Effects of Rhizobia Inoculation and Phosphorus Levels on Soybean (Glycine Max L.Merril) Yields and Selected Soil Properties in Embu County, Kenya

Abstract

Soybean (Glycine max L. Merril) productivity in Kenya is adversely affected by low yield, poor fertilizer use, ineffective inoculation, and soil fertility management constraints. The empirical understanding of the interaction between native soil rhizobia, inoculation, nitrogen fixation, soil fertility management practices and how these factors influence soybean productivity is insufficient. The general objective of this study was to investigate the effect of rhizobium inoculation and P fertilizer on nodulation and soybean yield on smallholder farms in Embu County, Kenya. Three P-fertilizer rates were evaluated with or without rhizobia inoculation in a randomized complete block design (RCBD) with six replicates. The P-Fertilizer rates included 0, 30 and 60 kg P ha-1. Soils were sampled thrice; before experimentation and after harvesting in 2 seasons (SR 2012-Oct, Nov, Dec; LR 2013-Mar, Apr, May). Nodulation was determined at 50% flowering (40 days after emergence -DAE) while biomass (above and below ground) was measured both at 50% flowering and at physiological maturity. Phosphorus use efficiency (PUE) was determined as a ratio of yield and P fertilizer input while grain yield was extrapolated from plot quadrats after moisture correction. Data was recorded and subjected to analysis of variance (ANOVA) and means separated by the least significance difference (LSD) at P≤ 0.05. Paired t-test was used to compare soil chemical properties before and after the experiment. The results indicated a significant yield increase in fields with rhizobial inoculation and application of 60 kg P ha-1 compared with the control treatment Total stover weight, number of nodules, total dry matter (DM) (shoot dry weight+root dry weight) and grain yield were significantly higher (p<0.05) in inoculated than in non-inoculated treatments in both seasons. A few nodules were present in control plots, indicating the presence of native soil rhizobia. PUE was higher in inoculated plots and in treatments with 30 kg P ha-1. There were beneficial soil changes arising from fertilizer application and inoculation for K, TOC, pH and CEC. A high decline in soil P after the first season was observed, partly attributed to the initial soil P demand for biological nitrogen fixation in soybean. Total soil nitrogen increased with cropping seasons, indicating the potential benefits of biological nitrogen fixation for soil fertility. Nutrient uptake by soybean plants was significantly enhanced by P fertilizer (p<0.05) and inoculation (p<0.05). Overall, better soybean performance and PUE was recorded in SR2012 compared to LR2013, due to higher rainfall. Soybean inoculation is an important and cost-effective legume N2 fixation enhancement method, suitable for farmers in sustainably supplementing N in their farms. The study therefore recommends soybean farmers to consider use of B. japonicum and P fertilizer at the rate of 60 kg ha-1 in their production system, coupled with efficient soil moisture management systems.