The Most Asked Question. What question do we receive the most frequently from ECHO's network? Easily it is some variation of, "What crops can people consider for the region where I work?" This is usually followed by some description of climate, soils, etc. Often some especially difficult condition is outlined--too much or too little rain, farms that are too small, steep, rocky, hot, infertile, swampy, or remote.
This is also one of the most frustrating questions to try to answer. We asked Dr. Frank Martin to put together something that would help you answer the question for yourself. He found it the most difficult assignment we have given him. "It should be possible to characterize soil and climate so that areas that are similar, even though widely scattered, could use the same technology. In practice this has proven very difficult." He knows of two large projects which tried to accomplish this, but neither turned up anything that appears to be useful. "The old-fashioned technique of a variety trial is still the best method to determine the value of a particular crop for your region."
The article that follows contains three levels of complexity. In one table, the most complex, he pulls together 140 crops, including both annuals and perennials, and vegetable, field and fruit crops. It will be useful as a rough screen to chose or eliminate crops you might consider. Other tables list several plants based only on rainfall amount and distribution and on temperature. No attempt was made to prepare an exhaustive list. For each of these climates, he has chosen several useful and probably familiar plants that would be well worth a try. Return to TABLE OF CONTENTS.
"What crops can I grow?" Consultants in tropical agriculture often receive letters from Peace Corps volunteers, teachers, missionaries, students, and those who have followed their careers to the tropics, with the question, "What crops can I grow?" Leaders and literate farmers often look for new alternatives to basic crops that do not bring in the income desired and write, "What other crops can I grow?" Conscientious persons from the developing world, and even from academic institutions in the United States ask the same question. Choosing the right crop or crops for a particular place is a common problem, and the information necessary for answering the question is not widely available or easily found. Knowledge of agriculture tends to accumulate in regional pockets that represent ecological zones. While the majority of those that write may understand their own area quite well, they are much less familiar with the broader situation or the whole of the tropics and subtropics.
Improvement of Local Agriculture. It can be taken for granted that agriculture can always be improved. Quantity and quality of agricultural produce, and usually the diversity as well, can always be improved. Nevertheless, it is a mistake to assume in any situation that improvements are easy. Agricultural systems represent evolutionary adaptations to particular ecological systems. This is not only biological but also socio-economic and technical evolution. Agricultural systems are almost always followed because they work. Under the local circumstances they work, or at least they work better than easily visualized alternative systems.
It is sometimes relatively simple to improve the technology of Third World agriculture, yet investigators are often puzzled why the technology is not readily adopted by farmers. Usually the answer is in the socio-economic aspects of the system, which are frequently overlooked. In highly technical systems, yields and quality might already be high. Improving such agriculture is like shooting at a moving target, hard to achieve a hit.
The newcomer in the Tropics is likely to need some changes in his way of thinking even before the question of the proper crop is asked. Therefore, we recommend the reading of ECHO Basic Document #1: Beginners Guide to Small Scale Agriculture in the Tropics.
Traditional approaches to answering the question. The most obvious and useful technique to answering the question, "What crop can be grown?" is to observe and talk to local farmers. They can be considered wealthy in appropriate technology with deep and sometimes almost poetic understanding of their particular crops and production systems. Following farmers' techniques, especially those of farmer-leaders or farmer-innovators, one is practically assured of a crop.
Yet, farmers have their roots in tradition. Excellent farmers frequently are unaware of what farmers do in an adjacent valley or region. They may not know of improved varieties or technological advances. They will seldom be aware of the world situation, or at times even the local market, and how it affects their crops. Thus, the expertise of farmers is valuable but limited.
A second source of information is that of agricultural statistics. While few countries have as extensive a system as that found in the United States, all countries maintain some sort of records of production, and these clearly show what crops are grown, and usually what acreage and what yield. Somewhere there is a tax collector who must get his share. If a crop is already grown in substantial quantity in a region, then you can be sure it is a crop that not only can be grown, but that can also make money -- and that it can be improved.
A third technique is to talk to the local agricultural agent, or, if possible, the nearest agricultural extension office or experiment station. The structure of the system developed to help local agriculture varies from country to country and at times even from region to region within the country, and these people have at least a rudimentary knowledge of the crops. They will know which crops the government emphasizes (usually the money makers) and often the improved varieties and technology. Do not underrate them and their potential answers to your queries.
While the above traditional sources of information may not be adequate, it would be foolish to start any serious long term endeavor without consulting these sources.
Still a fourth technique has been suggested, the use of observations of the wild plants on the land as an indication of what crops can be successfully produced. This technique has not been developed to the extent that it would be a useful tool, and has as a disadvantage the requirement of special knowledge of the flora. Furthermore, in some regions the original native flora has been destroyed. Return to TABLE OF CONTENTS.
An integrated approach. This kind of approach tries to use local, national, and internationally available information to answer the question, " What crop can be grown? ". In this publication the ecological situation is emphasized . If one can learn to distinguish ecological zones and if one learns the ecological requirements and preferences of crops of the world, then one can match crops with zones with a high degree of confidence that a given crop can be grown in a given locality. But, even so, one must always remember that there are other questions to be asked (see "The Next Questions"). Return to TABLE OF CONTENTS.
The principal factors that determine crop potential are both internal (genetic) and external (environment). Not only do species of plants vary with respect to their genetic potentials and responses to environment, but even within a given species different varieties or different individuals are distinct in adaptation. Return to TABLE OF CONTENTS.
