Overstory #29 - Tropical Green Manures/Cover Crops
In Overstory #20, Roland Bunch shared his five principles for maintaining farm fertility. In this issue, Roland Bunch summarizes the most important aspects of what is known about green manure/cover crops in the tropics for maintaining or improving farm productivity.
Tropical Green Manures/Cover Crops
What are green manures/cover crops? The terms "green manure" and "cover crops" originated from practices of using primarily leguminous plants and plowing them under to fertilize soils. However, as the practice has spread to the tropics, different conditions have generated different uses, and the practice has changed. The terms remain although many tropical farmers do not use the vegetation green, nor do they normally plow it under as one would a manure. The terms now refer to a series of plants, mostly leguminous, which are used by farmers for a whole range of purposes, one of which is the fertilization and improvement of the soil by applying the vegetation to the soil surface. In the following, the term green manure is used to cover both green manure and cover crops.
Advantages and Disadvantages of Green Manures for Village Farmers
The proven advantages of green manures (gm's) are numerous. The first six of those listed below would apply to virtually all leguminous gm's, while the last two would apply only to selected ones:
- + Organic matter. Gm's are capable of contributing up to (and occasionally more than) 50 T/Ha (green weight) of organic matter to the soil during each application. This organic matter has a whole series of positive effects on the soil, such as improving its water-holding capacity, nutrient content, nutrient balance, friability and pH.
- + Nitrogen. This organic matter also adds significant quantities of nitrogen (N) to the farming systems. Common levels of fixed N reported are about 150 kg/Ha, with some species yielding even more. This means that farmers can, even allowing for significant loss of N to the air, add to their systems quantities of N that would cost them at least US $75/Ha in the forms of chemical fertilizer.
- + Reduced costs. These additions of organic matter and N are achieved with no transportation costs; they are produced right in the field. Gm's require no capital outlay once the farmer has purchased his/her first handful of seed.
- + Weed control. Gm's can also be an important factor in reducing weed control costs, especially when used as mulch. Thus, the use of gm's can not only reduce the use of chemical fertilizer, they can also reduce or eliminate the use of herbicides.
- + Soil Cover. The soil cover provided by many gm's can be very important for soil conservation. A careful study has shown that farmers cultivating monocropped maize on 35% slopes with a 2000+ mm rainfall in northern Honduras are actually INCREASING the productivity of their soil year by year, with no conservation practices except for the fact that the soil is covered by velvet bean ten months out of the year.
- + Transition to zero-till. The use of gm's allows farmers to switch to zero-till systems while maintaining productivity on land. Since plowing and weeding are two heavy operations that have always provided a major advantage to those farmers capable of mechanizing their agriculture, and since gm's are often capable of eliminating both of these operations, it is quite possible that non-mechanized and/or hill-side farmers once again have the chance of competing with their wealthier, mechanized competitors. In an age of falling trade barriers, this could mean the economic salvation of millions of the world's small farmers.
- + Gm's can be effective in ending migratory ("slash and burn" or "swidden") agriculture and agriculture burning by improving soil fertility and controlling weeds.
- + Gm's can also provide food for people and fodder for animals, though the latter practice will, of course, reduce its value to the soil.
Each of these advantages should be analyzed and weighed when we choose gm's. It is quite infrequent that farmers are primarily attracted by the gm's ability to increase soil fertility. Much more commonly, farmers are motivated by the gm's multiple uses, for human food, weed-control capabilities, or the possibility of no longer having to till the soil.
Thousands of farmers in Honduras, Guatemala, and Mexico traditionally use gm's regularly, and the practice has often spread, gradually but spontaneously, without any outside encouragement. But, in general, and in spite of the advantages named above, the use of gm's is still surprisingly rare. Why?
- - Farmers will not plant gm's where they could plant either subsistence or cash crops. This means that the land they use to grow gm's must have no known opportunity cost.
- - The improvement of the soil is a long-term factor which is not immediately noticeable to the farmer. Usually, significant improvement in productivity does not occur until after incorporation, which means visible results are not apparent until well into the second cropping cycle or second year. This slow appearance of a result that is difficult to believe anyway, complicates the adoption of gm's.
- - Often gm's must either continue to grow or form a mulch during the dry season. Grazing animals, agricultural burning, termites and a host of other problems may prevent their lasting very long during this period.
- - Conditions such as extreme drought, extreme infertility, extremes in pH, severe drainage problems, and other problems common on poorer village farms, will affect gm's almost as much as traditional crops, thereby reducing the impact of gm's. In an increasing number of cases, we can overcome such problems (for instance, we have just heard that Desmodium triflorum is flourishing in soils with a pH lower than 4.0 in Belize), but often such solutions are achieved at the cost of reduced biomass production, reduced N fixation, and/or reduced number of niches in which the gm's can fit.
