An inoculant product should be used when there is reason to believe that soil populations of Rhizobia are low or ineffective.
All legumes have the capacity to fix their own nitrogen.
Legume growers see fixation in their soybeans, peanuts, clovers, alfalfa, dry beans and other legume field crops. It has been suggested that Rhizobia become 'lazy' or less effective as they reside in the soil for extended periods. Thus, there is the potential yield-increasing benefit of adding high levels of newly selected strains of Rhizobia each time a legume is planted by using an inoculant. Large amounts of excess nitrogen in the soil, either from an applied fertilizer, manure, or residual left from previous years, can inhibit nodule formation on legume plants even though there are plenty of the appropriate Rhizobia available to the forming root hairs. The plants will look green and healthy, at least through the first part of the growing season, until the soil nitrogen is used.