. Wash the body with soap and water, and if the eyes are affected, rinse them for 5 to 10 minutes with plenty of clean water. If the person swallowed pesticides try to make him vomit. Then get him to drink a lot of either flour mixed with water, or beaten eggs, or milk. Continue to alternate this with trying to induce vomiting. Seek medical care if at all possible.

13.3 Examples of pest and disease management

The following are examples of the management of insect, nematode, and large animal pests, and crop diseases. These examples are intended to help readers understand the ecological approach to pest and disease management and inspire them to develop methods appropriate for their specific needs. This understanding will also help them assess the potential of introduced pest and disease management methods.

13.3.1 Insects

Arthropods are small animals with segmented bodies and hard outer skins (exoskeletons). An insect is an arthropod with a three-part body and six legs (Figure 13.5). Insects are the most abundant type of arthropods with several hundred thousand different species, most of which are neutral or beneficial for gardens and people. For example, insects are extremely important in pollination, and due to pesticides that have inadvertently killed many pollinators, numerous crops are now suffering from lack of pollination.35 In temperate regions honeybees (Apis mellifera) are probably the most important pollinators along with bumblebees (Bombus spp). Leaf-cutting bees (Megachile spp.), mason bees (Osmia spp), honeybees and flies are the main pollinators in the tropics. Other minor pollinators include ants, beetles, thrips, and moths. The benefit for the gardener of having pollinators and pest predators is a strong argument against using synthetic chemicals that indiscriminately kill all insects.

Some noninsect arthropods that can be pests in the garden include sowbugs and mites; spiders, centipedes, and others are beneficial. Most arthropods in the garden will have no noticeable effect on production, and many are just visitors passing through.


Figure 13.5 Insect Anatomy


Figure 13.6 Ants can Quickly Destroy Small Plants

CHEWING INSECTS Insects can cause damage in the garden by chewing leaves, stems, flowers, fruit, and roots, but as we mentioned in section 13.2.1, in some cases light insect “damage” to a plant may actually increase production. Ants are a common garden insect worldwide, and can cut down young seedlings, damage tree bark, or cut and carry away sections of leaves from mature plants (Figure 13.6). Gardeners may need to protect seeds and young seedlings from ants until they are big enough to survive an attack.

Ants are difficult to control because their nests can be several feet underground. Citrus oil is toxic to ants and fresh, mashed citrus peels may help repel them, as will garlic. Starting seeds in containers or easily protected nursery beds is a good way to avoid losing seedlings to ants (section 8.2). The gardener can also try to slow the ants down, discouraging them from doing any major damage while the plants are young and most vulnerable. Pouring boiling water down their holes is one way to do this. We have seen small-scale farmers in southern Mexico placing piles of Bermuda grass over ant holes to distract the ants from the crops. Once plants are mature, most ants are a much less serious problem. In addition, some ants help pollinate crops like squash.

Grasshoppers and the closely related locusts will feed on just about any plant, but prefer young shoots. Larger grasshoppers do the most damage and if there are not a lot of them, can be controlled by hand-picking, especially in the early morning when cooler temperatures slow them down. Sprays made with hot chili peppers and soap (Box 13.2) may help repel them when there are large numbers.

In southern and central Nigeria variegated grasshoppers (Zonocerus variegatus) cause considerable damage to dry-season garden and field crops such as banana, citrus, cowpea, maize, okra, yam, and other vegetables.36 Based on local farmers’ careful observation of this grasshopper’s life cycle (Figure 13.7) an experiment was conducted using a simple, cooperative control technique. In two 200,000 m2 (49 a) test sites farmers dug up the egg pods on their land, killing them by exposing them to the sun. This control measure resulted in an 80% drop in variegated grasshopper populations on those sites. The key to this technique is cooperation because individuals acting alone cannot protect their gardens and fields from a large population of grasshoppers, which can move quickly and easily.

Caterpillars are the larvae, or immature form, of moths and butterflies, and many of them chew leaves, stems, and fruit (Figure 13.8). Some are general feeders while others are limited to a single crop. Many feed in the open on leaves and can be easily spotted and picked off, but others are well camouflaged. Chewing marks on the borders of leaves, small, often black particles of feces on the ground or on leaves below the damaged area are signs of caterpillars. Some caterpillars roll up leaves in webs and are easy to find and dispose of.

Like weeds, some insect pests including grasshoppers, termites, ants, and certain kinds of caterpillars and beetle larvae (grubs), can be food for humans or domestic animals. For example, each July in savanna West Africa, a certain caterpillar invades the shea butter tree and eats most of the leaves.37 However, the caterpillars are eaten dried or roasted, or used directly in sauces, and they provide a significant source of income for those who harvest and sell them.


Figure 13.7 Life Cycle of Grasshoppers


Figure 13.8 Life Cycle of Moths

Cutworms are the caterpillars of various moths. These worms eat many crops and different species are common pests in drylands all over the world. In warmer areas there can be as many as four generations per year. The adults are up to 5 cm (2 in) long, and a dull pinkish grey color, sometimes with markings. Cutworms feed during the night, often attacking new seedlings and cutting them off at the ground. But they also feed on roots, tubers, stems, and leaves. For example, the common cutworm (Agrotis segetum), which occurs throughout dryland Asia, Africa, and Europe, cuts off seedlings of many crops at ground level and also feeds on roots, potatoes, carrots, and other crops. During the day cutworms rest curled up under debris on the soil surface or up to 10 cm (4 in) below it at the base of the stem, where they can be found and destroyed. Collars of cardbord or of metal cans which extend 5-13 cm (2-5 in) into the ground and about 10 cm (4 in) above the soil surface are an effective control (Figure 13.9). Thoroughly flooding the garden bed drowns the cutworms or at least forces them to the surface where they are easier to find and destroy.38


Figure 13.9 Collars as Protection from Cutworms


Figure 13.10 A Wasp Laying Eggs in an Aphid

SUCKING INSECTS Sucking insect pests have piercing mouth parts which they use to penetrate plant cells and suck out the contents. Sucking insects include whiteflies, aphids, scale insects, thrips, plant hoppers, and leaf or plant bugs. They often carry and spread bacteria and/or viruses that may cause disease. Some mites also damage plants this way. Many other sucking insects and predatory mites kill pests in the garden and are thus beneficial. Common signs of damage caused by these animals are leaves that are curled, twisted, or have dry spots, and abnormal-looking twigs. Thrips, for example, are common on onions; they eat the leaves from the inside which creates transparent, thin spots eventually causing the leaves to fall over.

Tapping the plant to dislodge mites and small insects onto a cloth or other clean surface makes them easier to see as they crawl for cover. If a hand lens or magnifying glass is available it can be used for identification. Larger ones can be controlled by hand-picking, smaller ones by crushing, washing them off the plants with water, or spraying with soap or other home mixtures.

Aphids are very common light green insects 1-2 mm (0.04-0.08 in) long (Figure 13.2 in section 13.2.2). Adults and nymphs suck plant juices from the undersides of leaves, causing wilting and distorted growth. Aphids also transmit viral diseases such as mosaic viruses in squash, melons, and lettuce.39 Aphid “shells” are the remains of aphids that have been eaten by the larvae of parasitic wasps (Figure 13.10), an example of biological control. The wasps can be encouraged by growing plants like cilantro, fennel and dill, members of the umbellifer family, which the adult wasps feed on.


