Bio - prospecting for effective rhizobia isolates for Soybean production in Ghana

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Biological nitrogen fixation is considered an inexpensive means of soil fertility replenishment and as such needs to be exploited. Wild – legume rhizobia have been reported to form successful symbiosis with some important grain legumes like soybean. Hence this study sought to obtain wild – legume rhizobia (indigenous isolates) from wild and uncultivated legumes such as Desmodium spp, Centrosema spp, Mimosa spp, Crotolaria spp, Calopogonium spp and Ceasalpinea spp. A bio - prospecting activity was carried out to collect nodules from uncultivated legumes in parts of Ashanti and Northern regions of Ghana. After culturing and characterization of the indigenous isolates in the microbiology laboratory (SRI), 85 isolates showed characteristics similar to rhizobia on YMA (CR and BTB) media: 65 isolates showed fast growing character while 20 were slow growers. The 85 indigenous isolates were further tested for their infectivity (ability to nodulate legumes) on soybean in sterile river sand and non – sterile soil media. Eleven of the isolates were infective. Symbiotic effectiveness index, SEI showed that isolate NAG 218 was highly effective (SEI > 80), while NAG 150, NAG 155, NAG 180 and NAG 181 were effective (SEI between 50 – 80%) and NAG 152, NAG 168, NAG 170, NAG 171, NAG 173 and NAG 211 were classified as lowly effective (SEI between 35 – 50%). The symbiotic potential of the isolates on the promiscuous soybean varieties in non – sterile soil showed that isolate NAG 152 performed relatively better (11%) than the Legumefix strain. NAG 171 on the other hand produced statistically similar nodule dry weight as Legumefix strain. Shoot dry weight produced by NAG 168 and NAG 211 were statistically at par with the shoot dry biomass of Legumefix strain and USDA 110. Effective isolates at high cell concentration levels (108 and 104 –CFU) were not competitive. Isolates NAG 152, NAG 168, NAG 171 and NAG 211 showed potentials for fixing nitrogen thus increasing the possibility of obtaining local strains for soybean production.  
A Thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the degree of Master of Science in Soil Science, 2014