Soybeans are not native to the United States therefore when soybeans are planted in fields that have not been in soybeans, the bacteria need to be added to the soil. In some cases, soybean yields can be improved when Bradyrhizobium are applied to the seed coat at planting. Commercial inoculants vary in the type of bacteria strains; carrier; and other characteristics. The objective of this study was to evaluate the performance of selected commercial soybean inoculants on soybean yield in Kansas.

Ten treatments (Table 1) were utilized to assess the impact of seed applied Bradyrhizobium japonicum on soybean yield. The soybean variety Midland '8390' was utilized throughout this study. Soybean seed was coated with inoculants just prior to planting as directed by commercial suppliers.

A randomized complete block resign with three replications was utilized. All plots were planted May 16, 2000 in creek bottom field containing a Muir silty clay loam. The plot area was located under a center pivot sprinkler irrigation system. Previous crop rotation in this field was corn in 1999, corn in 1998 and winter wheat in 1997. A Hege plot drill was used to establish plots measuring 10 ft wide and 30 ft in length. Soil temperature at planting depth (2 in) was approximately 76°F.

Grain yields were determined on September 28, 2000 by harvesting 50 ft2 of plot area in each plot. Grain yields were adjusted to 13% moisture. Analysis of variance was used to determine treatment differences.

Soybean yields in northeast Kansas were extremely low in 2000 as a result of less than 2 in of precipitation and 18 days with maximum temperatures over 100°F during July and August (data) not shown. Supplemental irrigation alleviated much of the moisture stress experienced in the area as indicated by the 50.5 bu/acre plot yield (Table 1).
The importance of inoculants in a crop rotation that has not recently included soybeans is illustrated by the low yields of the untreated control. There were no clear differences between the inoculant treatments, with all except the Cell-Tech 2000 + Genstein + Nod-Factor (rate 2) and Cell-Tech 200 alone having yields greater than the control.