Figure 13.11 A Fruit Fly Trap Made from a Plastic Water Bottle

Aphids secrete a sugary, high-protein substance called honeydew which some kinds of ants harvest from them. This honeydew also encourages the growth of a black mold which can eventually cover the leaf surface, destroying its ability to photosynthesize.

Most aphid attacks can be controlled by hand crushing and washing, or removing heavily infested leaves and burying them in the compost pile or ground. Timing is important, so that the population is reduced before it has a chance to become large and spread (section 13.2.2). Aphids are attracted to yellow objects, and a bowl of soapy water with something yellow resting on the bottom is a trap which will attract and drown them. Soapy sprays or washes may be helpful if aphid populations are growing rapidly.

BORING INSECTS Boring insects make holes in stems, roots, and fruits and do most of their damage from the inside. Often a small hole and perhaps some frass (feeding debris and feces) will be the only evidence on the outside. Fly larvae (maggots), adult beetles and grubs, and caterpillars are among the most common boring insects. Spiking boring grubs that were preying on coffee bushes by pushing a bicycle spoke into their holes has been successful.40

There are many species of fruit flies (Dacus spp. and Ceratitis spp.) that together cause damage to a variety of host crops including mangoes, citrus, peaches, guavas, olives, cucurbits, and coffee.41 Female fruit flies lay their eggs under the skin of ripening fruit, where the larvae feed on the fruit. Often the spoiling fruits drop from the tree and most species then pupate (undergo the transformation from larvae to more mature, winged forms) in the ground nearby. The adult flies emerge from the ground and continue this cycle.

One way of controlling fruit fly damage is covering the fruits to prevent egg-laying (section 13.2.2). Chickens, which eagerly eat fallen fruit and the larvae inside, control fruit flies biologically.42 Another technique is to trap and eliminate the flies before they lay their eggs. According to some researchers, simple traps can be made using plastic water bottles (Figure 13.11) or jars containing bait and hung in the garden.43 See Box 13.2 for ideas for bait recipes.

13.3.2 Nematodes

Nematodes are roundworms, of which there are several thousand species. Most live freely in the soil and feed on microscopic plants and animals. Some cause human disease (section 11.3), others parasitize insects and are an important natural control, while several hundred species feed on plants and cause disease. Some are widespread and serious pests of many annual and perennial garden crops in the drylands.

Most nematodes that attack plants are microscopic and live below ground where they feed on roots, although some feed on flowers, seeds, and leaves. They also spread viral diseases and interact with some disease-causing fungi and bacteria, making damage to the plants greater than the sum of each separately.44 Identification of particular species requires microscopic examination by trained observers, but nematode problems can often be identified by gardeners, who can then take some simple actions to control them.

Some nematodes feed on plant roots from the outside, while others live inside of the roots, causing root galls, root lesions, excessive branching, injured root tips, or root rot. Aboveground symptoms are chlorosis (yellowing) of the whole leaf, wilt, failure to thrive, and poor yield. One of the most common and destructive nematodes, especially in gardens in warm or hot areas with mild winters, is the root knot nematode (Meloidogyne spp.). This nematode causes knots or galls on the root (Figure 13.12) which, unlike nodules caused by nitrogen-fixing bacteria (Box 9.7 in section 9.5.2), will not rub off without breaking the root apart. Also, unlike the nodules, a layer of soil often sticks closely to the knot galls. Sweet potatoes and other tubers have cracked skin when infested with nematodes.

In many areas where nematode pests are present, traditional gardening and farming practices, such as mulching, burning crop residues, rotation, fallowing, and making mounds or ridges for planting keep nematode numbers low.45 Often in these farming systems some loss due to nematodes is acceptable, but this balance is upset by the introduction of new crops, varieties, pests, or management techniques. Nematode problems in the garden can be managed using a number of low-cost techniques that help keep nematodes out of uninfected areas, and when present, keep their numbers in both soil and plants low enough that a sufficient harvest is still produced.

EXCLUDING NEMATODES FROM CROPS AND GARDEN PLOTS Nematodes travel through the soil at a rate of only 1-3 m (3.3-10 ft) per year on their own, however, they can be transported much further if infested water, soil, or plants are moved from one area to another. They can also be transported by insects. This means preventing the movement of soil, transplants, and irrigation water from infested to noninfested areas, for example, by maintenance and design of irrigation systems, and by cleaning soil from tools and gardeners’ hands and feet.


Figure 13.12 Tomato Plant with Root Knot Nematodes

Planting material for vegetative propagation (Chapter 7) should also be from nematode-free plants. In some plants there is a choice of planting materials. For example, sweet potatoes can be propagated by stem cuttings which do not become infested with nematodes, rather than tuber cuttings which do. If planting materials are infected, or might be infected, and if no other materials are available, nematodes can be killed by placing the planting materials in hot water. Care should be taken that the water is not too hot, because it can kill the planting material. Some approximate guidelines for a few crops are: banana, citrus (bare root), and garlic cloves: 25 minutes at 55°C (131°F), and sweet potato: 65 minutes at 50°C (122°F).46 A researcher in Nigeria found that putting infected yams in 50-55°C (122-131°F) water for 40 minutes was very effective for killing the nematodes.47 This technique worked very well on yams that had been in storage for two to six months, however, treating freshly harvested yams with hot water caused them to rot.

KEEPING NEMATODE NUMBERS IN SOIL LOW Once an area has become infected with nematodes, it will probably always be infected. Even highly toxic synthetic pesticides cannot completely eliminate nematodes, and most of these pesticides have now been banned in some countries, such as the United States, because they are dangerous to human and environmental health.

Nematode pests must feed on plants to survive, so nematodes in infested beds or areas of the garden can be starved out by fallowing for a year or two. The fallowed area should be kept weed free, since weeds can also provide food for the nematodes, allowing them to survive and multiply. If nursery beds are kept in the same location for many years, populations of nematodes, especially root knot nematodes, can build up and infect transplants (section 8.2.1).

Nematodes are very sensitive to heat and drying. Therefore turning the soil regularly during the hot, sunny, dry season while the plot is being fallowed, helps reduce nematode populations even more. Wetting the soil occasionally will increase the effect, because it causes nematodes to change to more active states, making them more likely to be killed when the soil dries and heats again. Up to 80% of nematodes can be killed in less than 1 month using these combined methods.48 Roots of plants can be exposed to the sun to kill internal nematodes like root knot.49 Burning crop residues helps to heat the soil, and also kills varieties of nematodes that live in aboveground plant parts.

Some researchers advocate killing nematodes by first moistening the soil and then covering it with a thin, transparent polyethylene sheet. The sheet traps the heat and the water increases its movement. However, simply turning the soil may work as well, and does not require purchasing poly sheeting (made from nonrenewable resources) which litters the garden area as it is broken apart by sun and heat.

When there are crops growing in the garden, adding a lot of organic matter to the upper layers of the soil and mulching heavily helps to keep nematode populations low. This is probably because several varieties of fungi, insects, and other nematodes, whose reproduction is encouraged by organic matter, compete with or attack pest nematodes.

