Green manure crops are crops that are [often times in North America] 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, including:
They provide large quantities of nitrogen for the soil.
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.
Green manure crops present no transportation problems, in contrast to compost and chemical fertilizers.
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.
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.
The cover they provide for the soil protects it from wind and water erosion.
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).
Some green manure crops provide human food, including various kinds of edible beans, peas, and pods.
Green manure crops can provide income, by selling firewood, food or feed (and maybe seed).
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.
Some green manures, when intercropped with basic grains, can control weeds, thereby eliminating costly weeding operations