Effect of Inoculum Rate on Competitive Nodulation of Glycine max L. Merrill. I. Greenhouse Studies1

Abstract To study the competition between inoculated rhizobia and soil rhizobia in forming soybean (Glycine max L. Merr.) nodules, the effect of soil rhizobial population, inoculum strain, and inoculum rate on nodulation of soybeans was determined. Soybeans were grown under greenhouse conditions on 22 soils that contained populations of Rhizobium japonicum ranging from less than 2 to greater than I million/g of soil. Liquid inoculum containing up to 1 billion rhizobia was added to each 2.5 cm of planted row. After 6 weeks of growth, the soybean plants were harvested and the number of nodules, nodule fresh weight, and serotype of rhizobia forming the nodules were determined. Nodule numbers were not increased by inoculation when the soils contained more than 1,000 rhizobia/g of soil. With higher rates of inoculation, the proportion of nodules formed by the inoculum strain was greater. Multiple regression equations indicated that number of cells inoculated and number of soil rhizobia present accounted for 75% of the variation across soils in the percentage of nodules competitively formed by inoculum strains. Soybeans grown on soils containing 1,000 or more rhizobia/g are not likely to be extensively nodulated by inoculum rhizobia applied at normal rates (less than 10,000/seed).