Green manure crops
Updated February 1993
Tremendous advantages to the small farm in the third world
Jack bean. Canavalia ensiformis.
Velvet bean. Mucuna deeringiana.
Lablab bean. Dolichos lablab.
Sunnhemp. Crotalaria juncea.
A Poor Man's Plow.
Green manure crops are crops which are grown to be turned under to increase soil fertility. Leguminous green manure crops, i.e. those which can make nitrogen fertilizers from atmospheric nitrogen, can offer small-scale Third World Farmers a tremendous number of advantages:
1) They provide large quantities of nitrogen for the soil.
2) They add many tons of organic matter to the soil, thereby improving topsoil depth, water- holding capacity, nutrient content, friability, and texture of the soil.
3) Inasmuch as the green manure crop grows in place, it presents no transportation problems, in contrast to either compost or chemical fertilizers.
4) Green manure crops require absolutely no capital outlay after the initial purchase of a handful of seed. Because they require no chemical inputs, dependency on outside sources of fertilizer, nutrients, and pesticides is reduced.
5) Green manure crops can shade the soil up to eleven months out of the year, a factor extremely important in tropical climates for preservation of soil moisture and organic matter.
6) The cover they provide for the soil protects the soil from wind or water erosion.
7) Green manure crops provide generous amounts of high protein fodder for animals, which can be especially valuable if it is available during the last months of the dry season (inasmuch as fodder at this time of year is the limiting factor in traditional animal-raising in much of the Third World).
8) Some green manure crops provide human food, including various kinds of edible beans, peas, and pods.
9) Green manure crops can provide a cash income, by selling firewood, food or feed (and maybe seed).
10) They often provide an incentive for people to abandon harmful traditional practices, such as burning crop residues or letting animals loose in the dry season to devour everything in sight.
11) Some green manures, when intercropped with basic grains, can control most weeds, thereby eliminating costly weeding operations.
Something like 30% of all the increases in harvests achieved by small farmers in the third World during the last three decades has been achieved through the use of chemical fertilizers. Should petroleum prices shoot up once again, as could easily happen sometime in the next decade, prices of chemical fertilizers could easily become too expensive to be economically feasible for use with traditional basic grains. Almost overnight, Third world basic grain production could plummet, causing famines the extent of which would make the present situation in Africa seem mild by comparison. Widespread use of green manure crops could avert much of this impact.
Comparison with Compost
1) Compost merely decomposes the organic matter one already has, whereas a green manure crop can often add over 40 tons of additional organic matter per hectare. Inasmuch as organic matter is often in short supply on villagers' farms (or is already being recycled), this is an important consideration.
2) At best, compost will return to one's field about 98% of the nitrogen one started out with. A green manure crop, however, will add considerable quantities of new nitrogen to the system.
3) A compost heap takes a tremendous amount of work, as anyone who has personally made one can attest. Though compost will often pay in a vegetable garden, it is not economical when used on basic grain crops such as corn or millet. On the other hand, although a green manure crop takes a bit of labor to plant (using a dibble stick) and a fair amount of labor to incorporate, it takes nowhere near the labor a compost heap does. And in some cases where the green manure crop is intercropped among traditional crops (such as corn, sorghum, or millet), it covers the ground so well that one or even two weeding operations can be eliminated, thereby actually bringing a net savings in labor.
4) A compost heap requires water. This often means it is made near a water supply but at a fair distance from where it is to be applied. Green manure crops are planted to take advantage of available rain water, and are planted right where they will be used.
5) Compost cannot be used as a food source, either for animals or humans.
A Few Ideas About What to Look For The major problem with green manure use around the Third world is that village farmers cannot afford to give up land in order to just grow a soil amendment. Or when they have the land, they cannot spare the labor. However, there are three ways in which these objections can be overcome. In many situations only one of these will be appropriate, in others two. Experience so far seems to indicate that only rarely is none of them appropriate.
1) Green manure crops can often be planted amount traditional row crops, especially corn, sorghum, and millet, without decreasing the production of the main crop at al the first year, and usually with major increases in the major crop in succeeding years. The major instance in which this is not possible is when people are already intercropping two or three other crops with their major grain.
