Influence of P sources and rhizobium inoculation on growth, nodulation, N & P uptake and yield of three soybean genotypes in Tanchera soil series of the northern Guinea savannah zone of Ghana

Soybean (Glycine max L) constitutes an important grain legume crop in semi-arid areas of sub Saharan Africa. The crop yields are usually low and falling relatively due to low P levels of the soil and the inability of the crop to fix nitrogen. Genotypes that make use of P applied to the soil and fix nitrogen could represent a key step in improving the productivity of soybean in the savannah zone. The objective of the study was to evaluate the effect of two P sources and Rhizobium inoculation on the growth, nodulation, N and P uptake and the yield performance of three soybean genotypes in Tachera soil series from the Northern Guinea savannah zone of Ghana. The P sources were Triple superphosphate (TSP) and Morocco phosphate rock (MPR) while the three genotypes were Jenguma (TGX1448-2E), TGX1904-6F and TGX1955-4F (new genotypes). The field experiment was carried out at Tili in the Bawku West District in the Upper East region during the 2015 cropping season. The experiment was repeated in the greenhouse at the School of Agriculture, University of Ghana, Legon. The field experiment was laid out in a split-split plot design with four replications and the greenhouse experiment was conducted using completely randomize design. The soybean genotypes constituted the main plot, the two sources applied at the rate of 30 kg P/ha and a control as sub-plots and rhizobium inoculation entries were sub-sub-plots. Parameters measured were indigenous rhizobia population (IRP) counts, plant height, nodule number, nodule dry weight and effective nodule number, yield and yield components, and nitrogen (N) and phosphorus (P) uptake and concentration in the shoot and in the grain. The results of the Most Probable Number (MPN) showed that Jenguma the local genotype contained 3.1 × 10² cells g^-1 soil while the new genotypes contained similar indigenous rhizobia of 1.7 × 10² cells g^-1 soil in TGX1904-6F and TGX1955-4F respectively at start of the experiment. The results indicate TGX1955-4F>TGX1904-6F>Jenguma in terms of growth, yield and uptake of phosphorus. The grain yield ranged between 694 - 1033 kg/ha in the field and 2.31 - 2.35 g/pot in the greenhouse respectively. Grain yield and harvest index correlated positively with plant height and nodule dry weight in the field experiment. Genotypes showed similarity in nodule number and effective nodule number, but varied in nodule dry weight.
Rhizobium inoculation did not have positive effects on yield and yield components in this study but increased nutrient (N & P) uptake and concentration. Application of TSP had significant pod yield of 1265 kg/ha compared to that of MPR and the control which was positively correlated to plant height in the field experiment. Nitrogen and P uptake was higher in TSP than in MPR and control. The P uptake trend followed TSP (49.93 kg P/ha) > MPR (32.05 kg P/ha) > C (29.58 kg P/ha). P sources interacted with rhizobium inoculation to significantly influence most of the parameters measured in both experiments. The Tanchera soil has high potential for soybean production with considerable grain and biomass yield. These two soybean genotypes (Jenguma and TGX1955-4F) that have thrived well in the low P soil with the potential of taking up their phosphorus from the readily available P source (TSP). More P was taken up from TSP than MPR. These two gynotypes TGX1955-4F and Jenguma performed better in terms of yield, nutrient concentration and uptake than TGX1904-6F; but overall TGX1955-4F could be recommended to farmers in northern Guinea savannah zone of Ghana.