In Nigeria a researcher compared the effectiveness of several different treatments on yam production in soil infested with the yam nematode (Scutellonema bradys).50 The treatments were as follows: 1 and 2) each a different synthetic chemical pesticide, 3) chemical fertilizer, 4) coating the yam setts with wood ash before planting, 5) mixing 1.5 kg (3.3 lb) of manure into each yam heap or planting site, and 6) a control plot, with nothing added. The results showed the highest yields from the manure treatment, which also suppressed the nematodes. The wood ash treatment showed the next highest yield, followed by the control. Yields from the plots treated with the chemical pesticides and fertilizer were lower than those from the control plot. Even though one of the pesticides did reduce nematode populations, under these three treatments yields suffered.

KEEPING NEMATODE POPULATIONS LOW IN PLANTS Because nematodes move so slowly, population densities can vary greatly over short distances. Therefore, replanting in another part of the garden may improve production, at least for that season. Nematodes multiply faster when the weather is warm, so nematode-sensitive crops grown in the cool season will probably do better than those grown in the warm season.

Observation in the garden will reveal that certain crops are more resistant to some nematodes than others. In addition, nematode resistance can often be found in local crop varieties.51

Some plants repel nematodes or are toxic to them. Crops like sesame, mustard, and asparagus are resistant.52 Asparagus releases compounds from its roots that kill some nematodes in the soil. Neem trees (Azadirachta indica) also release substances from their roots that have been observed in India to reduce the numbers of at least six genera (the plural of genus) of nematodes parasitic on tomato, eggplant, cabbage, and cauliflower.53 Azadirachtin, the compound found ill the leaves and seeds of the neem tree, introduced from India into many dryland areas, is said to kill nematodes.54 There is little information about what kinds of nematodes are affected and in what form the neem should be applied. Since the seed contains the highest concentrations of azadirachtin, gardeners can experiment with incorporating ground neem seeds into nematode-infested soils. Even if this treatment is not successful for reducing nematodes it will benefit the garden soil because neem seeds are high in nitrogen and other plant nutrients.55 The castor bean (Ricinus spp.), a common plant in many drylands, appears to be effective in reducing populations of root knot and other nematodes when interplanted with tomatoes.56

Some varieties of domesticated marigolds may reduce populations of root knot nematodes. In India, where marigolds are traditionally planted among other crops, African marigolds (Tagetes erecta) intercropped with tomatoes or okra, were found to be effective for reducing six kinds of nematodes including the root knot nematode, and for increasing the quality and quantity of crop yields.57 There appear to be several ways in which marigolds affect nematodes: substances exuded from marigold roots are nematocidal, the nematodes’ life cycle is interrupted because nematode larvae have difficulty penetrating marigold roots, and nematodes are unable to develop once inside the roots. Mulches made from marigold leaves and stalks have also reduced nematode populations.


Figure 13.13 A Walled, Dry-Season Garden in Northeast Ghana

13.3.3 Large Animals as Pests

Because of their size, hungry rabbits, goats, pigs, or cattle, can quickly ruin a garden. Not only do they eat garden plants and fruits but some large animals can trample a whole garden, destroying everything they do not eat, including perennials. Some birds can also be bad garden pests, eating seeds, seedlings, leaves, and fruit.

REPELLENTS One way to prevent large animals from eating garden plants is to cover the plants with a repellent. In many places gardeners and farmers use mixtures containing feces as repellents, with good success. Hopi farmers in the United States told us that they mix dog feces and water to make a foul-smelling liquid which they spray or splash on tender bean plants to keep rabbits from eating them. Some farmers add other ingredients to their mixtures, like rabbit intestines or garlic.

In Ghana, farmers and gardeners use the feces of the pests to make a repellent for protecting young trees.58 For example, if goats are the problem then goat feces are used. The fresh feces are mixed with enough water to make a soupy liquid, which is left standing to ferment for about 3 days. The gardeners then paint the mixture all over the tree. These repellent mixtures will wash off with rain or may eventually be blown off in a dry, windy area, and will need to be reapplied.

BARRIERS TO ANIMALS If domestic animals cannot be tethered or confined, or if there are wild animals that eat garden crops, then fences or other barriers will have to be built around the garden or gardens to protect them. In northern Ghana, for example, traditional law calls for animals to be controlled by their owners in the rainy season to protect the field crops, but the animals roam freely in the dry season, so that anyone with a dry-season garden must build a stout wall of sticks and earth topped with thorn branches (Figure 13.13). Even then, animals may break in if the garden is not guarded.

Fences can be made from a variety of different materials, depending on what works best for the gardener (Figure 13.14). In the Sonoran Desert of northern Mexico farmers have been planting living fences in the beds of rivers for at least 100 years, where they grow many beans, squash, and other vegetables.59 The fences keep out cattle in addition to protecting the fields from erosion by the river, and capturing sediment to enrich and enlarge the fields. The fences are made mostly of willows (Salix goudingii) and some cottonwoods (Populus fremontii). Cuttings are taken from trees in existing fences, side shoots and leaves are pruned off, and the cuttings are planted in trenches. Cuttings from trees and shrubs are then interwoven to provide a solid barrier to keep out cattle. Farmers say that birds living in the trees eat insect pests. The trees also provide fuel and cuttings for new fences.


Figure 13.14 Gardeners Make Fences out of Many Different Locally Available Materials (1)


Figure 13.14 Gardeners Make Fences out of Many Different Locally Available Materials (2)


Figure 13.14 Gardeners Make Fences out of Many Different Locally Available Materials (3)


Figure 13.14 Gardeners Make Fences out of Many Different Locally Available Materials (4)


Figure 13.15 A Young Mango Tree is Carefully Protected from Animals in Northern Ghana

In Burkina Faso market gardeners have traditionally used millet stalk fences to protect their gardens from cattle. However, these fences must be repaired or replaced because of termite damage, and many millet stalks are required. These stalks have other uses as fuel or organic matter for the soil. A recent Forestry Department project is working with gardeners on experiments with living fences made from the local tree Acacia nilotica.60 Once established, the living fences require little maintenance, they act as windbreaks, their roots hold soil and control erosion, and their leaf litter contributes organic matter to the garden soil.

Gardeners in Zimbabwe have used sisal (Agave sisalana) for fences.61 Sisal is a fiber plant related to agave with sharp, pointed leaves. The gardeners find that the sisal plants are very effective against goats, although goats are still the main reason for the small size of gardens in some areas.

Barriers can be used around individual plants such as important trees in northern Ghana (Figure 13.15), or even fruits or seed heads. In southern Iran women make loosely woven baskets from fan palm fronds and use them to cover and protect ripening dates from birds.62 The baskets are carefully designed so they will not impede air circulation, which would cause the fruit to rot. We have had success using cloth and paper bags to protect seed heads of sunflower, amaranth, and sorghum from birds in our garden.

FRIGHTENING AWAY Fences are useless for protection against large wild animals like elephants or hippopotomi, such as along the Zambezi River in northern Zimbabwe and southern Zambia. Gardens must be guarded by household members at certain seasons of the year to scare off the intruders. When the garden is close to the house this is much easier.

