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Corn+Soybean Digest

Soybeans, Friendly Bacteria Hit It Off Through Inoculation

If it's possible for an agricultural crop to have friends, the soybean plant can count a bacterial organism as its best buddy.

Bradyrhizobium japonicum, commonly known as rhizobia, shares its nitrogen-utilizing skills with soybean plants. Soybeans need nitrogen (N) for proper development but are ill equipped at turning it into a useable form. In exchange for its kindness, soybeans nourish rhizobia and give the bacteria a home on its roots.

In cropfields where soybeans have not been planted for at least a year, rhizobia usually are introduced through inoculant - a material that contains the bacteria. Soybean seed can be treated with inoculant at planting or rhizobia can be applied directly to the soil. Either way, research indicates that inoculating soybeans increases crop yields, says Shawn Conley, Purdue University Extension soybean specialist.

Before they invest in inoculant, however, growers should consider whether yield response might be good enough to warrant the extra production cost, Conley says.

Conley and retired Purdue soybean specialist Ellsworth Christmas outline the pros and cons of soybean inoculation, and how inoculant is used, in Purdue Extension publication SPS-100-W - "Utilizing Inoculants in a Corn-Soybean Rotation." The publication can be downloaded online at

"We've had 10 years of data to analyze soybean inoculants," Conley says. "The data has suggested that over years, locations, environments and cropping systems in Indiana, inoculation has been shown to increase yields an average of about one bu./acre.

"We're not necessarily recommending that every grower in Indiana put inoculant on every acre of soybeans, but we are suggesting that some of the newer technologies are making it more grower-friendly. Some of those newer inoculant technologies can be applied up to 30 days before planting." Regardless of what method they use, growers can expect inoculation to add to their soybean production costs, Conley says.

"On average, the cost to put inoculant on soybean plants ranges from $1.50 to $2.75/acre. So the cost is reasonable based on some of the other inputs we add to the soybean crop," he says.

Conventional wisdom says inoculation is unnecessary if a well-nodulated soybean crop has been grown in a field within the past five years and rhizobia levels have built up in the soil, Conley said.

In fields where soybeans are grown every year, inoculation probably won't improve yields, Conley said.

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