Response of soybean to rhizobial inoculation and nitrogen management options in the Southern Guinea savannah zone of Ghana

Field experiments were carried out in the Savelugu/Nanton and Yendi Municipalities in the northern region of Ghana in 2014 to investigate the effect of inoculation, rates and time of nitrogen fertilizer application on growth, yield, agronomic efficiency of mineral N and biological nitrogen fixation of soybean. The experiment was laid out in a split-split plot, arranged in a randomized complete block design with four replications at both locations. The main plot factor was inoculation (legume-fix inoculants), the sub-plot factor was nitrogen rates (0, 10, and 20 kg ha^-1 N) in the form of sulfate of ammonia fertilizer (21 % N) and the sub-sub plot factor was time of nitrogen application (either starter N or late N application). Triple superphosphate (TSP) 46% P₂O₅ was applied as basal fertilizer to all treatment plots at the rate of 30 kg P ha-1. Weed fallow was established at both locations as reference for the determination of N-fixation using nitrogen difference method (NDM).The crop parameters were evaluated on the basis of plant height, number of nodule and nodule dry weight at 50% flowering, number of nodule and nodule dry weight at R4 (full pod stage), dry matter yield at R4, pod number, pod weight, grain yield, mean hundred-grain weight, amount of N₂ fixed (BNF) and agronomic efficiency of mineral N fertilizer application in soybean production. The results showed that inoculation of soybean with rhizobia inoculants had no significant (p > 0.05) effect on all the response parameters measured at both locations. N rates at 20 kg ha^-1significantly (p < 0.05) increased dry weight of nodules at 50% flowering, hundred-seed mean weight and grain yield by 29.88% at Puriya over the control. At Bunlong, the N rates only significantly (p < 0.05) increased dry weight of nodules at 50% flowering. Starter N significantly increased nodule number at R4 stage by 19.8% and dry weight by 20% at Puriya but had no effect on other response parameters. At Bunglong however, starter N only increased the nodule number. Late application of N only increased nodule
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dry weight at Bunglong. The interactions among the factors (inoculation, rate and time of N application) did not significantly influence biological nitrogen fixation and growth at both locations. The agronomic efficiency of the mineral N fertilizer applied was significant with the 20 and 10 kg N ha^-1 rate increasing yield at Bunglong and Puriya respectively. It was therefore concluded that inoculation of soybean with rhizobia inoculant and nitrogen application did not increase growth and BNF. However, mineral N fertilizer application increased grain yield and agronomic efficiency in soybean production.