Rhizobia are responsible for biological nitrogen fixation (BNF) when in symbiosis with a host legume such as soybean. Though evidence suggests that legumes prefer to use mineralized sources of nitrogen (N) in the soil before spending energy on a symbiotic relationship with rhizobia, total plant N derived from BNF is typically between 25 and 75% for soybean (Zapata et al., 1987). Rhizobia inoculant application is the primary strategy employed by soybean producers to promote adequate levels of BNF. Inoculant recommendations differ by state and are largely driven by crop history. Conley and Santini (2007) conducted a survey of 1,134 farmers in Indiana and found 18% to use rhizobia inoculants. Wisconsin inoculant use was shown to be much higher, at 85% (n=168) (Conley, unpublished data, 2008).
Soil Science Extension
University of Wisconsin Madison