Abstract:
Agriculture is the dominant source of livelihood in rural areas of sub-Saharan Africa. However, in semi-arid eastern Kenya, smallholder farm productivity has been diminishing due to declining soil fertility and frequent droughts. Nitrogen (N) is the principal nutrient element limiting crop productivity in the region, followed by phosphorus (P). The use of N fixing legumes in intercrop with cereal crops is among the cheaper-viable options available for the resource-constraint farmers to enhance soil fertility and improve grain yield. A one-season study was carried out in 2 sites during the short rain season of 2012, to determine the effects of Rhizobial inoculation and inorganic P on soil chemical properties and productivity of sorghum-cowpea intercrop in Kitui and Makueni Counties, Kenya. The treatments were inoculation with Rhizobium (with and without inoculation), two rates of inorganic P fertilizer (0 kilogramme P ha-1 and 30 kilogramme P ha-1) and three cropping systems (sorghum - cowpea intercrop, and two controls of sorghum sole crop and cowpea sole crop). The treatments were laid out in a Randomized Complete Block Design (RCBD) with three replications. Inoculation and inorganic P markedly enhanced residual soil N and P after harvest. The combined use of Rhizobial inoculants and inorganic P significantly (P < 0.05) improved nodule production, N and P uptake, growth and yield. Performance of the studied treatments were in the order of inoculation + inorganic P > inorganic P > inoculation > control. For instance, I-C-P plants had 1055% and 3277% nodules per plant higher than the control (C) in Mbuvo and SEKU, respectively. On the other hand, I-C-P recorded 157% and 356% in Mbuvo, and 599% and 812% in SEKU, grain and straw P uptake, respectively higher than C. Moreover, sorghum in I-SC-P treatment in Mbuvo gave 124% and 106% grain and straw N uptake, respectively above SC treatment while in SEKU, I-SC-P sorghum plants registered 149% and 328% grain and straw N uptake, respectively above SC sorghum plants. I-C-P cowpeas in Mbuvo gave 244% and 354% grain and straw yield increment, respectively while in SEKU I-C-P resulted in 184% and 224% grain and straw increase above C, respectively. In Mbuvo, sorghum plants in I-SC-P treatment gave 183% and 74% grain and stover yield increase, respectively above SC sorghum plants. On the other hand, in SEKU, I-SC-P sorghum plants gave 170% grain yields increment while stover yields improved by 101%. Intercropping significantly (P < 0.05) enhanced grain yield potential of sorghum while cowpea grain yield reduced significantly (P < 0.05) in both sites. The Land Equivalent Ratio (LER) values were > 1, indicating that intercropping was more beneficial than sole cropping in the study areas. Based on the findings of this study, the use of commercial Rhizobial inoculants and inorganic P was recommended to enhance soil fertility and sorghum-cowpea productivity in eastern Kenya. Intercropping of cereals with legume crops is also recommended to improve the productivity of smallholder farm lands in the region. Further work should investigate the persistence of the introduced commercial Rhizobial strain in the soil, the possibility of enhancing yields with other rates of P application, and the economic viability of the system under smallholder farming systems‟ situation.