RESUMEN
The competitive interactions among four strains that nodulate soybean and cowpea rhizobia were studied in soybean plants grown under pot and field conditions. All the possible combination between the four strains; Bradyrhizobium japonicum USDA 110, Rhizobium fredii USDA 191, Rhizobium spp. [Arachis] strain 1 and Bradyrhizobium spp. [Vigna] strain 2 were studied to obtain single, double, triple and quadruple inocula. Developed plants were examined after 45 and 90 days from planting to record number, dry weight and nitrogenase activity of the root nodules as well as the dry weight and N-content of shoots. Nodule occupancy by the tested strains was also determined by agglutination and double gel diffusion techniques. Data showed that dual inoculation with R. fredii USDA 191 and B. Japonicum USDA 110 showed maximal effects on all tested parameters and provided soybean plants with 75% of its N-requirements. However, no nodules were developed on soybean roots inoculated with Rhizobium spp. [Arachis] strain 1 in single or combined inocula, Bradyrhizobium spp. [Vigna] strain 2 formed effective nodules in both cases