Abstract

Limited information is available on optimizing potassium (P) and phosphate (K) nutrition with rhizobia in a soybean-sorghum intercropping system. A two-year field experiment was conducted to study the effects of Rhizobium inoculation with P and K on root length, symbiotic performance and yield. The split-split plot designs with 2 × 4 × 7 factorial arrangement with three replicates were used. The main plots had rhizobial inoculation, while the sub plots included four cropping systems namely sole sorghum, sole soybean and two intercropping levels. The sub-subplots were control; 20K; 40K; 26P; 52P; 26P + 20K; 52P + 40K. Plants were sampled at 50% flowering and shoot processed for N fixation. Nitrogen fixation was estimated using total nitrogen difference method where the total nitrogen obtained from non-fixing plants was subtracted from total nitrogen obtained from fixing plants. The results showed that cropping systems, inoculation and P and K fertilization influenced root length, number of nodules, nitrogen fixation and yield in soybean. Intercropping increased the number of nodules relative to sole cropping. P and K fertilization increased nodulation, nitrogen fixation and yield over the control. The use of combined fertilizers at the 20K+26P improved nodulation and N fixation while fertilization of 52P influenced yield.

Key words: Biological nitrogen fixation, inoculation, intercropping, nodulation, phosphorus, potassium.