Cropping History, Commercial Inoculums, and Starter Fertilizer Effects On Soybean Growth and N Nutrition.

Planting soybean (Glycine Max L.) for the first time on soils with a history of cotton production raises questions about inoculation management. This greenhouse study evaluated the effects of previous cropping history of soybean or cotton only, starter fertilizer (10-34-0), and commercial inoculums (‘Optimize 400' and ‘Launcher Pro’) on inoculation, growth, and N accumulation of soybean. Nodule counts were greatest at 42.3/plant with a history of soybean compared to 4.7/plant with a history of cotton. It appeared that commercial Rhizobium strains were ineffective against competing native Rhizobia in soybean soil as evidenced by their small size and dispersal throughout the root system as opposed to large nodules clustered at the base of the hypocotyl or upper radicle in the cotton soil. Soybean dry matter production was 0.58 g/plant on the cotton soil compared to 0.15 g/plant on the soybean soil. Starter fertilizer increased dry matter production from 0.31 g/plant to 0.41 g/plant averaged across soils. Total N accumulation was 1.82 mg/plant on the cotton soil and 0.5 mg/plant on the soybean soil. Starter fertilizer increased N accumulation 0.31 mg/plant. Native Rhizobium strains seemed more competitive than commercial inoculums in the soybean soil which had a pH of 5.2, but were not as effective in N₂-fixation. Inoculation was successful on the cotton soil where native strains were absent.