Responses to rhizobial inoculation by two promiscuous soybean cultivars in soils of the Southern Guinea savanna zone of Nigeria

TGX soybean lines were bred at IITA Ibadan for promiscuity with indigenous rhizobia in Nigerian soils. Two cultivars, TGX1456-2E and TGX1660-19F, were tested in a 2-year trial for their response to rhizobial inoculation in five farmers' fields within a 60-km radius of Minna town, in the Southern Guinea savanna zone of Nigeria. Using the ELISA method, the competitiveness and persistence of the two elite strains of rhizobia contained in the inoculant mixture were also studied. There was a close relationship between nodulation and the size of resident rhizobial populations, with wide variation in nodulation across the various sites irrespective of the treatments. Cultivar effect on height and nodule number was significant only in the first cropping season of the trial. The inoculant strains appeared to be less competitive, but more effective, than the indigenous populations. The proportions of the nodules occupied by the inoculant strains were 17% in the first cropping season, and 24% in the second. Inoculation with rhizobia increased the percent arbuscular mycorrhizal infection by an average of 50%. Although grain yield varied between sites, no significant cultivar effect was observed. However, inoculation increased grain yield by 40% in the first cropping season, while no such yield differences occurred in the second season. The proportion of nitrogen derived from N-2 fixation ranged from 27% to 50% in both cropping seasons, and this was dependent on crop management on farmers' fields, rather than any cultivar or inoculation effect.