Evaluation of herbaceous legumes for soil Fertility improvement in maize/cassava cropping systems in coastal lowland Kenya

Poor soils and erratic rainfall limit crop yields in coastal Kenya. Legume cover crops and intercrops have potential to improve land productivity by increasing soil fertility through nitrogen (N) fixation and decomposition of foliage green manure. This study evaluates the effects of harvest frequency and stage of development of annual and perennial forage legumes on biomass and nutrient accumulation of legume residue, rate of nutrient release from legume residue, availability of residue derived nutrients to maize and/or cassava and soil 'Water availability during the cropping season. Agronomic experiments were planted at KARl Mtwapa. The site was in coastal lowland agro-ecological zone 3 (CL3). The soils are orthic Acrisol to Luvisols, sandy loam to sandy clay loam. The experimental design was split-plot, where the main plots were planting in pure stand of legume, maize and cassava and their combinations. The subplots were legume harvesting management: clitoria cut at two months and then after 6 or 10 weeks, and dolichos cut at 2 or 4 months after planting (MAP). The control plots were with or without inorganic fertilizer. Data is reported on the yield of maize, cassava and legumes during the first year of experimentation, and measurement of soil inorganic nitrogen (inorganic N) at two dates. The legumes exhibited different growth rates, underlining the importance of developing appropriate management systems for various legume species. During the long rain (LR) season when clitoria was establishing, intercropping maize with clitoria led to 27% more maize grain yield than maize without organic or inorganic fertilizer input. The reverse was true during the short rain (SR) season, when the presence of already well-established clitoria reduced maize grain production by 78%, such that the cumulative maize grain produced from the two seasons was 30% lower than the no input control. The high green manure production, as a result of dolichos fast growth during the LR season led to high plant nutrient supply, 106 and 253 kg ha'! for 2 and 4 MAP, respectively. The shading effect by dolichos on maize and cassava reduced the LR season maize grain yield by 27 and 49% when cut at 2 and 4 MAP, respectively. Cassava fresh tubers yield was also reduced by 50% when dolichos harvest was delayed to 4 MAP. However, during the SR season when moisture was limiting, the nutrients were available to the maize crop, such that the cumulative maize grain yield from the two seasons was 37% higher than theJlo input maize pure stand control. Compared to no. input control, fresh cassava tuber ~eld at 10 MAP was not affected by the presence of clitoria or dolichos harvested at 2 MAP. There were measurable changes in soil inorganic N concentration. The soil inorganic N was mainly in the form ammonium (~-N) and was abundant in the topsoil (0-20 cm) compared to the subsoil (20-40 cm) depth. These preliminary results indicate that the legumes, and the foliage harvesting management strategy influenced the maize grain and cassava production and the soil inorganic N con ten t.