Soybean ( Glycine max (L.) Merr.) response to application of mineral nitrogen and bradyrhizobia on Nitisols of Teppi, Southwest Ethiopia

Soybean (Glycine max (L.) Merr.), an important crop grown for its protein source for humans and livestock, is widely introduced in different parts of Ethiopia. However, the productivity of the crop is far below its potential in the country due to different factors, among which low soil fertility is a major contributor. Hence, this field experiment was conducted with the objective of determining the optimum rate of starter nitrogen (N) and bradyrhizobium inoculation on yield and yield components of soybean in the 2019 and 2020 cropping seasons. Two levels of bradyrhizobia (inoculated and uninoculated) and six levels of starter nitrogen (0, 9, 18, 27, 36, and 54 kg N ha-1) were arranged in a factorial design. The result showed that soybean grain yield increased by about 60 % with inoculation of bradyrhizobia applied with low rates of starter nitrogen fertilizer, regardless of cropping seasons. Application of a nitrogen rate above 18 kg N ha-1 leads to yield decline and has no significant variation from bradyrhizobia inoculation only. Regardless of the cropping seasons, elevated levels of starter nitrogen beyond 27 kg ha-1 suppressed nodulation and nodule dry matter. Starter N at a rate of 9 and 18 kg N ha-1 improved soybean nodulation by 125-130 % over control and 95 % over bradyrhizobia inoculation alone. Thus, it was recommended to apply bradyrhizobia strains with 9 or 18 kg N ha-1 starter nitrogen for better yield of soybean as well as adequate nitrogen fixation in Nitisols having moderate soil nitrogen levels similar to the Teppi areas.