Abstract:
Smallholder farming systems in Njoro are characterized by the application of sub- optimal rates or non-utilization of inorganic fertilizers due to their exorbitant prices vis-à-vis low financial returns from crops. Efficient application of natural materials in agro ecosystems, viz. the use of Minjingu phosphate rock (MPR) as a fertilizer is an attractive alternative to inorganic fertilizers, however, releases phosphorous slowly due to its low solubility. The use of the legumes apart from fixing nitrogen (N) can enhance MPR solubility through the release of citrate. The objective of the present work was to enhance phosphorus release from MPR and increase soil available N content using white lupin and chickpea for increased sorghum yields. The study was carried out at the Egerton University agricultural field experimental site during the long and short rain seasons of 2012 and Long rain season (LRS) of 2013. The experimental design was split plot arranged in a randomized complete block design. The main plots were three cropping systems; sorghum monocrop, legume- sorghum rotation and legume- sorghum intercrop. The subplots comprised two P sources; triple super phosphate (TSP) and MPR both applied at the rate of 60 kg P ha-1. Soil samples were collected at seedling, flowering and maturity stages of sorghum and analyzed for pH, organic C, and available P and N. Plant samples were also collected at the same time periods as for soils and analyzed for total N and P. Sorghum grain and dry matter yield were determined at maturity. N and P balances were also measured at the end of the experiment. Results on soil pH indicated a lower plant and soil response with the use of MPR at a pH greater than 5.5. The use of MPR also led to a rise in soil pH due to its liming effect. The use of TSP resulted in a decrease in pH in the long run to below 4 and this impaired N fixation. TSP application resulted in significantly higher P concentrations in the soil and plant tissues than MPR addition in the first season since the latter has low solubility in water. Comparison of the two legumes showed that both were competitive in enhancing MPR solubilization, with a greater potential in lupin. Higher soil available P led to higher levels of soil and plant available N and also to higher accumulation of dry matter and grain yield and upon incorporation of crop residues in to the soil and their decomposition, high levels of organic C was realized. Higher levels of plant N and P was also realized with the use of MPR. N, P and K nutrient balances showed a negative balance. All the cropping systems showed significant results but intercropping systems was the most effective for this research by the end of the LRS 2013. There is a need for more research to improve the poor performance of MPR in the first season and to identify other mechanisms of P mobilization from MPR. Thus MPR could be a viable alternative to TSP in supplying P to both soil and plants.
