Green Manure / Cover Crop Legumes

Introduction

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 datasheet 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.

How to start

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:

Example - Crotolaria as given below:

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=8,000 kg. about 4 % of 8,000 kg =320 kg 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

Best-Bet Green Manurel Legumes and their Characteristics

Mucuna Pruriens

© Joseph G. Mureithi, KARI, Kenya

Crolataria Ochroleuca

© Joseph G. Mureithi, KARI, Kenya

Crotolaria Ochroleuca

© Joseph G. Mureithi, KARI, Kenya

Crolataria Ochroleuca - Sunhemp seeds

© Joseph G. Mureithi, KARI, Kenya

Canavalia ensiformis seeds

© Joseph G. Mureithi, KARI, Kenya

Canavalia Ensiformis

Lablab Purpureus

Lablab purpureus

© Joseph G. Mureithi, KARI, Kenya

Lablab purpureus seeds

© Joseph G. Mureithi, KARI, Kenya

Lablab Purpureus 

Desmodium uncinatum

© Joseph G. Mureithi, KARI, Kenya

Desmodium uncinatum seeds

© Joseph G. Mureithi, KARI, Kenya

Desmodium Unicatum

Neontonia wightii

© Joseph G. Mureithi, KARI, Kenya

Neontonia wightii seeds

© Joseph G. Mureithi, KARI, Kenya

Neontonia Wightii

Potential Benefits of Green Manure Cover Crop Legumes

Potential for soil improvement through integrated nutrient management

Higher maize yields are obtained by combining green manure legume with FYM and inorganic N. Combining green manure legume with inorganic fertilizers saves on fertilizer requirement (Figure). Application of 15 kg N ha-1 to the GM treatment increased maize yields by 38%, but doubling the N rate only increased yields by an additional 3%. On the other hand, maize grain yield in FYM treatments only increased by 4% after application of 15 kg N ha-1, while doubling the N rate to 30 kg ha-1 increased yield by 23 %. These results suggest that maize grain yield can be greatly improved by integrating GM, FYM, and inorganic nitrogen. 

Green manure legumes can increase plant nutrient supply in the soil. Incorporating mucuna biomass (4 - 11 t DM ha-1) into the soil increased maize yields by between 40 and 120% in five Network sites. The additional labor required for incorporating mucuna into the soil was compensated by increased maize yields. 

Response of maize grain yield to inorganic NFYM, green manure legumes and their combination in Kakamega,  Western Kenya

© Joseph G. Mureithi, KARI, Kenya

Potential for soil erosion control 

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.

Table: Cumulative soil loss (t ha-1) and percent crop cover two weeks after the on-set of the rains during the experimental period.

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

Potential for soil moisture conservation

Leaving legume biomass on the surface as mulch gave better yields than incorporating it into the soil in the Machakos site, which is in semi- arid environment region. This was attributed to an effect on moisture conservation. 

Potential for feeding livestock

Dairy cows fed on mucuna and lablab forage had a daily DM intake of about 9.2 kg cow-1, which was similar to cows fed on Gliricidia sepium forage, a proven fodder tree for the coastal Kenya. Milk yield (6.5 kg day-1) was only 8% less than that produced by cows fed on gliricidia forage.

Goats supplemented with Neonotonia weightii gained on average 16.37 g while those on basal diet alone lost 23.81g daily in Katumani, Machakos.

Post harvest legume cover

© Joseph G. Mureithi, KARI, Kenya

Cropping Systems/Niches

Intercropping green manure legumes with maize

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. 

Vetch/Maize intercrop (left) and Mucuna/Maize intercrop (right)

© Joseph G. Mureithi, KARI, Kenya

Late intercropped legumes in maize

In the regions receiving unimodal rainfall, the only maize crop of the year is usually planted in April. The legume green manure system for such a region requires that legumes are planted in August between maize rows and that lower maize leaves are pruned to reduce competition for light. The maize is harvested in November/December and the legume is left growing in the field as short-term fallow until land preparation for the long rainy season maize crop. This system suits sites such as Kitale.

Late planted mucuna into maize

© Joseph G. Mureithi, KARI, Kenya

 Intercropped with vegetables 

Some farmers indicated that legumes could be intercropped with vegetables like kales. Crotalaria was most preferred for this purpose because it grows upright and does not coil on the kale plants.

Crolataria kales intercrop

© Joseph G. Mureithi, KARI, Kenya

Rotational short-term fallow with green manure legumes

Regions that receive bimodal rainfall but have an unreliable second season or where farmers do not grow maize during the second season are suited for a rotational system. The maize crop is planted as a pure stand during the long rains in February/March and harvested in July/August. Green manure legumes are planted in September as a short-term fallow crop and incorporated during land preparation in January/February of the following year. This system is practiced in areas like Kakamega.

Crotolaria short term fallow before maize

© Joseph G. Mureithi, KARI, Kenya

Cover crop for conservation agriculture in dry areas

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)

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Lablab cover crop

© Joseph G. Mureithi, KARI, Kenya

Furrow for direct palnting in lablab

© Joseph G. Mureithi, KARI, Kenya

Cover crops for plantation

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

Neontonia in sisal

© Joseph G. Mureithi, KARI, Kenya

Mucuna in citrus

© Joseph G. Mureithi, KARI, Kenya

Mucuna in coffee

© Joseph G. Mureithi, KARI, Kenya

Garden pea in papaya

© Joseph G. Mureithi, KARI, Kenya

Cover crop as a nurse crop for establishing plantation crops

When establishing plantation crops (e.g., coffee, tea, coconut, citrus, sugarcane etc) cover crops can provide excellent ground cover for weed suppression and control of soil erosion. For example lablab was used as nurse crop in establishment of sugar cane in western Kenya (see photo)

Lablab as nurse for sugar cane

© Joseph G. Mureithi, KARI, Kenya

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

Information Source Links

• Joseph G. Mureithi , Kenya Agricultural Research Institute