Optimizing soybean yield, phosphorus use efficiency, and economic returns using phosphatic fertilizer and inoculation with bradyrhizobium japonicum in lira city, Uganda

Abstract

Soybean (Glycine max L. Merril) is among the key oil seed crops worldwide providing several benefits from human consumption to the enhancement of soil productivity. The major bottleneck to the crop’s production in the tropics is the decreasing soil fertility, mainly caused by not only low nitrogen (N) but also phosphorus (P) levels in the soil. There is a high potential for supplying N from the atmosphere through biological nitrogen fixation (BNF), a natural process mediated by the symbiotic bacteria Bradyrhizobia Japonicum, which requires optimum P levels for effective N fixation. The study focused on assessing the synergistic effect of inoculation and phosphatic fertilizer application on nodulation, yields, economic returns, P uptake and use efficacy of Maksoy 5N soybean. The study was conducted at Ngetta Zonal Agricultural Research and Development Institute (NGEZARDI) located in Lira City, Uganda. The study had the experiment laid out in RCBD under a split-plot arrangement with the main plots being two inoculation levels (with or without inoculation) and the sub-plots being phosphatic (TSP) inorganic fertilizer at four P levels: 0, 7.5, 15, and 45 kg P ha-1. The treatments were replicated three times. The data collected included growth and yield parameters, such as the total number of root nodules, number of effective root nodules, number of seeds per pod, number of pods per plant, biomass yield, stover weight, grain yield, and 100-grain weight. Production cost and yield data were used to calculate economic returns. Data collected were subjected to analysis of variance in Genstat software where the treatment means were further distinguished at a 5% level of probability using Fishers' least significant difference. The total and effective root nodules increased with inoculation with a peak in fertilized plots with 15 kg P ha-1 (11.8, 13.3). Under inoculation and P application, treatments that received 7.5, 15 and 45 kg P ha-1 recorded 9, 30, and 25% higher yields than the control. The highest net income and benefit: cost ratio were recorded in the inoculated and fertilized plots with 15 kg P ha-1 (US$ 414.93, 1.87) and (US$ 390.23, 1.76). With inoculation, P use efficiency based on yield (PUEY) and economic returns (PUEE) decreased with increasing P levels in the orders of 165 > 73 > 54 > 21 kg grain kg P supplied-1 and 51 > 24 > 20 > 8 US $ kg P supplied-1 for the control, P7.5, P15, and P45 respectively. This study indicated that for optimal growth, productivity, and economic benefits, soybean ought to be inoculated with Bradyrhizobium coupled with the application of phosphatic fertilizer at the rate of 15 kg P ha-1.