In smallholder agriculture, green manure legumes can play a major role in conservation agriculture and improve farm productivity. They are fast growing, hence accumulates high biomass within a short period. They can increase plant nutrient supply in the soil especially nitrogen through biological nitrogen fixation and improve crop yields. They are deep rooting and thus lead to improved soil structure and recycling of nutrients. They provide good ground cover, which minimizes soil erosion through reduction of raindrop impact and runoff and improves soil physical properties. They smoother weeds, even the obnoxious ones like Striga hermonitica, thus reducing labour for weed control. They are an important source of human food and livestock feed.

The Legume Research Network Project was started in 1994 (by then known as the Legume Screening Network) to evaluate and identify suitable legume species for different agro-ecological environments and to subsequently incorporate the "best bets" into the existing farming systems. Initial network activities included screening of about 40 legume species, among them, green manuring species, food legumes and forage species, in 11 sites across Kenya, especially where soil infertility had been identified as a major problem for crop production. Over the years, the network extended its activities to cover research on legume residue management, integrated nutrient management, livestock feeding studies and to assess the value of the legumes in controlling striga (Striga hermonthica), a noxious weed common in western Kenya.

Presented in this manual are the best-bet green manure legumes for the different agroecological zones in Kenya, research highlights of potential benefits of the legumes, potential niches for GM legume technologies including cropping systems for incorporating the legumes into smallholder farms.

In the following descriptions of different legumes yield is given in t DM. This means tons of dry matter. N content in leaves is given in percentage.
It is therefore possible to calculate how much nitrogen a well grown cover crop will add to the soil as follows:

Time to maturity 3-4 months
Biomass yield 8 t DM ha-1. within 3-5 months
N content in leaves 3.6 - 4.7%
8 tonnes=8000kg. about 4 % of 8000kg =320kg N per hectare if the whole crop is incorporated into the soil. This is a lot - most crops need less than 100 kg/ha of Nitrogen to grow well. So this type of green manure yield allows for partial harvest before incorporating the rest of the crop as green manure into the soil

A study was conducted from 1998 short rains to 1999 long rains to evaluate the effectiveness of green manure legumes inter-cropped with maize in controlling soil erosion. In 1999 long rains when the legume was establishing, soil loss was almost similar in all treatments (see table). At the onset of the SR 1999 there was post-harvest crop cover from the 1999 LR season and this dramatically reduced soil loss in the first two weeks of the rains (see Table and photo). Another major result is that eroded sediment contained higher levels of organic C, total N, available P, and K than the original soil surface confirming that erosion removes soil particles enriched with plant nutrients.

M- Pure stand maize, M+M- maize intercropped with mucuna, M+V- maize intercropped with purple vetch, M+D- maize intercropped with dolichos.

Intercropping of green manure legumes with maize is feasible for the regions receiving bimodal rainfall where farmers plant two maize crops in a year. In this system green manure legumes are either planted at the same time with maize or planting is delayed by one to two weeks to reduce competition. The legume is planted in between maize rows. After the maize is harvested, the legume is left to continue growing during the short fallow period preceding land preparation for the following crop. As the land is prepared, the legume biomass is incorporated into the soil.

Lablab purpureus var. Rongai has been tested successfully as a cover crop for conservation agriculture in the dry Machakos district Kenya. Lablab established well and provides complete ground cover in three months. The legume is left on the surface as mulch. During planting of maize a furrow is created in the mulch for direct planted of the seed (photos below).

Green manure legumes are planted in plantation crops to provide ground cover for control soil erosion and weeds. Examples are given here.

Recommended Spacing and Seed Rate for Green Manure Legumes for pure Stand Establishment