Biomass Production in the Short Rains and Its Influence on Crops in the Long Rains: A Systems Approach in Organic Farming

Production of legumes for purposes of obtaining biomass is often restricted to seasons that are less optimum for economic crops, thus limiting the possibility of accumulating substantial biomass. A three and half-year study was conducted at Egerton University, Kenya, with the aim of determining the biomass and grain yield production of lablab (Dolichos lablab) during the short rains and the effects of lablab biomass incorporation (LAB) or replacement with farm yard manure (FYM) (2.5 t ha-1 or 5 t ha-1) on subsequent organic maize, potatoes and legume intercrops grown during the long rains. Results shown that the rainfall in the short rains season was highly variable and ranged from 164 mm (in 2005/06) to 744 mm (in 2006/07) and so did lablab biomass (0.04 - 2.41 t ha-1). Lablab biomass and grain yield were closely related to the total amount of rainfall received. Moreover, it was showed that a minimum of 2 t ha-1 lablab biomass could provide sufficient N (60 kg ha-1) removed by subsequent maize grains; yet to generate this amount of biomass; about 637 mm of rainfall is required under the trial conditions. Maize grain yields following LAB and FYM were 4.5 t ha-1 and 4.9 t ha-1 respectively (P = .05 in 2005), while potato tuber yields were 18.8 t ha-1 and 21.2 t ha-1 respectively (P = .05 in 2005) about 10% less yield in LAB compared to FYM. Both LAB and FYM increased the available N soon after incorporation, but reduced the soil pH over the trial period. Lablab grain yield and biomass were less by 17% and 14% respectively when planted after maize than after potatoes. The study concluded that rainfall largely affects the amount of green manure biomass which in turn affects the amount of N recycled, N availability and yield response of subsequent crops.