Most recent research has found that inoculants provide the most benefit when used on land that has not had soybeans before or has not been inoculated in a long time. Some research from other states shows that over the long term, use of inoculants may increase soybean yields by about 1 bushel per acre. These studies were conducted over a range of fields under a range of conditions.
In a drought there might be some differences, but my informed opinion is that the bacteria are resistant to one year’s bad conditions, and so the drought would have negligible effect. We conducted a study near Tekamah and Brownville last year on ground that had been under water for an extended period due to Missouri River flooding. This would create harsh conditions for microbes that need oxygen to survive.
At Tekamah our yields for soybeans grown without inoculant averaged 61 bu/ac compared to 60 bu/ac with traditional inoculant, and 57 bu/ac with another inoculant that was an enhanced product. At Brownville the uninoculated soybeans yielded 57 bu/ac and one inoculated treatment yielded 64 bu/ac and the other 70 bu/ac. At Tekamah the least significant difference (LSD) was 9 bu/ac and at the Brownville site, the LSD was 12 bu/ac. The LSD indicates the yield difference that is needed to have confidence that the difference is due to the treatment and not chance. Yields at both sites were very variable.
SOURCE: Charles Shapiro, UNL Extension Soil Specialist, Haskell Ag Lab