Birds can also cause major damage to many garden crops such as sunflowers, figs, peaches, dates, maize, and young seedlings, especially in the early morning and late evening. Scaring birds away is easier when the garden is near the house and is the center of many other activities. Scaring them before they have discovered there is something good to eat, and have encouraged other birds to join in is best. Anything that will flap or make noises in the wind like strips of cloth, clusters of sticks (Figure 13.16), gourds, or the tape from spoiled tape cassettes can be fastened to poles or tree branches in the garden to help scare off birds.


Figure 13.16 Bird Scarers

In Sierra Leone some gardeners and farmers scare birds away from crops using tin cans with pebbles inside.63 Tall, flexible poles are driven into the ground in the garden and rope is tightly strung in a web from one pole to another. Many short (20-30 cm, 8-12 in) pieces of string are hung from this web, and old tin cans with a few pebbles in them are tied onto their ends. A long cord is tied to the web so that someone can jerk it when birds are in the garden, shaking the poles and cans and making a lot of noise that scares the birds away.

13.3.4 Diseases

In plants disease is abnormal growth or functioning that harms the plant. Diseases can be infectious or noninfectious. Examples of noninfectious diseases are nutritional deficiencies, sunburn, and severe drought stress. This section is about infectious plant diseases, those caused by pathogens. While some crop diseases are fairly easy to identify, there are many that can only be identified by plant pathologists (specialists in the study of plant diseases) using specialized equipment and processes, and even they are not always successful. In the tables at the end of this chapter we list a few dryland garden crop diseases that are relatively easy to identify and for which there are specific responses. However, in most cases precise identification is not necessary because it will not help gardeners respond to the disease. Instead, we provide basic guidelines for coping with diseases identified by symptoms.

Fungal, bacterial, and viral pathogens causing disease in plants are different from those causing disease in animals and people and therefore most diseased plants and food harvested from them will not harm people or animals. Important exceptions to this generalization are some fungi that grow on plants and produce toxins that are poisonous to animals. Aspergillus flavus, for example, infects grain and legume seeds while still in the field and can produce levels of the poison aflatoxin that cause severe illness in humans and other animals (section 2.10).64

Fungi (the singular is fungus) are nonphotosynthesizing organisms that obtain their food through the decomposition of organic matter or from living organisms. Fungi secrete a sticky substance that helps form water-holding aggregates in the soil. They are also important in the garden soil because they break down organic matter into forms that are easier for plants to use. Some also attack and devour nematodes, including harmful ones, while others help to control disease-causing fungi by competing with them. Mycorrhizae are an extremely important group of fungi that grow in and around the roots of most flowering plants including all garden crops and help them to take up nutrients (Box 9.5 in section 9.5).

Some fungi, however, are parasites that attack garden crops, causing diseases. In fact, most plant diseases are caused by fungal pathogens. In the dry season, most fungal pathogens occur below ground in the root system, but are expressed above ground as wilting or chlorosis. Fungal diseases can be spread by wind, water and by contact from insects and other vectors. Generally they are encouraged by warm, moist conditions which in dryland gardens occur near the soil surface and in the middle of lush garden beds.

Bacteria are simple, one-celled organisms. They are the most abundant organisms in the world, and play an important role in gardens. Bacteria are very small (0.001-0.003 mm, 0.00004-0.00012 in), about the size of clay particles. Most bacteria found in the garden are beneficial, helping decompose organic matter in the soil, and making nutrients more available to crops (section 9.6 and Box 9.7 in section 9.5.2). Some make their own food from mineral nutrients in the soil, often transforming them to forms that are more available to plants. There are some, however, that harm the growth and productivity of dryland garden crops. Some of these bacteria live only as parasites in plants, while others can also survive for a period in the soil, living off dead organic matter. In warm, moist environments bacteria can multiply extremely rapidly and are easily spread by insects, people, and water.

Fungal and bacterial diseases often appear first on lower stems, trunks, fruit, and older leaves near the soil surface. On leaves they show up as specks or spots that are water-soaked, dark green, or brown. Lesions may look like a target, having alternating dark and light concentric circles, or may have a furry, moldy appearance. The fruit may feel like a bag of water, look rotten and moldy, or have a scablike wound, depending on the disease.

A virus is a parasite that can only reproduce by invading and taking over cells of other organisms. In plants viruses are spread mainly by infected seeds, cuttings, grafts; and by sucking insects or people touching infected plants and then uninfected plants. Viruses cannot live without a host nor can they lie dormant in fallowed soil. Viruses harm their hosts by diverting the resources and processes of those cells into the production of more viruses. In humans viruses cause many different diseases like AIDS (acquired immune deficiency syndrome), influenza, and harmless skin warts. In plants there are viral diseases that affect only specific crops and others with a wide range of hosts. Often viral symptoms in plants are most obvious on new growth as deformities, dieback, and discoloration. Most importantly, plant diseases caused by viruses are almost always systemic.

Distinguishing between systemic and localized plant diseases can help gardeners plan their disease control strategies. Localized diseases are only active in certain parts of the plant such as the roots or leaves and fruits. Systemic diseases, including vascular wilts and almost all viral diseases, are spread throughout the plant, for example through the vascular system (section 5.2). Some localized diseases or pest problems, especially in the roots, may first become evident to the gardener in symptoms that look systemic, like wilting. Checking the roots of garden plants periodically helps the gardener detect these problems early, before they cause too much damage. Checking roots for signs of disease is discussed in section 13.4.

If caught early in their development, localized fungal and bacterial diseases can be controlled by removing the infected parts and using other management strategies. For example, if a fungal leaf spot is found on a few tomato leaves in the garden, these leaves can be removed, the plants pruned and staked, and organic matter added to the soil. However, if leaves on the growing tip of a squash plant appear mottled and deformed, signs of a systemic viral disease, removing those leaves will not provide control because the entire plant is infected.

Management strategies for systemic disease will vary depending on the crop and severity of the disease. Many systemic plant diseases can be transmitted by insects and humans to other plants and this should be considered when deciding whether or not to keep a diseased plant alive. If other, healthy plants of the same crop are growing in the garden, removing the diseased plants helps control further spread of the disease.

Some crop varieties may be able to overcome or tolerate systemic diseases, especially if the infection is not severe. We have seen both bean and squash plants infected with viral diseases survive to produce good harvests. In general, it is a good idea to remove from the garden young plants with signs of systemic disease infection. It is often not worth the effort to keep sick young plants alive until they become productive, and removing them prevents the disease from spreading to other young plants. The gardener must decide if the plant is capable of surviving, if it is worth any extra work to save it, and if leaving it will lead to further spread of the disease.

It is also useful to check the area surrounding the garden for weeds that may serve as alternate hosts for systemic plant diseases. Experienced local gardeners may know of weeds associated with plant disease problems in their area. Removing those weeds before planting can help prevent or control disease.

Using locally adapted, disease-resistant crop varieties is one of the best ways to control disease problems. Local varieties may have been selected for resistance to diseases as an adaptation to local growing conditions. Familiarity with local crops and their growing habits helps the gardener take advantage of a crop’s ability to resist disease and the environmental conditions that make this resistance most successful.

