Biomass accumulation and nitrogen fixation of soybean varieties, grain yields of soybean and maize intercropped at different planting densities in western Kenya

Biomass accumulation and nitrogen fixation of soybean (Glycine max) and grain yields of maize (Zea mays) and soybean intercropped at different planting densities was assessed in two villages of Butere and Mumias district of western Kenya during the short rains of 2011 and long rains of 2012. A randomised complete block design with a factorial arrangement replicated thrice was used. Three soybean varieties NAMSOY 4m, SC SQUIRE and TGx 1987-18F were intercropped with imidazolinone- resistant maize (IR maize). Five maize: soybean planting densities; sole maize (D0), 1:1 (D1), 1:2 (D2), 1:3 (D3) and sole soybean (D4) were tested. Above ground biomass was determined by destructive sampling of plants, nitrogen fixation assessed using the 15N natural abundance method and yields were assessed from the net plot at harvest. There were significant differences in above ground biomass, N fixation and grain yields, between planting densities and varieties (P< 0.001). Treatment D3, for TGx 1987-18F variety produced the highest above ground biomass (3.8 t ha-1) and the highest N fixation (104.2 kg ha-1) in Manyala in the short rain season. Grain yields of soybean were high in D3 for SC SQUIRE variety (1.8 t ha-1) in Manyala, in the long rain season. Treatment D1 with TGx1987-18F variety recorded the highest grain yields of maize (4.7t ha-1) in Manyala in the long rain season. The lowest above ground biomass and N fixation was recorded in treatment D1 with variety NAMSOY 4m in Lubino during the long rain season, 0.8 t ha-1 and 21 kg ha-1 respectively. Low grain yields of soybean were recorded in treatment D1 with variety NAMSOY 4m in Lubino during the short rain season (0.1 t ha-1) while low maize yields were recorded in treatment D3 with variety NAMSOY 4m in Manyala during the short rain season (1.0 t ha-1). Treatment D3 with TGx 1987-18F variety was the best in terms of biomass accumulation and N fixation, however, to build soil fertility while at the same time optimising yields, treatment D2 with SC SQUIRE variety, above ground biomass (2.6 t ha-1), N fixation (61 kg ha-1), soybean and maize grain yields of 1.2 t ha-1 and 2.8 t ha-1 respectively is worth adopting in the smallholder farming system of western Kenya.