(updated June 14 2002)
For low elevations (0-1500 meters) or warm weather:
1) Velvet bean (Mucuna pruriens) The most popular is being used by Central American traditional systems and development programs worldwide (Central America, Brazil, Africa, India, etc.). It is one of the best nitrogen-fixers (150 kgs/Ha), the best weed suppresser, and grows in very poor soil being resistant to drought and heavy rains. Problems include need for pruning and cannot be grown with low-stature crops. Must be processed for human consumption or boiled if used as animal feed.
2) Lablab bean (Dolichos lablab, or Lablab purpureum, also known in Asia as "hyacinth bean") It is edible without special processing with a 23% protein content, grows as well as mucuna producing nearly as much biomass, perennial, and more drought-tolerant than mucuna. However, in some cases, needs good soil fertility and has suffered insect attacks.
3) Jack beans (Canavalia ensiformis) or Sword beans (C. gladiata) By far the most hardy of the known green manures, the jack bean will grow in dry climate and poor soils where virtually nothing else will grow. For either of these conditions, it is an excellent beginning green manure. It also fixes more N than the velvet bean (230 kgs/ha) and is perennial. And the tender pods can be eaten like string beans. In some areas of Honduras, farmers who started with velvet bean intercropped with maize are now converting to jack beans in order to avoid the pruning work. However, jack beans produce about 10% less biomass than velvet bean and do not control weeds as well. Also, jack bean plants and seeds are not palatable for animals, nor is it advisable to use jack beans for human consumption, unless boiled three times, discarding the water.
4) Vigna species Various local vignas are used traditionally and as introduced green manures, and their use should be promoted. These include cowpea, mungbean, rice bean, etc.
5) Other legumes used in Brazil and other areas include various crotalarias, the pigeon pea (Cajanus cajan), sunhemp, and many others. We are not yet using perennial soybeans and perennial peanuts (Arachis pintoi), because farmers in Nicaragua and Brazil complain about not being able to get rid of them.
6) Under trees (either bananas, oil palms, citrus, etc.), another series of legumes are useful. Usually these are slower growing perennials that climb less vigorously. Among these, the most prominent is the tropical kudzu (Pueraria phaseoloides) which is used widely to control weeds and provide N in Central America under fruit trees of many kinds. Tropical kudzu MUST NOT be confused with the common kudzu (Pueraria lobata). The latter should probably NEVER be introduced anywhere, as it rapidly becomes a very serious pest.
For intermediate elevations (1500 to 3000 meters)
Much less is known about these species, although there are a good number of them:
1) Scarlet runner bean (Phaseolus coccineus) is grown as an intercrop in maize by farmers in highland areas from northern Mexico to Peru. It requires about 5 to 6 months to grow, but reseeds itself naturally and does not need to be pruned.
2) Sweet clover (Melilotus alba) has been found to be a very good perennial soil improver and forage crop, and can be intercropped well with maize. Can be extremely good for marginal drought areas used only for forage.
3) Other cool-weather green manures used in Brazil and other
areas include two non-leguminous green manures that have
become popular, the forage turnip (Raphanus sativus) and
various kinds of oats (Avena spp). Other green manures from
this region include peas (Pisum sativum) and Vicia species
(especially V. sativa and V. villosa).
[Bunch29 1995] Bunch, Roland, The Overstory #29--Tropical Green Manures/Cover Crops
The Overstory is published by Permanent Agriculture Resources, P.O. Box 428, Holualoa, HI 96725 USA; Tel: 808-324-4427; Fax: 808-324-4129 email: email@example.com; site: http://www.agroforester.com/