Some diseases only affect certain crop families, for example tobacco mosaic virus (TMV) only occurs in solanaceous crops (tomato, pepper, eggplant, potato, tomatillo), and the fungus causing clubroot only on crucifers (mustards, cabbage, cauliflower, broccoli). Planting crops from different crop families is important for controlling these disease pathogens. In the case of TMV, nonsolanaceous crops can replace those that have succumbed to virus, although solanaceous crops may be tried again the following season because the virus does not remain in the soil. However, in the case of a fungi such as clubroot, which can live on in the soil, it is more appropriate to plant a rotation of noncrucifer crops for several seasons. In fact, if the pathogens are well established, planting nonsusceptible crops may be the only control method available short of abandoning the site.

Interesting new work is being done on using watery compost extract (WCE) as a treatment to encourage crop plant resistance to some fungal diseases.65 Watery compost extract is a solution made by soaking one part well-rotted compost in six parts water for about 1 week. The compost must include some animal manure, although the best kind of manure and its proportion in the compost have not been explored. Spraying WCE on leaves of garden crops like fava beans, tomatoes, and grapes has helped control fungal disease in those plants. One of the pathogens controlled is Uncinula necator, powdery mildew of grapes, a common dryland fungus. In addition, researchers suggest soaking seeds overnight in WCE before planting to prevent damping-off fungus (section 13.4). Since WCE itself has no fungicidal properties, it is thought that it stimulates the plant to produce fungicidal substances.66

Fungal root rots can occur when crops are grown out of season. For example, cool soil temperatures favor the fungi more than the roots of cold-sensitive squash varieties. Maize should be planted in warm soils (20-25°C, 68-77°F) and wheat in cool soils (15-20°C, 59-68°F) to control seedling blight caused by the same strain of a fungal pathogen.67 This is because the different soil temperatures favor seedling growth of the two crops differently, and vigorous seedlings resist penetration by the pathogen. So, if grown under unfavorable soil temperatures, seedling growth is very slow, and the plants succumb to fungal disease.

Mechanical and environmental controls of plant diseases focus on preventing the spread of those diseases. Removing and destroying diseased plants or plant parts is advisable to prevent the spread of disease by vectors like wind and insects. Some insects that commonly spread plant pathogens are aphids, leaf-hoppers, cucumber beetles, and whiteflies. If the presence of any of these insects coincides with the onset of disease in the garden it is likely that they are the source, and control measures should be taken. Familiarity with life cycles of insects that are disease vectors helps in timing plantings to avoid the insects’ active periods.

While not all pathogens can be spread from infected plants or plant parts, many can. Burning these plants or burying them away from the garden is recommended. With systemic diseases the entire infected plant should always be destroyed when it is removed from the garden. Although only some diseases are seed-borne, it is wise not to use seeds from diseased plants or fruit, especially in the case of systemic diseases. People can also spread plant diseases from infected to uninfected soil or plants. This can be prevented by carefully washing hands and feet with water and soap, if possible, and cleaning tools with bleach or soapy water after contact with a source of disease and before touching healthy plants or entering an uninfected area.

Many disease pathogens thrive under moist, warm conditions, so staking or trellising plants and pruning lower leaves helps because it allows better air circulation, reducing some of the moisture. Placing dry mulch or sticks under fruit in the garden keeps the fruit from lying on the moist soil surface. We sometimes place sticks or a flat stone under ripening melons in our garden to keep them from rotting due to fungal or bacterial infections (Figure 13.17). Immediately removing and destroying infected fruit and leaves prevents spread of the pathogen by insects.

Traps are mechanical control measures that help prevent the spread of pathogens by controlling their insect vectors. The yellow aphid traps described in section 13.3.1 attract and drown aphids. When approaching a garden or field, aphids will stop first to feed on a tall trap crop such as corn planted around peppers, beans, or squash. In doing so, many of the aphids lose any pepper-, bean-, or squash-infecting viruses they may have been carrying, thereby greatly reducing the amount of virus reaching those crops.68


Figure 13.17 A Flat Stone Keeps Ripening Fruit Off Moist Soil and Prevents Rotting


Figure 13.18 Careful Observation is the Best Tool for Diagnosing Garden Problems

Suppressive soils are those that inhibit the growth of pathogens because of the microorganisms contained in the soil.69 Often it is the diverse mixture of soil microorganisms that seems to suppress growth of pathogens. For this reason, some fungal and bacterial diseases can be controlled biologically by adding lots of organic matter to the garden soil. This encourages growth of fungi and bacteria that compete with and thus control some disease pathogens. For example, studies in Mexico of soil in indigenous fields showed that those soils suppressed damping-off disease caused by fungi significantly more than did soils from industrially farmed fields.70

Another way to biologically control diseases is by using predators to control those insects and other animals that spread the disease pathogens. An example of this is the parasitic wasps that prey on aphids (section 13.3.1).

13.4 Diagnosing pest and disease problems

The key to diagnosing problems in the garden is careful observation that leads to an understanding of the ways in which plants, animals, insects, microorganisms, and their environment interact. Traditional gardeners and other small-scale food producers are usually excellent observers of the world around them, and are continually testing, evaluating, and redesigning their management strategies in response to garden conditions. Always look first to local gardeners for help with diagnosing, understanding, and managing problems in the garden.

When trying to diagnose problems, it is important to look at the garden as a whole. Is there a pattern among the plants that are not doing well? Are there pests or eggs on the undersides of leaves, in the soil and mulch around the plants (Figure 13.18)? Night is an active time for many insects and other animals in hot, dry areas and is a good time to check the garden for them. Although not necessary, a small hand lens or magnifying glass can sometimes help, for example, in determining if leaf spots have small insect holes in the middle. Are the affected plants young or old, scattered or concentrated, of one kind or different kinds? How does the distribution pattern of plants with the problem correspond to that of water drainage, soil, shade, wind? What is the history of the garden site? What was grown there before? Was anything buried there? Have chemicals been applied nearby? Has there been standing water?

It is a good idea to plant a few “extra” plants of each crop in the garden for digging up occasionally to check on progress or to diagnose problems below the soil surface. Many problems - especially soil conditions, fungal and bacterial diseases, and some insects - cause damage below ground in the root system. These problems are then expressed above ground in the stems, leaves, and fruit. (See Chapter 5 for definitions of plant anatomy terms used in this Chapter.)

Learn what a healthy root looks like by digging up a healthy plant and washing the roots carefully in water. A rotted root system may have fewer lateral roots, be off-color (tan or brown), and collapse when squeezed between a finger and thumb (Figure 13.19). Cortical sloughing is the loss of cortex (cortical tissue) on the roots and occurs in several plant diseases such as fungal root rot. If a plant is suffering from a disease that causes cortical sloughing, but that tissue has not yet been shed, it can be easily tested by pinching the root lightly and pulling toward its tip. This will pull off a “sleeve” of cortical tissue, revealing vascular tissue underneath (Figure 13.20).


Figure 13.19 Comparing Healthy and Diseased Root Systems (1)


Figure 13.19 Comparing Healthy and Diseased Root Systems (2)


Figure 13.20 Testing for Cortical Sloughing

It is not necessary to dig up a whole tree to examine the roots (Figure 13.21). At a point at or just inside the drip line (the line indicating the maximum spread of aboveground growth from the central stem or trunk), a 20- to 50-cm (8- to 20-in) deep hole can be dug and roots about the diameter of an adult’s small finger exposed. These can be checked for cortical sloughing, discoloration, and deformities.

