Most of us associate nitrogen (N) needs to corn, but soybeans need N, too. In fact, they can use twice as much N during the growing season as corn.
Most growers don't need to supply soybeans with N like they do corn because soybeans have a unique way of pulling nitrogen from the atmosphere, rather than using nitrogen reserves in the soil. The mechanism is a soil bacterium, rhizobia (Bradyrhizobium japonicum). It turns nitrogen gas in the soil into ammonium, a form the plant can use. The process works this way:
Unless there's a lot of N in the soil, the soybean plant will send out a chemical signal that attracts rhizobia bacteria to the roots. The bacteria invade the roots and establish colonies in rounded nodules on the roots. The plant supplies carbohydrates and minerals to the rhizobia. The bacteria "fix" the N as ammonium, which fuels plant growth and protein development.
"The fixed N isn't free," says Shawn Conley, associate professor at the University of Wisconsin. "It can take 10 pounds of carbohydrates from the plant to produce a pound of nitrogen. But this still results in higher yields for the plant."
Rhizobia aren't native to U.S. fields. However, in fields where soybeans are grown regularly, large populations of rhizobia usually are present.
"Rhizobia populations have to be established in fields," notes Keith O'Bryan, Pioneer agronomy research manager. "Growers will need to inoculate fields with no recent history of soybean production to obtain decent yields."
Even in fields producing soybean regularly, populations may be too low to supply nitrogen needs. "We recommend inoculating fields every three years," O'Bryan says. "In light, sandy soils, populations may decline more quickly. Inoculating every year may be beneficial."
In the Dakotas, some growers use rotations that include soybeans only once every three or four years. These fields may need inoculation with every soybean crop.
Even with a strict corn-soybean rotation, rhizobia populations may drop below optimum levels under certain conditions. Drought or excess moisture can limit N fixation.
"During a drought, the plant may not supply enough nutrients to rhizobia," Conley says. "Our research is trying to determine how rhizobia populations decrease under drought stress."
Too much water also restricts rhizobia. If water limits the amount of air in the soil, it limits rhizobia numbers and how much N the bacteria can fix.
Today's growers want to maximize yields, which may require more N per acre. "We have to find the best way to get extra N," Conley says. The answer likely won't be to spread more N. "Inoculants seem to be a safe way to ensure N is available to feed soybean yields," he says.