I agree with the comments above. Besides the N content of the residue, soil moisture, temperature , and pH are other factors that will affect  the decomposition rate of the crop residue. Soil acidity (lower pH) which can be an issue in most topical soils can limit microbial activity.

Hello Yolani,

Adding to the responses above,onsite farmland decomposition rates also depend on management regimes,landscape type and production systems.For instance, are the residues being used on slopes, gullies, marshes,ankara,slash and burn, mixed cropping, integrated,intensive,extensive or pasture? I will suggest you consider this point in your study.

Goodluck.

Among the crop residues, legume residues are easily decomposed material. Because of high N content in legume residues, the decomposition rate will be faster. Hence, the legumes mentioned above will decompose faster under favourable condition.  

As already mentioned by others, the residues of cowpea and lablab are expected to enhance the soil fertility of the soil particularly in terms of availability of nitrogen and phosphorus. They will add N to soil through the process of symbiotic N fixation. But the success of N addition will depend on the status of native population of specific strains of  Rhizobium bacteria in soil and the purpose for which the crop is grown - whether for grain of fodder. The addition of N is likely to be more when grown for fodder purpose instead of grain purpose. The organic acids released during decomposition of legume residues help to solubilise the fixed or insoluble forms of phosphorus in soil.

In addition to atmospheric nitrogen fixation, Cowpea has strong and long roots. These roots break soil layers and allow following crop cultivated in rotation benefit from this disturbance: more moisture content, presence of macrofauna etc.. In long term trial in Burkina Faso, sorghum roots cultivated in rotation with cowpea were measured up to 40 cm deep while in other plots (without cowpea), 80-95% of sorghum roots remain in upper layer (

I also had an experience using lablab in pastures as intercroping with increased yields on the companions plants. It seems to fix a godda amount of nitrogen there is a lot of literature abut it. I can sed you some.

Factors affecting decomposition are well covered above.  In addition, in relation to soil fertility, consider what factors determine the amount of N-fixation in the first place. 

First you need effective nodulation, which can be checked by carefully digging up roots and looking for large pink nodules close to the crown  (use a razor blade to cut the nodules).  Failure to nodulate may mean that inoculation is necessary, but first check that high quality inoculant is available and that farmers are able to apply it effectively.  If not, look for genotypes that nodulate well with Rhizobia presently in the soil.

Second, the legume needs to grow well, generally meaning that other nutrients are adequate (like P) - a small plant will fix little N.

Third, soil mineral N needs to be low - a fertilised crop like maize can leave residues of N in soil that inhibit nodulation/N-fixation.

Finally, if grain is harvested, the legume may add little N to the soil, although it should have grown without further depleting N in the soil.

Nitrogen rich crop residues enhance the status of soil N and thereby soil fertility. Cowpea, and lab lab bean is not exception. There are several literature where beneficial effects of legumes have been documented.

Dear Yolani

As a leguminous crop cowpea residues would enhance the soil nitrogen status (Organic matter content) besides several other nutrients to the soil at varied quantities. In general the C:N ratio is a major driving factor for its decomposition. Since legume C:N is inherently low, this factor need not be considered. Other essential factors are soil microbial load, temperature, sol moisture, incorporation status would decide its decomposition and nutrient release. 

I think that previsous answers already gave you many informations. I would therefore recommend you, if posible for you, to get a copy for the following book which is still a reference in this topic and that will give you many detailled answers about biochemical parameters to take into acount to estimate plant decomposition and potential impact on soil and synchronicity with crop needs.

Title  : Driven by nature - plant litter quality and decomposition

Eds G Cadish and KE Giller

CABI publishing, 1996, 409p.

Both of these plants are legumes, therefore symbiotic N fixation will occur provided there are no stresses like salinity/sodicity/ water deficiency/ very high or very low temperature/disease or pest attack. The result will be addition of N into the soil and improvement of fertility status. If the residues of these plants are incorporated into the soil, the organic matter content will also increase resulting in improvement of physical properties of soil like: CEC, water holding capacity, porosity etc.  

Interesting response Dr Nazir. The most important of all is the stage at which cowpea is harvested and turned into the soil , to decide the mineralisation rate and simultaneous release of nutrients to have  an eventual effect on soil fertility . i think, this will have entire of soil properties with complete cascading effects , right from aggregate stability to water transmission properties to improvement in plant available pool of nutrients..since copea is liguminous in nature with narrower C:N ratio compared to non-liguminous palnt residues , facilitate  accelerated  mineralisation rate..