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Green Manure: What should farmers know before application?

Soil degradation is one of the principal cause of continuous decrease in crop production and the consequences of this is the reduced economic incomes and increased poverty levels among small scale farmers. The decrease in crop productivity is partly due to the declining soil organic matter levels resulting in low release of nutrients upon mineralization and also reduce nutrient use efficiencies when fall below 3.4%. The strategies adopted by farmers to improve crop yields involve application of fertilizers which in turn increases the production costs which majority cannot afford. So how farmers are reacting to declining soil fertility and crop yields?

Management strategies to maintain soil fertility on small scale farms should therefore be inclined to practices that utilize green biomass - cheap and sustainable! The practice can be incorporated well together with crop rotation and no-till systems for more sustainable farming. Green manuring involves turning under or incorporation of any green manure crops while at vegetative phase or soon after they flower into the soil. These materials can be used to provide soil cover thus protecting soil from erosion, reduce weed infestation and improve soil structure. It is noticeable to say that since green manure performs the roles listed above they increase economic returns, reduce dependence on herbicides and thus improve yield.

Decomposition of green manures changes the soil environment that results in poor crop germination and establishment and increased occurrences of plant diseases. Researchers have argued that during decomposition of leguminous green manures, plant community structure is affected thus limiting the diversity of productive environments through variety of mechanisms such as mechanical impediment and allelopathy. Some of these chemicals produced interfere with root development and enhance population of plant pathogenic fungi. Fresh plant residues stimulate the population of fungal pathogens such as Pythium, Fusarium and Rhizoctonia spp. and this depends on the degree of decomposition, thus increasing the incidence of plant diseases. Fresh green manure residues are rich in cellulose thus act as food source for many plant pathogens while mature composts of green manure origins provide conducive environments for growth of beneficial microbes such as Trichoderma, Penicilium, that outcompete or are antagonistic to the pathogens. The initial phase of green manure decomposition basically consists of residue breakdown and subsequent release of cell contents. At this stage the inhibitory effects on germination of crops by the applied green manure is high for majority of the residues, however, the inhibitory effects rapidly decrease during the period ranging from 14 days to 28 days for lablab green manure. Therefore, a quantitative assessment of the inhibition period can be based on the observation of germination and crop establishment window. The successful adoption of green manure for soil nutrients enhancement therefore depends on identifying green manures that do not suppress establishment of crops or one should practice relay planting system for about 28 days to reduce the effects of chemicals released during decomposition of green manures.