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Corn+Soybean Digest

Necessary Inoculation

Many southern growers have switched from cotton to soybeans, and their whopping price. So spending an extra couple of dollars per acre to help assure the yields are there is a no-brainer, right?

That's about what growers pay to inoculate soybeans with the bacterium Bradyrhizobia japonicum, which allows the soybean plant to fix nitrogen (N) from the atmosphere, says Dan Poston, Mississippi State University soybean specialist in Stoneville.

Soybean planting usually continues through about April 10 in many southern regions, but may be stretched with the shortage of seed seen across much of the bean belt. And to Poston, not paying enough attention to inoculants is costing producers a lot of money.

“We encourage growers to do whatever it takes to ensure adequate nodulation, especially in long-time cotton fields that are transitioning to soybeans,” says Poston. “Too much of our cotton land doesn't contain the bacterium or fertility balance needed to assure good soybean nodulation.”

Poston also notes that other fields should be targeted for inoculants, including winter-flooded fields, fields that have not been in soybeans for the past five years and fields with a history of poor nodulation.

Nicholas O'Neal, whose family farms at Greenwood, MS, says they inoculate every bean they plant. “They perform better,” especially on long-time cotton ground that doesn't have the nutrients needed to push beans along.

The prime target land for inoculation usually has years of mostly continuous cotton, or a cotton-corn or cotton-rice rotation. The N-fixing bacterium is often missing.

Bacteria of the genus Rhizobium play a very important role in agriculture by inducing N-fixing nodules on the roots of legumes, such as soybeans. This can relieve the requirements for added N fertilizer. Soybeans are usually more vigorous in growth than cotton, which is being replaced on many southern acres this year. Failure to inoculate soybeans with N-fixing bacteria can reduce yields severely, says Poston, adding that soybeans can require 4-5 lbs. N/bu. of yield.

Along with the lack of N fixation on some long-time cotton land, Poston says the Reniform nematode can remain after cotton. “Reniform nematodes only make the problem worse by reducing the plant's ability to extract residual N and other nutrients from the soil,” says Poston.

“That's why it makes good sense to inoculate soybeans,” he says. “The cost of a typical inoculant ranges from $1.50 to $3/acre, depending on product and seeding rate.”

Seed can be treated with an inoculant or applied through the planter box.

O'Neal grows soybeans in 38-in. rows on irrigated land, as well as some twin-row beans and single, 19-in. rows on dryland. His family normally plants mid- to late Group IVs and average yields are about 50 bu./acre.

“We are going with beans treated with a fungicide and an inoculant that is labeled for 120-day treatment,” he says. “Even when we plant beans behind beans, we see a good result from inoculation. The nodules show up on the roots faster. Plants are healthier, quicker.”

Some would argue that inoculation isn't needed. Inoculant tests by Iowa State last year showed no overall yield advantage for inoculated seed (see results at However, if fields have been under the conditions described by Poston, growers should likely check with a local Extension agronomist or crop consultant to determine if inoculation is recommended.

“Inoculation is cheap compared to N,” says Poston. “We advise growers not to take chances with soybeans, especially on lighter-textured cotton fields that haven't been in soybeans for some time.”

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