The term 'root crops' is applied to plants which produce subterranean structures that may be used for human or animal food. They are normally perennating organs, storing plant nutrients through a resting period (dry season or winter) which are used in the regrowth of the plant when growing conditions are again favourable. The word 'root' is often a misnomer, as in many cases the storage organ may be morphologically a modified stem, eg a swollen rhizome or corm, or a tuber such as a potato, rather than a swollen root as in carrot or sweet potato. All these swollen underground organs are commonly spoken of as 'tubers'.
Root crops are the second most important source of carbohydrates in the world's food: FAO figures for world production in 1981 showed 1,661 million tonnes of cereals and 561 million tonnes of root crops. The tropical world, however, where root crops are proportionally much more important, produced 82 million tonnes of root crops and only 42 million tonnes of cereals. In many tropical countries where rice is not grown they are the staple diet. In general, protein content is low, but some, for example Solanum tuberosum (potato) and Dioscorea spp. (yam), provide significant amounts of certain vitamins.
The following pages briefly describe 42 root crops, the most important of which are Manihot esculenta (cassava), Solanum tuberosum (potato), Ipomoea batatas (sweet potato), Colocasia esculenta (taro), Xanthosoma spp. (tannia) and Dioscorea spp. (yam). Many others are of only minor or local importance, but have been included to make this compilation as comprehensive as possible.
Each crop is listed alphabetically under its first common name, followed by local names. In the index of 'trivial' names (Appendix E) these local names are cross-referenced to the first common name. Selected literature references are given, but are not exhaustive; some crops, eg Manihot esculenta (cassava) have been the subject of so much study that lengthy bibliographies for them have been published, and such bibliographies are included in the relevant lists of references.
Data about each crop are arranged under the following headings:
Widely used English names are given, the first being printed in capitals and being used for the alphabetical arrangement of the entries and for cross-referencing.
Botanical name; Family
Nomenclature closely follows the Index Kewensis and its Supplements
Most plants have a wide range of local names, and many of these are listed, with the country or language to which they normally apply being appended in parentheses. Less common English names are given without the country being indicated.
A short description of the plant, its form and habit, varietal differences and systematics where appropriate is given.
Origin and distribution
Brief particulars of the origin and distribution of the crop are given.
The main climatic regions in which it is possible to cultivate the plant are given in accordance with van Royen and Bengtson. The climates of the world are divisible into tropical, subtropical, intermediate or temperate and polar types.
Tropical climates have an average annual temperature of above 25°C, no month having an average temperature below 18°C. Subtropical climates have short, mild winters and long growing seasons. There is a period of 1-2 months when freezing temperatures may occur, though the average temperature of the coldest month is above 6°C. The summer temperatures may be as high as those of the tropical climate. Intermediate or temperate climates, ie those between subtropical and polar, have cold winters and warm to hot summers. They vary from areas where the winters are short to those where they are long and severe. All intermediate climates have a season of frost as well as a frost-free season.
The humid tropical climates are tropical rainforest, tropical monsoon, and tropical savanna. The tropical rainforest has no pronounced or prolonged dry season, an annual rainfall of 200-400 cm or more, a relative humidity of around 80 per cent and a high and uniform temperature with annual means ranging from 25 to 26.5°C with little seasonal variation. The tropical monsoon climate exhibits marked daily and seasonal temperature changes, has an annual rainfall of 100-200 cm with abundant rainfall during the wet season, alternating with a period of drought lasting 4-6 months or longer. The tropical savanna climate has a rainfall often exceeding 100 cm annually, well spread over 120-190 days, with a prolonged drought often lasting 6-7 months. The climate is hot with a moderate range of temperature. The dry tropical climates are subdivided into semi-arid or steppe type and arid or desert type. In the areas of tropical steppe climate the rainfall is occasional, though seasonal and commonly averages 20-50 cm or more annually; the temperature is variable but high at all seasons. The desert climate has a rainfall usually averaging less than 20 cm per annum, and a daytime relative humidity (RH) commonly less than 50 per cent.
