Optimum rate and timing of supplemental nitrogen for soybean inoculated with bradyrhizobia- USDA 110
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Soybean (Glycine max (L.) Merr.) is a legume crop which is increasingly being adopted by farmers in Uganda. Given its versatility as human food, an industrial raw material and an animal feed, the crop has a proven potential of addressing income and food insecurity problems of rural livelihoods. However, soybean production in Uganda remains suboptimal significantly due to its nutritional requirements that are still hazy. A field experiment was conducted at Makerere University Agricultural Research Institute Kabanyolo- MUARIK to examine the effect of inoculation, rates and time of nitrogen fertilizer application on yield of soybean. Composite soil samples were taken from the experimental field in the different blocks prior to establishment of the experiment to assess the soil physical and chemical properties. The experiment was laid out in a Randomized Complete Block Design with a split-plot arrangement having three replications. The main plot factor was inoculation (Mak- Bio-fixer) while the subplot factor was nitrogen rates (0, 5, 10, 15, and 20 Kg N/ha) in the form of Urea (46% N). These were applied at planting and pod fill stage of soybean growth. The crop parameters were evaluated on the basis of nodule number per plant at 40 and 60 days, effective nodulation, above ground biomass at 50% anthesis, nitrogen recovery, pod number per plant, seeds per pod and plot yield. Processed data were subjected to statistical analyses using Genstat 14th edition statistical package and thereafter, the impact of nitrogen supplementation to soybean productivity examined. The study revealed that only nodule number amongst the agronomic parameters was significantly influenced by the treatments. However, yield and yield parameters were more responsive to nitrogen level, rhizobia inoculation and time of nitrogen application. 10 Kg N /ha application rate showed the highest nitrogen recovery and pod number per plant. Additionally, nitrogen application at planting time was associated with better response. It was concluded that 10kgN/ha application rate applied at planting time has a promotive effect on soybean growth and yield.