Agronomic benefits of rhizobial inoculant use over nitrogen fertilizer application in tropical soybean

Abstract

This study compares the performance of inoculated soybean and the fertilized crop with a control. Soybean was cultivated with the use of inoculant or nitrogen (N) fertilizer application in five farmers’ fields in the dry zone of Sri Lanka. Farmers’ practices without the inoculant or fertilizer N served as the control. Plant density, nodule and plant dry weights were measured at flowering. Seed yield and seed N were measured at crop maturity. The study reveals that crop performance of tropical soybean could be increased by using greater plant density with inoculation. However, fertilizer application did not show such a trend. High plant densities reduced nodulation due to a limitation of native soil rhizobia to nodulate. This adversely affected seed yield. Both inoculation and fertilizer application increased plant growth and seed yield of soybean, as compared to the farmers’ practices. Fertilizer application at the present rate did not suppress nodulation. This study shows the importance of biological nitrogen-fixation (BNF) even under fertilizer application in tropics. Future studies should be focused to evaluate optimal plant densities for maximum yields of soybean under inoculation.