Although these species are the best known to date, we should all be constantly looking for alternatives.
For low elevations (0-1500 meters) or warm weather:
- Velvet bean (Mucuna pruriens) This is by far the most popular of the gm's used both by Central American traditional systems and development programs worldwide (Central America, Brazil, Africa, India, etc.), and for good reasons. Of the legumes, it is one of the best N-fixers (150 kgs/Ha), and one of the best weed suppressors. It grows well in very poor soil and resists both drought and heavy rains quite well. Its biggest problems are that it is an aggressive climber (must be pruned often, or associated with maize, and cannot be grown at all among shorter-stature crops); it is already too widely used; we must avoid its becoming so widely spread and frequently used that it develops major insect or disease problems; it is perhaps not advisable for human consumption; and the seed must be boiled for most animal consumption.
- Lablab bean (Dolichos lablab, or Lablab purpureum, also widely known in Asia as "hyacinth bean") The lablab bean has a whole series of advantages over the velvet bean: it is edible and good tasting without any special processing; it grows just as well as the velvet bean and produces nearly as much biomass; it is the palatable for animals of all the gm's we use, with a 23% protein content; it is perennial, often staying green and producing seeds through four or five months of drought; and it is significantly more drought-tolerant than the velvet bean. Nevertheless, in some cases (perhaps in the absence of rhizobia bacteria) the lablab bean has been fairly demanding of good soil fertility, and in others has suffered insect attacks that have significantly retarded biomass production.
- Jack beans (Canavalia ensiformis) or Sword beans (C. gladiata) By far the most hardy of the known gm's, the jack bean will grow where either the climate is so dry or the soils so poor that virtually nothing else will grow. For either of these conditions, it is an excellent beginning gm. 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. Furthermore, there are both bushy and climbing varieties, although all the bushy varieties we know of do some climbing when grown under shade. 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 beans plants and seeds are not palatable for animals, nor is it advisable to use jack beans for human consumption, unless heavily boiled.
- Vigna species. Various local vignas are used traditionally and as introduced gm's, and their use should be promoted.
- Other legumes used in Brazil and other areas include various crotalarias, the pigeon pea (Cajanus cajan), sunnhemp, 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.
- 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:
- Scarlet runner bean (Phaseolus coccineus) is grown as an intercrop in maize by farmers from northern Mexico and into Honduras. It requires about 5 to 6 months to grow, but reseeds itself naturally and does not need to be pruned.
- Sweet clover (Melilotus alba) has been found to be a very good soil improver and forage crop, and can be intercropped well with maize. However, it is difficult to get rid of, so we are not promoting it heavily as yet.
- Choreque (Lathyrus nigrivalvis) and other L. species. Choreque is used traditionally in one part of the Guatemalan highlands as an intercrop for maize which then grows exuberantly throughout the six-month dry season. It produces more biomass than any other gm we know and is excellent for dry season fodder production, but is severely limited by its environmental requirements: it needs a very fertile soil, does poorly the first years it is grown in a given soil, and requires cold climate but cannot withstand frost. Thus it can only be grown in quite fertile soils between 1800 and 2100 meters in elevation.
- Other cool-weather gm's used in Brazil and other areas include two non-leguminous gm's that have become popular, the forage turnip (Raphanus sativus) and various kinds of oats (Avena spp.) Other gm's from this region include peas (Pisum sativum) and Vicia species (especially V. sativa and V. villosa).
To sum up, green manure plants are usually valued for their multiple uses, especially for food, and secondarily for their long-term effects on maintaining or improving the productivity of the land. Experience leads us to believe that, with the possible exception of very intensive farming systems such as irrigated vegetables and rice, green manure and cover crops systems can probably be introduced into many, if not most, of the world's small-scale farming systems.
The above text was summarized from the following article by Roland Bunch of COSECHA, Honduras: The Use of Green Manures by Villager Farmers: What We Have Learned to Date, Technical Report No. 3, 1995, CIDICCO, Apdo. Postal 4443, Tegucigalpa MDC, Honduras C.A., e-mail firstname.lastname@example.org
Other selected references: The best book we know on this issue is Claudino Nonegat's Plantas do Cobertura do Solo, 1991. Claudino Monegat, Chapeco, SC, Brazil.
Achieving Sustainability, ILEIA Newsletter, October 1997, PO Box 64, 3830 AB Leusden, The Netherlands, e-mail: email@example.com
International Institute of Rural Reconstruction. Agroforestry Technology Information Kit, 1990. IIRR, Room 1270, 475 Riverside Dr., New York, NY 10115.
Related Editions to The Overstory
- The Overstory #70--Rhizosphere
- The Overstory #66--Carbon Sequestration: Storing Carbon in Soils and Vegetation
- The Overstory #28--Microlife
- The Overstory #22--Pioneering Difficult Sites
- The Overstory #20--Five Fertility Principles