Soybean Response to Soil Rhizobia and Seed-applied Rhizobia Inoculants in Wisconsin

Seed-applied rhizobia inoculants can be used to promote biological nitrogen fixation in soybean [Glycine max (L.) Merr.] production, yet their efficacy remains highly variable. The goal of this study was to improve the predictability of a positive soybean seed yield and quality response to inoculation by (i) characterizing inoculant product response in multiple environments, and (ii) quantifying the response of soybean to indigenous soil rhizobia populations. Two inoculants and a control were tested on three soybean varieties at nine locations in Wisconsin during 2009 and 2010. Soil samples were collected from each plot for the quantification of soil rhizobia. Seed yield, protein, and oil did not respond to inoculation across all 18 environments (P = 0.15, 0.38, and 0.30, respectively). Three environments responded positively to inoculation (P < 0.05), while the remainder showed no response. Yield of the control treatment was positively correlated with the soil rhizobia population level across all environments (P = 0.005), while the inoculated treatments showed no response. These data indicate a yield advantage for inoculation at lower rhizobia populations; however, given the large range of response with only three of 18 environments showing a positive response, a rhizobia population threshold to aid in the decision to treat soybean seeds with a rhizobia inoculant cannot be determined.