Selecting Suitable Tropical Crops

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.

• Technical Note: Selecting the Right Crop for Your Location in the Tropics or Subtropics, by Dr. Franklin W. Martin
• Technical Note: Comparison Charts of Tropical Crops, by Dr. Franklin W. Martin and Michael P. Fennema
• What Seed Would You Take to an Uninhabited Tropical Island?
• How Can I Garden in the Hot Humid Tropics?

Dr. Frank Martin is the author of several books and articles on tropical subsistence farming and a frequent consultant to ECHO. We received from him the following interesting note:

"If I were to go to an uninhabited island in the hot, humid tropics, taking with me the seeds with which I think I could best provide myself food, I think I would take the following:

• Roots and Tubers
  (1) sweet potatoes--the variety 'Gem' (orange-fleshed) and some white-fleshed types,
  (2) yams--Dioscorea alata and D. esculenta, selected varieties,
  (3) cassava--some true seed to start my own,
  (4) Queensland arrowroot (Canna edulis), very easy to grow and productive.
• Grains
  (1) corn,
  (2) okra, for edible seed and well as green fruit,
  (3) wax gourd (Benincasa hispida) for edible seed as well as squash-like fruit.
• Legumes
  (1) Catjang cowpeas (climbing, disease resistant forms),
  (2) winged bean,
  (3) Dolichos lablab beans,
  (4) asparagus beans.
• Leafy Vegetables
  (1) chaya,
  (2) sunset hibiscus,
  (3) Tahitian taro (Xanthosoma brasiliensis),
  (4) Tropical or Indian lettuce (Lactuca indica).
• Fruit Vegetables
  (1) tropical pumpkin,
  (2) okra,
  (3) small-fruited, indeterminate tomatoes,
  (4) hot pepper,
  (5) ensalada pepper, selected for its edible leaves.
• Trees
  (1) bananas,
  (2) breadfruit,
  (3) limes (West Indian, from seed),
  (4) tamarind,
  (5) papaya,
  (6) mangoes (from seed, turpentine type but selected)."

Several of Dr. Martin's publications (co-authored by Ruth Rubert‚) are available from ECHO. We are reprinting their book Edible Leaves of the Tropics (see chapter on Tropical Vegetables). Techniques and Plants for the Tropical Subsistence Farm (see below) is an excellent introduction to a wide variety of food plants adapted to hot, humid regions. 

Letters from EDN readers often contain questions similar to this. Even experienced gardeners can get discouraged when they move to the humid tropics from a temperate country and plant the vegetables they know from home. Others of you have not had gardening experience in any climate, but now face the need to learn quickly. The approach of the two books described below is so different that they complement each other very well.

The best way to begin gardening in hot humid regions is to try those plants that God has clearly made for such climates. However, temperate crops are often in demand because of the increased variety that they add to the diet and their value as a cash crop to replace imported vegetables. Techniques and Plants for the Tropical Subsistence Farm is oriented toward plants that are adapted to the tropics. Growing Vegetables in Fiji is more oriented toward growing temperate vegetables, with some discussion of other vegetables.

Dr. Frank Martin and Ruth Ruberte‚ with the USDA's Tropical Agriculture Research Station in Puerto Rico wrote a 56-page book called Techniques and Plants for the Tropical Subsistence Farm.  It is an excellent introduction to a wide variety of foods that are adapted to hot, humid regions. Its scope is a bit broader than only gardening, as the title implies. The table of contents lists: overall planning; vegetables and cereals (leaves, legumes, roots and tubers, fruit vegetables, cereals); trees (fruit trees, vegetable trees, leguminous trees, trees for wood); forage crops (site selection & preparation, planting, management, grazing, storage, selecting forages, grasses, legumes, misc. forages). Their approach to insect control is less specific with an emphasis on organic methods. (It is quite likely that insect control is less of a problem with the native tropical plants.) Here are some excerpts.

"Phosphorous is an essential, limiting element in tropical soils as often as nitrogen. It is important in stimulating root development and is necessary for fruit and seed development. Although it is common enough in the soil, most of it is insoluble, unavailable for plant use. Manure, compost and cover crops do not add enough phosphorous to the soil" and tend to become insoluble when they are added. He then discusses deficiency symptoms. Commercial fertilizers are one source. "Bonemeal is a useful additive though much of its phosphorous is insoluble. Marine organic materials (seaweeds, fish) are other good sources. There does not seem to be an easy solution to the problem, but a soil with adequate humus and good aeration slowly releases soluble phosphates from the insoluble forms."

"Indian or tropical lettuce (Lactuca indica) from Southeast Asia is considered by some to be the best lettuce for the hot humid tropics. Like all lettuces, it requires a fertile soil. Seeds are small and seedlings require careful attention. [Ed: So many plants come up wild we no longer need to replant, however.] The plants grow rapidly and produce large succulent leaves. These may be harvested individually, or the tops may be snapped off... [to be] replaced rapidly by new growth. Once flowering begins, it cannot be suppressed, but leaves may be harvested until exhausted. Year-round production is easily achieved by planting every 3-4 months. Yields are excellent and the plants can be grown in pots. The lettuce is somewhat bitter in taste, but its flavor and texture are perfect for mixed salads." [Ed: It is also excellent as a cooked green. ECHO has seed.]

Kirk Dahlgren worked as the Rural Development Director for the Peace Corps in Fiji. He wrote a 123-page book for the Peace Corps called Growing Vegetables in Fiji. This book is an excellent general introduction to gardening in the tropics (or elsewhere for that matter). We think so much of the book that we have reprinted it (US$5 plus postage). As mentioned earlier, a special emphasis is placed on growing temperate vegetables. The climate is hot and humid in much of the country, similar to conditions faced by many of you. His writing is clear and choice of subjects excellent. He has an unusual ability to get right to the most important points and to explain them clearly in as few words as possible. The table of contents lists: the Fiji vegetable crop environment; building and maintaining soils; cultural methods of vegetables; the garden crops of Fiji; growing vegetables under plastic; composting and mulching. Some examples follow.

"The culture of many of these new crops required the learning of a totally foreign propagation method--the use of seeds." Traditional Fijian crops are propagated vegetatively, i.e. a piece of the plant other than a seed is used to produce new plants. "The vegetable crops the missionaries brought evolved in temperate zones where plants needed to produce resistant bodies, i.e. seeds, to survive the harsh winters." Crops the Indians brought had a similar need to survive dry months. Because in Fiji the weather is always just right for the plants to grow, it is often advantageous for plants to reproduce through means other than seeds.

Here is an excerpt from the discussion of carrots. "Fiji relies largely on carrot imports to satisfy local demand. Carrots do well in Fiji, however. ...Carrots are small-seeded, slow-germinating, and slow to establish so require a steady supply of moisture and a high measure of weed control. Quick growth produces better carrots. Carrots are high in vitamin A and have good keeping qualities." He then gives recommended varieties and detailed cultural procedures. "Show extra attention to weed control. Weeds in carrots can be controlled by spraying with kerosene at the three-true-leaf stage. The rate is 450 liters/ha (45 ml/square meter). Spray on a sunny day for best control." Diseases, pests and their control [usually chemical rather than organic] are discussed for each vegetable. He lists three common problems: cavities caused by calcium deficiency, galls caused by nematodes, and split root tips caused by excess soil nitrogen.