Bean Booster

July 26, 2013 08:24 PM
Bean Booster

Soybean inoculants increase nodulation and fix nitrogen 

Weather extremes at planting, such as too little or too much moisture, are a sign your soybean crop might benefit from a commercial inoculant, a biological product made of living microorganisms called rhizobia.

Rhizobia help soybeans boost nodulation and fix nitrogen, says Russ Berndt, legumes and northern crops product manager with Becker Underwood, now part of BASF. "This leads to a more robust root structure, better nitrogen fixation and more yield potential," Berndt explains.

He estimates 50% of soybean seed in the U.S. is treated with a commercial inoculant, and that’s trending up.

Arthur, Ill., farmer Cory Green saw a return on investment from seed inoculant in 2012, despite the historic drought that plagued the Midwest.

"We saw a 1-bu.- to 2-bu.-per-acre yield increase," says Cory, who owns Heritage Family Farms with his brother, Justin, and dad, Mike.

The Greens have used an inoculant on seed beans the past three years. Their strategy is to use one on early planted soybeans and in soybeans after continuous corn. "That’s when we see the best yield results," Cory reports.

A yield increase of 2 bu. to 3 bu. per acre from an inoculant is common under either of those scenarios, he says, based on side-by-side comparisons of treated versus untreated seed.

The Greens do not use an inoculant for soybeans strictly planted in a corn–soybean rotation. "That’s not to say you shouldn’t, but we haven’t seen enough yield to justify it," Cory says.

Vince Davis, University of Wisconsin cropping systems Extension specialist, says a 1-bu.-per-acre yield increase from inoculant use is common. "With beans at $13 per bushel and a $2 to $4 per acre cost [for the inoculant], it takes very little yield benefit to pay for itself," he says.

Nodes for nitrogen. Inoculants also offer benefits beyond yield, Berndt notes. Well-nodulated soybeans produce an average nitrogen credit of 40 lb. per acre. This nitrogen is in the soil, in a highly stable form, and available for use by the next year’s crop.

The potential payoff from a soybean inoculant increases in one or more of these five scenarios,
according to Iowa State University Extension research:

1)The field has never been planted to soybeans.

2)Soybeans have not been grown in the field in the past three to five years.

3)The soil pH is below 6.0.

4)The soil has a high sand content.

5)The field has been flooded for more than a week, creating anaerobic conditions in the soil.

Because inoculants are living organisms, they must be handled with care. They can be easily killed by direct sunlight, heat and contact with caustic fertilizers and pesticides. For best results, Berndt says not to use an inoculant product that was carried over from the previous year.

"You want to use a product that is produced fresh each year because these are living creatures and they do die off over time," Berndt explains.

Alex Johnson, LG Seeds field agronomist, adds that inoculants are not all created equal, and their efficacy can vary considerably.

There are numerous inoculant products on the market. "Many can be mixed in the seed box with the seed or applied as the soybeans are transferred into the planter," he explains.

"Another convenient option is in combination with the talc or graphite that is already being used as a lubricant in the seed hopper." 

Performance Verification

Want to know whether your soybean inoculant is working? You can get a good sense if it is by evaluating the nodules on the plant roots once the crop reaches the vegetative stage, roughly six weeks after emergence, says Russ Berndt with Becker Underwood. Dig up the plant, being careful to leave a clump of soil around the roots in the process. Then, gently remove the soil from the roots by hand, or place the roots in a bucket of water to remove the soil.

Select a handful of the nodules, cut each one in half and look at their interior color. Nodules that are fixing nitrogen for the plant will be pink or a reddish color inside. If the interior of the nodules is green, grey or even white, your inoculant is not adequately working.

You can e-mail Rhonda Brooks at

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