2) Green manure crops can often be intercropped with basic grains toward the middle or end of the growing season, with the idea that their major growth would occur during the dry season, thereby using land that would not ordinarily be under cultivation.
3) Wherever multiple-year fallows and/or shifting agriculture is used, green manures can be planted on land the first year it is to go fallow. Thus the period of fallow can be cut to one year instead of three to fifteen years.
What characteristics should we look for, then, in a legume that will be useful under these circumstances?
1) It must be a non-woody annual with vigorous growth.
2) It should grow well in the poorest of soils in the area, without needing any kind of fertilizer.
3) One must be able to plant it in local fields with no special soil preparation, and either with a dibble stick or, preferably, by broadcasting the seed.
4) The plant must have few enough natural enemies that it will grow vigorously without the use of any pesticides or major labor requirements.
5) The legume should either be very shade- resistant (for intercropping) or drought-resistant (for growing into or through the dry season).
6) If possible, it should first cover the ground well, then climb any stalks that remain in the field.
7) If possible, the green manure crop should be edible by animals and/or humans.
Some Already Known Possibilities
1) Canavalia ensiformis(jackbean, etc.) is highly shade and drought-resistant covers the soil, climbs extremely well, and is edible by animals. It does well from sea-level to about 1,800 m. It has almost no natural disease or insect pests. It can be dibble-sticked (at 2 seeds/sq.m.) or broadcast (at 4 per sq.m.) in among other crops. I have seen it grow vigorously on soil so badly eroded and depleted that no weeds would grow there at all. A high-protein fodder, the pods and beans can also be eaten by humans if certain precautions are taken.
2) Stizolobium spp. (velvetbean) covers the soil and climbs much like the jackbean. It grows even more vigorously than the jackbean under less harsh conditions (in Yucatan, where droughtiness is always a problem the jackbean does better in years of low rainfall, whereas the velvetbean does better when rain fall is higher than normal, but still scarce). Be sure to use a variety that does not have the irritating itchy powder on the pod (we have such a variety from Honduras). It grows from sea-level to 2,000 m. So far no natural diseases or pests have been observed in Central America, where it is native. It can be dibble-sticked (2 seeds/sq. m.). A high-protein fodder, the beans can also be toasted and ground to make a tasty high-protein "coffee", or used to "stretch" real coffee.
3) Clitoria ternatea is even more drought- resistant then the Canavalia, although being small-leafed, it does not cover the soil well. We really do not have much experience with this plant yet. It grows well at sea-level.
4) Dolichos lablab etc. (lablab bean) also covers the soil and climbs, much like the velvetbean. On semi-fertile soils around 1,200 to 1600 m., it grows very well with good shade-resistance, and so fast that it should not be planted in corn until at least 2 months after corn is planted. I do not have wider experience with this plant yet. It produces good forage.
[Ed: Young pods of some varieties are quite tasty when cooked. Dr. Andrew Duncan recently told me that he saw a variety with an exceptionally wide pod growing on sides of village houses in Bangladesh.]
2) On very steep hillsides, something must be done to keep the organic matter from washing away. Piling crop residues along roughly contour lines can help, as can contour ditches. Another possibility is incorporating the green manure immediately after cutting it, but this is hard work before the rains come (if soil is a heavy one), and once the rains have come, people generally do not have extra time.
3) On flatter land, the green manure should usually be cut and allowed to dry for a couple of weeks before incorporating it (if during the dry season). The labor saved in incorporating it will be worth more to the farmer than the small amount of fertility lost. In one case farmers cut holes in the Canavalia cover to plant corn when the rains came, cut down the Canavalia entirely about two weeks later and replanted the Canavalia. Then, two weeks later, they incorporated the dead Canavalia vegetation. In this manner, they avoided both weeding operations in their cornfields!
4) Where weather is unreliable, a combination of similar plants, one of which is more drought- resistant (e.g. jackbean and velvetbean) reduces risk of total loss, yet assures a vigorous crop if rains are plentiful.
