October 3, 2005

Soybeans, friendly bacteria hit it off through inoculation

WEST LAFAYETTE, Ind. - 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 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, said 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 said.

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.

The publication is the first in a series Conley is developing under the heading of "Soybean Production Systems." Subsequent publications will deal with such topics as seeding rates, planting dates, row spacing, fungicides and soil fertility.

"We've had 10 years of data to analyze soybean inoculants," Conley said. "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 a bushel per 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."

One method of inoculating soybeans involves treating seed with powdered or sterile peat containing rhizobia, Conley said. The peat is mixed with the seed in the planter or drill box. Liquid inoculants, which are metered onto the seed at the base of the auger, are becoming more popular as bulk seed use increases.

Less common are soil-applied inoculants, Conley said. "It is important to maintain the recommended concentration per 1,000 feet of row to be effective," he said. "If row spacing is less than 30 inches, the cost increases accordingly, and may not be cost effective."

Regardless of what method they use, growers can expect inoculation to add to their soybean production costs, Conley said.

"On average, the cost to put inoculant on soybean plants ranges from $1.50 to $2.75 per acre. So the cost is reasonable based on some of the other inputs we add to the soybean crop," he said. "Another thing growers should consider is whether the cost benefit and time it takes to apply the inoculant is worth the potential gain of a bushel an acre. That will depend on each grower and how they've set up their system. It's going to be very site-specific."

Deciding whether and when to use inoculants are questions soybean growers should answer before planting, Conley said.

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. Recent research indicates that, depending on geography and the soil composition, inoculation can be beneficial if soybeans are grown every other year in rotation with corn, he said.

"These experiments have had variable results among states, suggesting that regional differences may exist," Conley said.

Another inoculation study, conducted by Purdue, compared two fields where soybeans had not been grown for at least 15 years. One soybean crop was inoculated and the other was not. The study found that the inoculated field produced a yield of 75 bushels per acre, while the uninoculated field yielded 63 bushels of soybeans an acre.

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

"We would not recommend utilizing an inoculant in a continuous soybean rotation," he said. "Since we are continuously producing new rhizobia every year that we're fixing nitrogen in a continuous soybean environment, it would most likely not pay.

"On the other hand, if we haven't planted soybeans for a year, there's some evidence that under certain situations it does pay for itself. It is very environment-specific."

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Shawn Conley, (765) 494-0895, conleysp@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
Agriculture News Page

Related Web site:
Purdue University Department of Agronomy

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