Survival of Two Strains of Rhizobium in Soil

A sensitive method was devised for determining the abundance of strains of Rhizobium meliloti and R. trifolii introduced into soil. This method relies on the use of streptomycin‐resistant mutants which, after inoculation into soil, are counted on a medium supplemented with streptomycin at concentrations sufficient to inhibit most soil microorganisms. Viability of streptomycin‐resistant strains of R. meliloti and R. trifolii did not fall markedly when the bacteria were introduced into moist soils. Various organic matter amendments did not significantly alter the decline rate of the rhizobia. R. meliloti populations did not lose viability readily in moist sand, in desiccated soils, or in solutions free of organic nutrients, but the population density fell rapidly when the bacteria were inoculated into desiccated sand. The rate of decline in cell number was enhanced with increasing temperature. Both rhizobia survived well in dry, nonsterile soil.