Date Published: July 01, 2018
Author(s): M. Thuita, Bernard Vanlauwe, E. Mutegi, C. Masso.
•Soybean yields in SSA are low and variable compared to North and South America.•Low soil fertility and poor quality rhizobia inoculants contribute to low yields.•Integrated soil fertility management (ISFM) packages for soybean to reduce yield variability.•Inoculation with Rhizobia and addition of Sympal gave yield of up to 4 t ha−1.•Yield increase of 35–70% required for profitability of ISFM packages.
Soybean grain yields in most of sub-Saharan Africa (SSA) remain relatively low compared to those achieved in South America and USA (Mpepereki et al., 2000). In other regions, annual average yield increases have been reported of 31 kg ha−1 in the United States (Specht et al., 1999) and 28 kg ha−1 worldwide (Wilcox, 2004). To achieve their high yield potential, soybean must sustain high rates of photosynthesis and accumulate large amounts of nitrogen (N) in seeds (Salvagiotti et al., 2008). Nitrogen exists in leaves primarily as ribulose biphosphate carboxylase/oxygenase and there is generally a strong relationship between N per unit leaf area and photosynthesis (Sinclair, 2004). Soil is the main source for most plants but N remains a major plant growth limiting nutrient in SSA (Sanchez et al., 1997). The alternative source for soybean is biological nitrogen fixation (BNF) through symbiosis with rhizobia. Worldwide some 44–66 million tonnes (t) of N2 are fixed annually by agriculturally important legumes with another 3–5 million t fixed by legumes in natural ecosystems, providing nearly half of all N used in agriculture (Smil, 1999; Graham and Vance, 2000). The contribution by BNF could be increased by improving the nutrition of legumes, attending to edaphic constraints such as soil acidity and drought, and breeding varieties that’s target the symbioses with rhizobia (Graham and Vance, 2000).
On average, yields of the TGx1740-2F variety of soybean were relatively low in Siaya County of western Kenya (2000 kg ha−1). Soil analysis showed deficiencies in most of the nutrients tested (e.g., N, P, K, and Ca). Thus, it was possible to raise soybean grain yields up to 4000 kg ha−1 through using rhizobia inoculation and by addressing selected nutrient deficiencies. For a sustainable economic return from using rhizobia inoculants in soybean production in Siaya County of western Kenya, we recommend that their use in the context of ISFM considers other potential limiting factors such as secondary and micro-nutrients, as well as soil organic matter and pH. The geographical spread of the study region which has three agroecological zones demonstrated the need for ISFM packages that can respond to limitations in soil fertility for increased nodulation and grain yields.
This work was supported by IITA through the COMPRO-II project funded by the Bill and Melinda Gates Foundation (USA) and the RCT study implemented by the Paris School of Economics and funded by the Conseil National de Recherche Scientifique (France).