The subtropical climate is subdivided into dry subtropical or Mediterranean and humid subtropical. The former has an average annual rainfall generally below 75 cm, in some places below 50 cm, with most of the rainfall occurring during the cool season. In some regions there is a moderate amount of summer rainfall, while others may be nearly rainless during this period. There are about 6-8 months with an average temperature below 18°C. In humid subtropical regions the rainfall averages above 75 cm per annum, with no pronounced dry season. There are generally 4-6 months with an average temperature below 18°C. In both types of subtropical climate frost may occur during the coldest period.
Humid intermediate climate has an annual rainfall which ranges from 50 cm in the drier parts to 200 cm in the more rainy sections. Dry intermediate climates have an annual rainfall which is commonly less than 50 cm. They may be subdivided into middle latitude steppe and middle latitude desert; the former having an annual rainfall of 15-50 cm and the latter less than 15 cm per annum.
Plant growth requirements are arranged under the main headings of temperature, rainfall and soil, with additional factors such as altitude and day-length noted where they are crucial. The possibility of growth under irrigation is mentioned when describing rainfall requirements and any positive evidence concerning the effects of fertilisers is included in the information on soil. The main climatic zones in which the root crops listed are generally grown are shown in Appendix A.
Information concerning the type or types of planting material is given, with brief mention of their relative merits, and with special emphasis on the preferred type, where more than one type of planting material is available. The usual methods of planting are given, together with details of field spacing and, where applicable, seed rate.
Pests and diseases
The most serious pests and diseases attacking the crop in various growing regions are noted, along with methods of control. (A list of the pesticides referred to in the digest is given in Appendix C.)
An approximate average or range of time lengths from planting to harvesting is quoted.
Harvesting and handling
The most common and best methods of harvesting, handling and storage are briefly indicated.
The part of the plant for which the crop is primarily grown and the form in which it is commonly marketed are given. Normally one form only has been selected and this is shown at the beginning of the heading; this form will be used as the basis for quantitative data given in subsequent headings, unless otherwise stated. In some cases where there are other main products, eg seeds or pods, these are noted separately.
A good average yield of the primary product is given. Yields obtained in different regions or circumstances may be separately quoted.
The main use or uses of the primary product are given.
Additional uses of the primary product are entered under this heading.
Secondary and waste products
Useful by-products resulting from the processing of the primary product or prepared from other parts of the plant are listed, together with their uses, etc. Major waste products which result from primary or secondary product processing are noted, with possible outlets where applicable.
Information is given on the chemical components of importance in the plant, and the main nutrients of the edible portion are listed wherever possible. The percentage composition often varies widely according to the variety, locality, conditions of growth, etc. so 'typical' figures are quoted, taken from the most reliable source available, eg FAO Food Composition Tables, recent papers, etc. Only a few publications quote ranges of high and low values; workers in the nutritional field are advised to seek local information for the food in question. Fibre and protein represent 'crude' fibre and 'crude' protein (N x6.25) respectively, unless otherwise stated.
In addition, the presence of constituents that may call for special treatments, eg toxins, are indicated, and also of those that may have value as drugs, antibiotics, etc.
The main processing operations through which the primary product may have to pass in order to produce a final marketable commodity, are listed. In certain circumstances, similar information may also be given for secondary products.
Production and trade
There is very little statistical information available regarding the production and trade of many of the individual root crops included in this digest, largely because most root crop production in the tropics is in smallholder units. Moreover, the major part of the production is consumed locally, since tropical root crops, owing to their high water content and perishability, have not assumed any great significance in international trade. Where information is available, details are provided of: (i) the estimated average world production of the crop; (ii) the output of the major producing countries; (iii) shipments from the major exporting countries; and (iv) shipments to the major importing countries. In some cases only very fragmentary information is available, in others none. In view of the extreme variations in exchange rates, commodity prices and general economic instability of recent years, prices have not been included as any such figures could be misleading.
Any factors which might have a significant influence on the future supply of and demand for the commodity are mentioned under this heading.
Particular attention is paid to possible competition from synthetic materials and other substitutes.
Textbooks, technical bulletins, research reports, papers given at recent symposia, articles in technical periodicals and bibliographies are cited. For some root crops literature is scant, but for many it is extensive, and the bibliographies for each individual crop are selective and by no means exhaustive. Emphasis has been given to material published between 1972 and 1982.