Incubation of soil with agricultural Lime and Phosphorus enhances Biological Nitrogen Fixation and yield of Soybean (Glycine max (L.) Merrill) in an Ultisol

Low pH and phosphorus are among major soil chemical constraints to Biological Nitrogen Fixation (BNF) and soybean production in sub-Saharan Africa. A screenhouse pot experiment was conducted during the 2016/2017 season at Sokoine University of Agriculture-Morogoro Tanzania, to examine the ameliorating effect of liming to pH, phosphorus uptake, BNF and yield of soybean grown on an ultisol. The experiment was a 33 factorial-laid out as a split-plot with two replications. Source of nitrogen (inoculation, non-inoculation and applied inorganic nitrogen) was main plot factor, while the subplots received combinations of lime and phosphorus. The levels of lime used for the incubation experiment were 0, 5 and 10 tons/ha while those of P were 0, 50 and 100 kg/ha. A commercial inoculant-Legumefix, was used for inoculation. Results indicated that liming significantly (P<0.05) raised the soil pH and that the interaction of lime and phosphorus showed significant (P<0.05) effect on nodulation and N-2 fixation. Overall, application of 5 tons of lime/ha and 100 kg P/ha gave the best-case scenario in terms of pH amelioration, P availability and BNF enhancement by the soybean-Bradyrhizobium symbiosis.