Farmers who have planted year after year of corn and are now contemplating a switch to soybeans should plant seed that has been inoculated with rhizobia to help rebuild the soil population of nitrogen-fixing bacteria, scientists say.
These naturally occurring rhizobium bacteria release compounds that cause legumes, such as soybeans, to develop nodules on their roots. The bacteria move into these nodules where they grow and multiply. In this protected environment, they also extract environmental nitrogen from the air and turn it into a nitrogen form the plant can use to grow and produce a crop.
"Typically, fields that have been in continuous corn for several years will have rhizobium cells, but the populations are very low and they don't generate enough nodules to supply adequate nitrogen for a high-yielding soybean crop," explains Jim Beuerlein, professor of agronomy and soybean research and extension specialist for The Ohio State University. "In this type of situation, it is very likely that a grower will see a large yield response from inoculation."
University studies show that growers normally can expect an average yield benefit of 2.5 bushels per acre from the use of inoculants in fields that are rotated to soybeans every other year. However, the yield benefit often is significantly greater in fields where soybeans have not been planted in several years, such as continuous corn or CRP acres.
There is an environmental benefit, too. "The production of synthetic nitrogen fertilizers generates large volumes of carbon emissions," Hale says. "Reducing the need for synthetic nitrogen fertilizer decreases greenhouse gas emissions. Also, by applying less nitrogen to their fields, growers lower the risk of nitrogen leaching into the groundwater."