Evaluation of Commercial Soybean Inoculants by Various Techniques

Much of the expanding production of soybeans (Glycine max L.) in the southeastern United States and in other countries is on soils without naturalized populations of Rhizobium japonicum. Effective inoculation is essential where soybeans are lanted for the first time. Considering the frequency of inoculation failures and the variety of inoculant products available to growers, research was initiated, to determine relative effectiveness of inoculants and to compare evaluation techniques. Commercial soybean inoculants sold in Alabama in 1976, 1977, and 1978 were evaluated by rhizobial plate counts in two laboratories, most probable number (MPN) determination, greenhouse pot test and acetylene reduction assay, and by field experiments in R. japonicum free soil. Serial dilution and standard plating techniques, along with MPN by soybean nodulation, provided measures of viability in the inoculants. Products were applied in greenhouse and field experiments according to manufacturer's instructions. Nodulation, N fiiation, and yield of soybeans were measured relative to numbers of rhizobia applied per seed by various products. More than 1,000-fold differences were found among products in viable rhizobia content. Plate counts adequate discriminated products when 10⁷ rhizobia per gram was considered to be the lower limit for acceptability. The close correlation of plate counts and MPN (soybean nodulation) indicated reliability of plate counts despite the difficulty of enumerating rhizobia among numerous contaminants. Nodulation of soybeans in greenhouse and field plantings in soil free of R. japonicum was directly related to the number of rhizobia applied per seed, with no nodulation produced by products supplying fewer than 108 rhizobia per seed, and generally abundant nodulation by products providing 10⁵ to l0⁶ rhizobia per seed. Some products performed poorly despite apparent adequate viability; for example, where a fungicide was added at planting. Various strains of R. japonicum and formulation processes, as well as number of rhizobia applied, may account for differences in nodulating effectiveness. In no case, however, was a product of low plate count found to be an effective inoculant. Plate count provides a relatively rapid, inexpensive evaluation that is supported by other techniques involving soybean nodulation response.