Maintaining adequate levels of rhizobia organisms in the soil can help increase nitrogen (N) fixation, potentially leading to higher yields, according to DuPont Pioneer agronomists. When a field is planted to soybeans in a corn-soy rotation, the rhizobia populations tend to remain adequate. However, when growers change from soybeans to other crops, such as continuous corn, rhizobia populations drop.

GOT RHIZOBIA? When a field is planted to soybeans in a corn-soy rotation, rhizobia populations in the soil usually remain adequate. But when you change from beans to other crops such as continuous corn, the rhizobia population declines.

"Too little or too much water in a field also may impact rhizobia populations, but the effects are difficult to detect," says Paul Carter, DuPont Pioneer senior agronomy research manager. "There are some indications drought may curb rhizobia numbers and limit the soybean plant's ability to fix nitrogen."

Where can using a rhizobia inoculant pay off?
There's no easy way to know a field's rhizobia levels. If growers are uncertain, especially if they're targeting yields above 50 bushels per acre, they may want to consider supplying additional rhizobia at planting. It can be a cost-effective way to help boost overall yields. Any field that's never produced soybeans or that's been producing continuous corn for three years is a candidate for treatment with rhizobia inoculums.     

Fortunately, growers have the option of planting soybean seed that's been treated with rhizobia inoculum. DuPont Pioneer provides a Pioneer Premium Seed Treatment, or PPST, offering, called PPST 120+, a premium on-seed, inoculant plus extender. PPST 120+ delivers a high concentration of beneficial rhizobia bacteria to the soybean plant.

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PPST 120+ is an excellent companion to PPST 2030 FST/IST (fungicide and insecticide seed treatment) and helps prolong on-seed rhizobia life for up to 120 days or more, says Carter. This provides added flexibility for growers, increasing effectiveness from the time the inoculant is applied until after the seed is planted.

As you push bean yields higher, crop needs more N
"A seed treatment such as PPST 120+ helps provide inexpensive insurance, some peace of mind that nitrogen fixation avoids limiting yields," Carter says. "As growers push yields to 60 or 70 or 80 bushels per acre, the crop will need more nitrogen. It's important they exploit the plant's ability to fix nitrogen as much as possible. This requires adequate rhizobia populations in the field."

Growers may not realize how much N soybeans use during the growing season. Soybeans pull some N from the soil, just as corn does, but they also have a unique way of pulling nitrogen from the atmosphere. The mechanism is a soil bacterium, Bradyrhizobium japonicum, or rhizobia, that turn nitrogen gas in the soil into ammonium, a form the plant can use.

Rhizobia help soybean plants fix N from the air
Unless there's an abundance of N in the soil, the soybean plant will send out a chemical signal that attracts rhizobia bacteria to the roots. The bacteria invade soybean roots and establish colonies in rounded nodules on the roots. The plant supplies carbohydrates and minerals to the rhizobia which then "fix" the N as ammonium, fueling plant growth and protein development.

"Rhizobia are living organisms," says Keith O'Bryan, DuPont Pioneer agronomy research manager. "Any condition that affects the amount of oxygen in the soil can limit their numbers. Stresses such as drought, water saturation, high soil pH or sandy soils can curb rhizobia populations. Talk to your Pioneer sales professional for more information on nitrogen fixation and rhizobia populations."