Many agricultural and other chemicals, including fertilizers, can cause damage to plants, and should be considered a possible cause of problems. Poisoning by herbicides such as 2,4-D (Box 13.3 in section 13.2.4) can cause symptoms that look like a viral infection. Symptoms include abnormal stem and leaf growth, and leaf chlorosis, curl, and mottling. Check for evidence of nearby spraying, or contamination by tools, containers, soil, or clothes brought into the garden. Be sure a poisonous chemical was not accidentally put in a bucket, sprayer, or on other tools used in the garden, and find out the chemical history of the garden plot. This is especially important in urban areas where empty lots are often used as waste dumps.


Figure 13.21 Digging for a Root Sample from a Tree


Figure 13.22 Diagnosing Problems with Seedlings and Recent Transplants


Figure 13.23 Signs of Damping-Off (1)


Figure 13.23 Signs of Damping-Off (2)

SEEDLINGS AND RECENT TRANSPLANTS (Figure 13.22) The younger and more tender its tissues the more susceptible a plant is to pests, diseases, and difficult growing conditions. As described earlier, some garden crops contain chemicals that repel or are toxic to pests, but only start producing these chemicals as they grow and mature. In addition, young plants with small root and shoot systems are much more likely to be killed by pest or disease damage than more established plants. This makes careful observation and quick diagnosis and action especially important when caring for young plants.

The most common disease problem in seedlings is damping-off, caused by several soil fungi and some bacteria that rot the plant roots or stems at the soil line (Figure 13.23). Infected seedlings wilt, the tops dry out, they fail to grow, or even fall over. Damping-off is diagnosed by digging out the seedling, washing it carefully in a container of water, and checking the roots and stem for soft brown areas and cortical sloughing. In advanced cases, the entire root system will be rotten and will disintegrate during washing.

There are three common causes of damping-off:

· An unusually large population of the fungi or bacteria that cause the disease due to insufficient competition from other microorganisms in the soil. Adding compost can reduce damping-off problems because compost contains many active soil organisms, some of which compete with and reduce the damping-off fungi.

· Waterlogged soil due to overwatering or poor drainage causes damping-off. This can be remedied by watering only as much as is necessary, and by improving soil drainage, for example, by adding sand to the soil.

· Soil temperatures too hot or too cold for the crop variety result in lack of vigor and susceptibility to disease. This can be avoided by modifying soil temperatures (use a covering of surface mulch to cool soil, remove it to allow soil warming), or replanting at a better time of year. Soaking seeds overnight in watery compost extract (WCE) before planting may be helpful for controlling damping-off fungus (section 13.3.4).


Figure 13.24 Problems with Established Garden Plants (1)


Figure 13.24 Problems with Established Garden Plants (2)


Figure 13.24 Figure 13.24 Problems with Established Garden Plants (3)

Problems with transplants are often caused by transplant stress or exposure to pests or diseases in the planting site. Transplant stress weakens the plant making it vulnerable to pests or pathogens which were not a problem before transplanting. Bruised and broken tissues from handling during transplanting create openings where pests and pathogens can enter the plant.

One of the best ways to overcome problems with seedlings and transplants is to just keep trying. For example, try replanting or transplanting when the soil is a little warmer or cooler, wait until insect populations have died down, or plant where there is more or less sunlight. Often these experiments lead to an understanding of how to work with the garden environment instead of fighting it, with better long-term results using fewer resources.

ESTABLISHED PLANTS Most of the symptoms of disease in established herbaceous annuals, such as wilt and chlorosis, will be expressed above ground (Figure 13.24). However, the majority of these symptoms, except those due to insects and viruses, are caused by problems below ground. Digging up an extra plant to look at its roots helps in diagnosing problems. Problems with trees and other woody perennials also frequently originate in the root zone, and checking the roots is important for diagnosing these symptoms.

If a tree seedling or cutting grown in a container was root-bound at the time of transplanting, the roots can twist around each other and in 3 to 4 years choke each other off. Above ground the leaves may become chlorotic, turn orange, or wilt. Spreading the roots at the time of transplanting prevents this problem (section 8.4.4). Verticillium wilt and a variety of nematodes can also cause problems in woody perennials. Impervious soil layers can slow or stop growth of perennials and can also contribute to waterlogging and salt buildup. Above ground the most common problems of woody perennials are insects (twig damage, leaf eating, or causing abnormal growth), powdery mildew, birds and other large animals, and wind damage.

FAILURE TO THRIVE All gardeners at one time or another have plants that just do not grow. They may look perfectly fine or show a variety of difficult-to-classify symptoms. Failure to thrive and grow may be due to a single factor, but just as likely it is the result of a combination of interacting factors. Planting again at different times of the year, in different soil conditions, checking the roots of a few plants to look for clues, and talking with other gardeners to see what is happening in their gardens all help. If other gardens have similar problems, then a common climate (section 5.7), water quality, or seed source may be the cause.

The following four sections include figures and tables to help in diagnosing garden problems according to types of symptoms. Figure 13.25 is a guide to these sections.


Figure 13.25 Diagnosing Problems in the Garden

13.4.1 Wilts (Table 13.1 and Figure 13.26)

When a plant wilts it loses its rigidity, its leaves droop, and its branches bend. In drylands the most common cause of wilting is lack of water in the root zone (section 10.3.1). On very hot days, with temperatures higher than 38°C (100°C), some plants like cucurbits, will wilt even with sufficient soil moisture, but will recover in the cool of the evening. Check the plants in the early morning during the hot season to see if they have recovered. Shading plants and providing windbreaks also helps. Plants may also wilt while being hardened off for transplanting. This is to be expected but should not be so severe that the plant’s growing tips die back or that it does not recover in the evening (section 8.4.4). Other important causes of wilting in drylands include fungal and bacterial wilts, insects, and nematodes.

In general we recommend adding organic matter to the soil when fungal diseases are a problem. Although this is not a cure for those diseases, in some cases the fungi present in the organic matter will compete with, and therefore help control, the disease-causing fungi in the future (section 13.3.4).

Larvae or grubs in the soil, which eat root hairs and even larger roots, will cause the plant to wilt, especially under hot, dry conditions. When these pests attack the storage roots of crops like carrots and Jerusalem artichokes, the plant may not wilt. However, those wounds are sites for diseases to become established which can destroy the root in the ground, or later in storage. Clusters of frass along a plant’s stem are a common sign of stem boring caterpillars.

13.4.2 Leaf Problems (Table 13.2 and Figure 13.27)

Many soil nutrient deficiencies (section 9.5) show up as discoloration of the leaves. A common sign of many problems is chlorosis, a light green or yellow color which contrasts with the normal, darker green leaf color. The pattern of the chlorosis is often a good clue to the cause of the problem. Frequently leaf chlorosis or other discolorations due to nutritional deficiencies are symmetrical, (the same on both halves of the leaf) and follow leaf vein patterns, but leaf discoloration caused by diseases is often asymmetrical (the opposite of symmetrical). Mottling refers to mixed patches of light green or yellow and normal green leaf color caused by viral diseases.

