RRL - FAQ

When a legume is introduced into a soil that has not been previously cropped to that species, it is unlikely that the soil will contain rhizobia, and response to inoculation is likely. Yield responses following inoculation may be of the order of 30%. Where such inoculation is properly carried out, senescence of the nodules at the end of the growing season will return large numbers of rhizobia to the soil, and should ensure that inoculation in future years will be unnecessary. A caveat here is that not all inoculant rhizobia are good soil colonizers, and situations are known where, despite all the advantages, the inoculant rhizobia disappear over time and are replaced by less effective seed-borne or aerial contaminants.

By contrast, where the crop is a traditional species in the area, it is likely that the soil will already contain abundant rhizobia. This is essentially irrespective of whether the land in question has previously been inoculated or even planted to that particular crop. These soil organisms will compete with inoculant strains, reducing the proportion of nodules formed by them. Since - on average - the soil strains are likely to be less effective in nitrogen fixation than are the inoculant rhizobia, the plant may not be able to derive a significant part of its N needs from fixation. This has happened with soybeans in the American Midwest, where many plants derive less than 50% of their N needs from symbiosis. Displacing these soil rhizobia is basically a numbers game; the inoculant rhizobia have a placement advantage around the seed, but the indigenous organisms are more numerous in the bulk soil. Some recent studies report a response to inoculation in areas where the soil already contains rhizobia, but only where high potency inoculants were used. Yield responses in the case of soybean of 1.5 to 2 bushels/acre have been reported, but response is likely to be extremely variable.

Studies are known where inoculant rhizobia dominate in soil, even fifteen years after inoculation. However, response to inoculation becomes more likely with time since the particular crop was last planted. On sandy soils in Nebraska, for example, the recommendation is to reinoculate after a lapse of three to five years between plantings. Another aspect of the numbers question mentioned above is that adverse environmental conditions between planting and actual nodule formation will reduce the number of viable rhizobia available, and possibly limit the number of nodules formed. Factors of this type include soil acidity (pH less than 5.5), high soil temperature, a prolonged period between seeding and germination, and seed treatments that bring the inoculant rhizobia in contact with fungicides, acidic superphosphate or molybdenum. Soil acidity and temperature in many Brazilian soils are such that annual inoculation is often recommended. Where soil conditions are unfavorable, it is wiser to use a higher potency inoculant or an inoculant treatment that allows a greater quantity of inoculant per unit area.

When is it advisable to inoculate?