Enhancing Biological Nitrogen Fixation in Promiscuous Soybeans through Rhizobial Inoculation in the Nigerian Guinea Savanna.

The Tropical Glycine cross (TGx) varieties of soybeans, commonly referred to as promiscuous soybeans, were developed in the 1980s by the International Institute of Tropical Agriculture (IITA) Ibadan, Nigeria, to nodulate and fix nitrogen with indigenous bradyrhizobia in tropical soils. However, studies have shown that biological nitrogen fixation by indigenous rhizobia may not often be adequately meeting the nitrogen needs of these varieties. Inoculation using elite rhizobial strains is one of the strategies that could be employed to enhance N₂-fixation and, hence yield, in these crops. This paper highlights the results of two years of inoculation activities in the Nigerian savanna. Estimates of rhizobial population in soil using TGx varieties as test crops range from as low as 90 cells g^-1 to as high as 8.0 x 10⁸ cells g^-1. In spite of their general permissiveness and the relatively large estimates of indigenous rhizobia in the soil, the TGx soybean varieties exhibited a high frequency of response to inoculation with average yield increases due to inoculation being about 30% and a range of 6-70% depending on planting date, soybean variety and agro-ecology. The magnitude of response to inoculation was influenced by the degree of symbiotic effectiveness, but not population size, of the indigenous rhizobia population and generally was higher in the Sudan savanna than in the Northern and Southern Guinea savanna agroecologies. The results also show a tendency for the soils of the Southern and Northern Guinea savannas to have higher rhizobial populations than those of the Sudan savanna.