Leaves that are yellow or have brown edges, and poor plant growth may indicate damaging amounts of salt in the soil or water, which is common in arid areas (section 11.2). Blowing sand, also common in some drylands, can tear and pit leaves with tiny scars. Fungi and bacteria affecting primarily the leaves are less common in drylands because of the lower humidity, and under dry conditions most of these diseases are primarily focused in the root system. An exception is a fungal disease called powdery mildew, which thrives in dry conditions. However, during the rainy season, or with watering methods such as sprinkling, fungi and bacteria, can be a problem especially on lower leaves. Insect damage of leaves can be severe, but can often be controlled by hand picking, washing the leaves, or replanting. Some of the sprays described in Box 13.2 in section 13.2.4 are also effective until they are washed off.

13.4.3 Abnormal Growth (Table 13.3 and Figure 13.28)

Abnormal growth means that the roots, stems, leaves, or flowers are not developing normally and are misshapen in some way. This growth is the plant’s response to stresses including pests, disease, the physical environment, management practices, or even genetic abnormalities.

13.4.4 Fruit Problems (Table 13.4 and Figure 13.29)

Fruit problems can be especially disheartening for the gardener because the harvest seems so near. Here we cover fruit problems of some herbaceous annual crops, as well as some perennial tree crops. Fruit can show abnormal growth and the mottling symptoms of virus infections. Fruit symptoms are usually associated with leaf symptoms (section 13.4.2). Some of these symptoms may also develop in harvested, stored fruits.

13.5 Resources

College and high school textbooks about entomology, the study of insects, are good sources for learning more about insects, their life cycles and habits. In addition, for understanding basic principles of some control methods, textbooks on diseases of plants, such as Plan Pathology (Agrios 1988) are useful. Unfortunately, many such texts most often recommend using synthetic chemicals as solutions to pest and disease problems.

Agricultural Insect Pests of the Tropics and Their Control (Hill 1983) seems to be one of the most thorough books on this subject in the English language. The discussions of principles and methods of pest control are useful, although the emphasis is on large-scale agriculture and using manufactured pesticides. The bulk of this book is a catalog of major pests organized according to order and accompanied by distribution maps and excellent drawings or photographs.

Pests and Diseases of Tropical Crops: Volume 1, Principles and Methods of Control (Hill and Waller 1982) is a useful, easy-to-read handbook. Again, the emphasis is on large-scale production and using manufactured chemicals. See Onwueme (1978) for tuber crops.

Natural Crop Protection (Stall 1987) focuses on solutions using local resources available to farmers and gardeners in the Third World tropics and subtropics. It is divided into sections for specific crops, and for specific methods in the field and in storage. Natural Crop Protection is really an annotated bibliography on the subject and contains some gaps and discrepancies. However, because this kind of research has been ignored for so long, these problems are to be expected. The author has tried to carefully document the techniques described and she invites readers to help improve future editions. We feel this book is a good first step in a new direction and includes helpful ideas and interesting references on the subject.

Developing Countries Farm Radio Network (DCFRN) has some program scripts that are useful for pest and disease control including: “Insect control” (#1-1), “Fruit and vegetable soft rot” (#8-2), “Pest life cycles” (#10-8), and “Preventing bird damage” (#12-10).

Table 13.1 Wilts

Observations

Possible causes

Suggested actions

SOIL AND TEMPERATURE

Heat stress

High temperatures and transpiration rates

Shade, mulch, check soil moisture, consider adjusting planting time

Soil in root zone dry

Underwatering

Irrigate, mulch, and shade

Water-saturated soil

Overwatering and/or high water table

Reduce watering, try raised beds for high water table, add organic matter for more open texture

Chemical injury

History of herbicide use in area

Move garden site or use containers and soil from other source

LEAVES

Many tiny insects on underside of leaves

Aphids, mites, thrips

Wash off, crush, spray, use a yellow trap, trap crops

Mottled, curled, misshapen,

Viral disease

Burn or discard plant, clean

dwarfed


hands and tools

Young tomato leaves stunted, curled inward, bumpy on lower surface

Curly top virus spread by insects (leafhoppers)

Shade tomato plants to discourage insects

Dwarfed, curled, misshapen

Chemical injury

Move garden site or use containers and soil from other source

STEMS

Insect holes on main stem

Stem- and vine-boring caterpillars

Remove borer, cover stem with soil; on squash stems encourage rooting elsewhere

Ring of vascular browning seen in stem cross section

Fungal vascular wilt

Plant resistant varieties, add organic matter, do not overwater, vary planting times

White ooze from cut stem, discolored tissue

Bacterial wilt

Rotate crop families, plant local resistant varieties, 1 year dry fallow

Chew marks on base of stem

Caterpillars, sow bugs

Hand pick, especially at night, remove mulch immediately around stem, use stem collars

ROOTS

Roots brown and soft, cortical sloughing

Fungal root rot

Add organic matter, do not overwater

Round knots, lesions or swellings

Nematodes

Rotate crops, add organic matter, use noninfested planting material, heat and dry soil

Club-shaped swellings (galls) on crucifers

Clubroot fungi

Rotate noncrucifers, resistant varieties, raise pH above 7.2

Small root system, few root hairs

Larvae or grubs eating roots

Dig around plants or turn soil to expose and kill pests, allow domesticated or wild birds to forage for larvae or grubs


Figure 13.26 Wilts

Table 13.2 Leaf Problems

Observation

Possible causes

Suggested actions

CHLOROSIS (leaf yellowing)

Whole leaf (old leaves)

Nitrogen deficiency

Add high N organic matter

Whole leaf

Nematodes

Check roots for swellings or knots, rotate crops, add organic matter, fallow and/or heat soil

Whole leaf

Fungal root rots

Check roots for browning and soft tissue, add organic matter, do not overwater

Whole leaf

Root-bound

Check roots for tangled, twisted growth, untangle roots, transplant to larger container or into the ground

Between veins of new leaves

Iron or zinc deficiency

Lower soil pH

Veins in old leaves

Viral disease or herbicide damage

Check for herbicide damage, burn or discard plant if young, clean tools and hands, control insect vectors, rotate in different crop family

Half of leaf and vascular browning

Fungal vascular wilts

Plant resistant varieties, rotate crop families, add organic matter

Mottled or mosaic pattern

Viral disease

If plant is young bum or discard it; clean tools and hands, control insect vectors, rotate in different crop family

OTHER

Leaves unusually purple

Phosphorus deficiency

Add high phosphorus organic matter

Black spots or rings

Fungal leaf spot

Remove and discard affected leaves, clean hands and tools, add organic matter to soil, rotate in different crop family

Salt burn: edges brown, white, or yellow

High salt concentration in soil or water

Water deeply, flush soil, check water quality

Dry, brown patches, holes visible with lens

Sucking insects

Pick off or crush, wash or spray with water or a repellent, use traps

Chew marks on edges, small holes in center

Chewing insects

Pick off, wash or spray with water or a repellent

Pitting and tearing

Windblown sand

Windbreaks

White, powdery spots

Powdery mildew fungi

Remove and destroy infected plants or parts, add organic matter to soil, plant resistant varieties

Small (5-mm, 0.02-in diameter) spots, dark with yellow margins

Bacterial spot

Remove and destroy affected leaves and fruit, stake plants, wash hands and tools

Rusty yellow, brown, or white spots, also on stems, can form galls

Fungal rusts

Remove and destroy affected leaves and fruit, stake plants, wash hands and tools, add organic matter to soil