5) In West Africa, we are trying a system of planting a perennial every sixth row (pigeon pea), and then gathering the corn or millet residues under the pigeon pea plants at the end of the year, to be distributed six months or so later when well-mixed with better C:N pigeon pea leaves. The presence of the pigeon pea trees (already known as a cash crop) will also prevent burning of residues.
6) On South and Southeast Asian hillside areas, Leucaene leucocephala is planted as a contour barrier and constantly pruned, thereby providing erosion protection, some green manure, and firewood (see the booklet produced by World Neighbors called Leucaene-based Farming). This produces less green manure than other systems, but can be sued where green manure cannot be intercropped among traditional crops.
7) We certainly would welcome any experience yo have in this subject. Much more information and experimentation must be done. We readily admit we are just getting started at this, but the positive response from hundreds of villagers and dozens of other programs has made us decide to share what little we know as soon as possible so we can all work together to learn more about it. I would think that, right now, the most important subjects we need to learn more about are:
a) What legumes will work above 1,800 m.?
b) What additional plants will work at any elevation?
c) Do adaptive research to see what of these technologies will work outside the Southern Mexico/Central American habitat that this information comes from.
d) Which legumes can be broadcast rather than planted with a dibble stick?
e) Which of these green manure crops provide the best yield increases with which basic grain crops?
f) What green manure crops would be best under high rainfall conditions?
g) Where can I get seed?
Thanks a million Rolland! (Martin speaking now). We have a modest amount of one vigorous variety of velvet bean that we can share. We will fill seed requests for Rolland's variety of velvet bean as well as Clitora ternatea, sword bean and jack bean in January if it does not freeze this year. We have plenty of lablab beans, pigeon peas and leucaena, including a variety that is hopefully less toxic because it is low in mimosine. If we cannot supply seed, we will see if Rolland can help, though note that he cannot meet the local demand. Please note that seeds for jackbean, swordbean, and velvetbean are quite large. We will send approximately 6 seeds of these, which will come in a very bulky envelope or a small package (if such packages do not get through to you, perhaps someone will be visiting that can bring them in). This will give you a few plants to begin increasing your own seed. Do not ask for pigeon pea if it is an important crop in your area, as you do not need it and we want to minimize risk of diseases of established crops. If you want more than 6 seeds of the larger seeded varieties, please send a couple dollars to help with postage. Some of you have ideas and experience with groundcovers. Let us hear from you.
A comment on green manure from zaire
Pete Ekstrand just visited us and had this account from the Paul Carlson Medical Program in Zaire. They have found that Pueraria phaseoloides (tropical kudzu or puero) grows vigorously and can even smother the vigorous native imperata grass if the grass is manually bent over. This is not the same kudzu that took over so much land in Alabama and elsewhere. They then cut circles perhaps 2 meters wide and plant fruit trees, coffee, etc. in the middle. It had not rained for 60 days when he visited and the ground in the circles was hard and dry. But one arm length under the ground cover the soil was moist and could be molded with the hand! We have not succeeded in harvesting our own seed, so let me refer you to the Yates Seed Co., P.O. Box 117, Rockhampton, 4700 Australia. Yates is an excellent source for a large number of tropical pastures.
update: what we have learned to date about green manure crops for small farmers
In spite of the advantages of green manures, their use has seldom become common among farmers in the Third world. They cannot afford to give up scarce cropland just to grow a soil amendment. If they do have the land, they cannot afford the labor. Nor are they generally willing to spend money to improve crops grown for subsistence, because they earn no money to improve crops grown for subsistence, because they earn no money from them with which to replace what they have spent. World Neighbors/Central America has found a number of ways to overcome most of these problems to the extent that farmers have accepted green manures faster than any other agricultural technology with which we've worked through the years. One program sold 65 pounds of seed last year to local farmers and 1500 pounds this year in the same area with minimal promotion. There are six ways to produce green manure without reducing at all the land used for other crops.
(1) Plant among traditional row crops.
(2) Intercrop near harvest of the first crop timed so green manure will grow primarily during the dry season.
(3) Where shifting agriculture is practiced, plant during the first fallow year to shorten the fallow period.
(4) Alley cropping.
In Central America our work has used the first four possibilities. We have had the most success with jack bean and velvet bean.
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