Availability of water. Water occurs everywhere, including in the driest desert. Nevertheless, not all water is available for plant growth. For example, water in the air is not available to most plants. Since almost all crop plants grow in the soil, water availability for practical purposes is the water available to plants in the soil. When excess water falls on the soil, a part may run off even before it can enter. Part of that which enters will be held in the soil by physical and chemical forces. The remainder will move deeper into the soil attracted by gravity until it comes to rest on an impenetrable basin or joins a downward flowing underground stream or aquifer.
Water is lost from the soil not only by percolation downward but also by evaporation on the surface. The rate of evaporation depends on the water holding capacity of the soil, and also on environ- mental conditions, chiefly temperature, relative humidity, and wind.
Plants growing in the soil can remove water from the excess flowing through the soil, from basins or aquifers, and from the water that is physically and chemically held in the soil, up to a limit. Even after plants have removed all the water they can from the soil, some water will remain that can be removed and estimated by other techniques.
In general, sandy soils hold the least available moisture, clay soils and soils of high organic matter hold the most. The water holding capacity of the soil can be increased, for practical purposes, chiefly by the addition of organic material to the soil.
From a practical standpoint, water availability to a plant is determined also by its ability to retrieve water, and this is associated with a large and efficient root system. In addition competing plants whether of the same or different species (weeds), reduce the water available to a particular plant.
In dry climates runoff can be reduced by contour planting, by furrows oriented crosswise to rain- carrying winds, by plowing, and by rototilling the soil surface. Furthermore, plants can be planted at the bottom of furrows or in pits to increase their chance to obtain water.
Seeds may need water almost continuously in order to germinate, and seedlings may need extra water to grow. The growing plant needs large quantities of water, but may be very adept at getting water because of its root system. The plant that is maturing seeds, fruits or tubers often needs less water. A plant that matures in a short period may avoid drought by its ability to mature when water is available.
Life zones (as defined by Holdridge, see Table I) depend in part on the amount of water received annually. The yearly average rainfall, much more than the extremes, dictates the kinds of woody perennial plants that can be grown without irri- gation in a particular zone. The suitability of an annual crop plant for growth in a particular region, however, depends not only on life zone, but also on the water availability through irrigation and through water conservation methods. Distribution of rainfall must also be taken into account in interpreting the life zones. If rainfall occurs over a relatively short period, followed by a dry season, some annual crops might not be able to mature. Return to TABLE OF CONTENTS.
Temperature. Temperature affects plant growth directly and indirectly. As temperature increases chemical activity increases and thus over a certain range temperatures increase growth. On the other hand, protoplasm cannot survive extremely hot temperatures, and many plants cannot survive temperatures below freezing. Special organs may be more susceptible to heat (reproductive organs, flowers) or to cold (succulent organs). Some organs, particularly some seeds, may resist both heat and cold. Furthermore, loss of water from plants and soil is increased by high temperatures (as well as by low humidity and wind.).
Plants are adapted to particular climates in part by their ability to grow and reproduce at certain temperatures. Among vegetable crops one can distinguish cool season crops (cabbage, lettuce) from hot season crops (corn, squash). Some crops grow best where days are hot and nights are cool (tomato).
Life zones as defined by Holdrige depend not only on annual rainfall but also on mean annual temperature. Return to TABLE OF CONTENTS.
Altitude. Altitude influences temperature and in this way affects plant growth. As altitude increases, temperature decreases. Latitude influ- ences temperature by influencing the amount of light intercepted by a unit area. It also influences daylength. Daylength influences plant growth through hormonal mechanisms which are part of a plants adaptability. For example, short day plants require or flower best in short days. Long day plants often flower best only during long days. Some plants are day neutral and their flowering is not influenced by day length.
Thus, life zones are influenced chiefly by annual rainfall and mean annual temperature. Some of the world's life zones as defined by Holdridge are given in the table. In any region of the earth a person should be able to determine the life zone by weather records. It may now be impossible to do so from the vegetation. Agricultural zones, however, are determined also by availability of irrigation water. Return to TABLE OF CONTENTS.
Soil acidity. The acidity of the soil, defined in terms of pH, is a third important factor deter- mining crop potential. While almost all crops grow well in soils with slightly acid pH (6.5), nevertheless crops differ in their tolerance of acidic (low pH) and alkaline (high pH) conditions. The acidity of the soil can be increased with the use of acid forming fertilizers (such as sulphates) and organic materials, or decreased with the addition of lime. These are common agricultural practices. Usually soils of the humid tropics are acid and those of the dry tropics alkaline, but there are exceptions. Return to TABLE OF CONTENTS.
Use of Table I and the Appendix. As a first step in determining whether a particular new crop (old crops are obviously suitable) may be suitable for your region, determine the life zone for the region from annual rainfall and temperature. Determine the normal pH of the soil of the region. Consult Table I and the Appendix. These will tell you only the crops which have been grown in similar regions. The tables cannot tell you if the crop will be a success in your particular region. Return to TABLE OF CONTENTS.
A second step. Classify your environment in a less formal manner than that of Holdridge. First, classify the environment during the period of maximum rains as follows: cool (C), intermediate (IT), or hot (H). Then consult Table II for annual crops and Table III for perennial crops.
Note in Table II that irrigation changes everything. If temperatures are favorable, all vegetables can be grown in a dry climate where water is added. This is probably true of fruits as well. Note also the other considerations in the case of some of the other crops.
Finally, you can use Table IV if your region falls into one of the following categories: hot humid tropics; tropical monsoon; dry tropics; beach climate; wet, cool highlands; and dry, cool low- lands. Consult the portion of the table which corresponds to your climate and find the grains, legumes, vegetables and other crops most likely to be a success. Return to TABLE OF CONTENTS.
Advance to Appendix (Species A - G)
Advance to Appendix Species H - Z)