ABILITY OF 2 BRADYRHIZOBIUM-JAPONICUM STRAINS TO PERSIST IN SOIL AND NODULATE SOYBEANS AFTER DUAL INOCULATION

The competitiveness of two Bradyrhizobium japonicum strains was determ ined under controlled conditions. The relationship between the ratio o f the nodules formed by the two bradyrhizobial strains and the ratio o f the two strains in the inoculum was established and used as an inter nal standard to evaluate the equilibrium ratio between populations of the two strains in the soil. The fluorescent antibody (FA) technique w as also used for counting the bradyrhizobial populations directly in t he soil. Results with the FA technique were analysed and compared with the estimate obtained through nodule occupancy. The initial ratio of the two bradyrhizobial strains in populations introduced simultaneousl y into the soil changed during incubation time and the two strains rea ched the same equilibrium levels after 53 days. Nevertheless, the stra in introduced into the soil as an overpopulation remained clearly domi nant for the nodule occupancy. We conclude that direct counts with the FA approach are selective and these counts could be mainly of the una ttached bacteria.