Figure 13.27 Leaf Problems

Table 13.3. Abnormal Growth

Observation

Possible causes

Suggested actions

ROOTS

Round swellings or knots

Root knot nematodes

Rotate crops, add organic matter, solarize soil, turn soil regularly during hot season, fallow

Spindle- or club-shaped swellings on crucifers

Clubroot fungi

Rotate in noncruciferous crops, raise pH above 7.2

Plant in container with misshapen, twisted, forked roots

Root-bound

Untangle roots, transplant into larger container or into ground

Plant in ground with stunted, twisted, forked roots

Impermeable layer in soil e.g., caliche

Dig out deep planting hole, build up topsoil

Misshapen, twisted, forked roots

Transplanting inappropriate crop e.g., carrot

Plant seeds directly instead of transplanting

STEMS Galls or swellings on crown, or higher, especially on stone fruits and grapes

Crown gall bacteria

Destroy infested young perennials, tools used near or on diseased plants should be thoroughly cleaned

Swellings or holes higher on stem, dieback above these

Insect eggs

Allow beneficials to hatch, remove others

Many buds

Damage from thrips, mites, and other insects, may carry viral diseases

Remove and destroy affected parts, eliminate insect vectors, clean hands and tools

Pale, internodes long and spindly

Insufficient sunlight

Gradually reduce shade and/or remove mulch

Large, dark, sunken spot at soil line

Collar rot fungi

Discard or bum plant, clean hands and tools, add organic matter to soil, do not overwater, plant resistant varieties

Swollen growths with black dust inside, at the joints, in maize and teosinte

Maize smut fungi

Remove affected stalks before growths open, discard fungus

LEAVES

Misshapen, curled

Mites, thrips

Remove and destroy affected parts, crush, spray, trap pests, encourage predators

Misshapen, curled, sometimes sticky

Aphids, other sucking insects

Wash off, crush, spray, use yellow trap

Dwarfed, misshapen, chlorosis

Chemical damage

Move garden or use containers with soil from another source

Misshapen, dwarfed mottled, especially the new growth

Viral disease

If symptoms severe or plant is young bum or discard it, clean hands and tools, use disease-free seeds/stock

Curled and brittle, thickened midribs, leaf hoppers active

Curly top virus

If symptoms severe, bum or discard plant, shade plants, plant resistant varieties

Misshapen, dwarfed with wilt or unilateral chlorosis, vascular browning

Fungal vascular wilts

Add organic matter, do not overwater

FLOWERS

In maize, sugarcane, sorghum: swollen black growths with dust inside

Smut fungi

Remove affected fruit before growths open, discard fungus


Figure 13.28 Abnormal Growth

Table 13.4 Fruit Problems

Observation

Possible causes

Suggested actions

FRUIT HARD

Mottled yellow/green, faint yellow rings

Viral disease

If severe, remove and destroy fruit and plant, control insect vectors, do not plant seeds produced in these fruits

Tomatoes: black/tan hard spot on end (blossom end rot)

Calcium deficiency

Water more regularly, mulch to maintain even soil moisture

Hard white lesion or scar on exposed part of fruit

Sunburn

Shade plant

Yellow spots with holes in middle

Sucking insects

Remove, trap or crush insects wash or spray fruit with water

Pitting with no holes in middle

Windblown sand

Windbreaks, harvest early if danger of rotting, replant if possible

Bruised or scarred, chunks of skin missing

Hail

Harvest early if danger of rotting, replant if possible

Small, scablike dark spots with yellow margins

Bacterial spot

Remove and destroy affected fruit and leaves, prune and stake plant, add organic matter to soil

Pomegranates: fruits crack open

Uneven or inadequate water supply

Establish regular, deep watering schedule beginning just after flowering through fruit ripening

FRUIT SOFT

Soft spots where fruit touches the ground, e.g., melons

Fungal soft rots

Elevate fruit above moist ground on stones, sticks, mulch, or trellis

Holes eaten in fruit, e.g., figs, dates, peaches

Fruit beetles or other boring insects

Harvest early, trap and remove beetles in morning, remove ripe, rotting fruit

Small holes on skin, inside eaten, rotten

Fruit fly larvae

Cover fruit to protect from egg laying, use homemade sprays, traps

Holes eaten in fruit

Birds

Frighten off, harvest early, cover fruit or whole plant

Fuzzy brown growth on stone fruits, also occurs in storage

Fungal brown rot

Remove and compost fruit before spores spread

Fruit feels like a bag of water

Bacterial soft rot

Pick and discard affected fruit to control fruit flies and other insect vectors, prune and stake plant to allow air circulation


Figure 13.29 Fruit Problems

References

1 van den Bosch, et al. 1982:16.

2 Hill 1983:63.

3 Hill 1983:242.

4 Cammell and Way 1987:4.

5 Hill 1983:49.

6 Hill 1983:39.

7 Hill 1983:42.

8 Hill 1983:43-44.

9 van den Bosch, et al. 1982:2.

10 van den Bosch, et al. 1982:22-23.

11 van den Bosch, et al. 1982:24-27.

12 Garcia, et al. 1988.

13 van den Bosch, et al. 1982:63-64.

14 Gould 1988.

15 Stoll 1987:138-139.

16 van den Bosch, et al. 1982:63.

17 Stoll 1987:138.

18 Based on Stoll 1987:84-86.

19 Based on Stoll 1987:96.

20 Based on Stoll 1987:140.

21 Based on Stoll 1987:91.

22 From Stoll 1987:130.

23 Pimentel 1988.

24 Levins 1986.

25 van den Bosch, et al. 1982:28.

26 Hill 1983:80.

27 Primarily based on IOCU 1984 and Hill 1983.

28 IOCU 1984:101.

29 Hill 1983:124.

30 IOCU 1984:105.

31 Abrahamse and Brunt 1984; Bull 1982:87-123.

32 Bull 1982:76ff.

33 Atteh 1987.

34 Werner 1977:103.

35 Hill 1983:16.

36 Page and Richards 1977.

37 Dupriez and De Leener 1983:66.

38 DCFRN #7-9B.

39 MacNab, et al. 1983:23,29.

40 Hill 1983:38.

41 Hill 1983:382-393.

42 DCFRN #4-9B.

43 Stoll 1987:129-132.

44 Agrios 1988:713-714.

45 Bridge 1987.

46 Radewald 1977.

47 Adeniji 1977.

48 Maas 1987.

49 Bridge 1987.

50 Adeniji 1977.

51 Bridge 1987.

52 Bridge 1987.

53 Rice 1983:59.

54 NAS 1980b:114.

55 Radwanski and Wickens 1981.

56 Rice 1983:58.

57 Rice 1983:51-56.

58 DCFRN #10-6.

59 Nabhan and Sheridan 1977.

60 Ouangraoua 1988.

61 Bell, et al. 1987:38,39.

62 FAO 1982c:225.

63 DCFRN #9-1A.

64 Agrios 1988:445-447.

65 ACPP n.d.; Weltzien and Ketterer 1986.

66 Weltzien and Ketterer 1986.

67 Roberts and Boothroyd 1984:204,230.

68 Agrios 1988:192.

69 Baker 1987.

70 Lumsden, et al. 1990.

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