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Corn+Soybean Digest

Should Soybeans Be Inoculated?

You hear a lot of tall talk about using inoculants on soybeans. New strains and formulations are supposed to kick up yields where previous products didn't.

But, in most cases, using an inoculant won't improve yields in fields that have been planted to soybeans in recent years.

The results from six years of research convinced Michigan State University crop scientist Maury Vitosh.

"The data suggest that soybean seed inoculation with Bradyrhizobium is only beneficial when soybeans are planted on new soybean ground," Vitosh says.

"We have evaluated Hi-Stick, Nitragin, Urbana and Sow-Fast inoculants," he continues. "Most of our trials have been on land that has had a history of soybean production - two or three years away from the previous soybean crop. We didn't get a yield increase due to soybean seed inoculation or liquid in-furrow treatments."

The increased popularity of no-till soybeans also has raised concerns about the performance of inoculants under those conditions.

"The concern is that the heavy residues, higher soil-moisture content and cooler soil temperatures may hinder efficient soybean inoculation," says Vitosh. "Our tests indicate that those concerns are unwarranted."

On the western edge of soybean country, where soybeans haven't been grown before or are planted every few years, however, inoculants do have an advantage.

"Most of this ground has never seen soybeans," says Dave Collins, a Gothenburg, NE, crop consultant.

"Using an inoculant increases yields 25 to 30 bushels per acre. We see another 5- to 7-bushel increase in fields where we use liquid inoculants placed in the seed furrow.

"We recommend that our farmers inoculate each time they plant soybeans," Collins adds. "We don't have a native population of rhizobium in our soils."

Several of his clients have used Lift inoculant the last two years. It's a liquid product that's placed with the seed. There's roughly a dollar difference between the cost of seed-box treatments and that of liquid in-furrow products.

"Several years ago we set up some planters to apply a soil amendment with the seed that was supposed to boost yields," Collins reports. "The product did not pan out, but the micro-injectors are perfect for applying Lift in the seed furrow.

"The amount of nodulation we get with this system is just amazing. The plants are darker green and outrunning anything else. We get nodule clusters two weeks before anything else."

Gothenburg grower Larry Gill is sold on the liquid inoculant.

"It probably costs more to use the liquid, in terms of equipment and product, but if you grow 300 or more acres of soybeans, it's absolutely worth it," says Gill.

"With the liquid, you don't have to take the beans out of the bag and go through the whole rigmarole of getting the inoculant on the seed," Gill points out. "What we spend extra for product we save in labor. And it does an excellent job. Our soybeans start to develop bacteria colonies almost immediately."

"Evidence of soybean nodules can be observed any time after plant emergence," says Vitosh. "Active N fixation, however, doesn't start until the V2 or V3 stage. If the soil contains large amounts of nitrate, or N fertilizer has been applied, nodulation will occur only after the soil nitrate level has been reduced by plant uptake or lost from the root area by leaching or denitrification."

If you farm low-pH soils, you might need to inoculate every soybean crop, says Vitosh.

"When growing soybeans in acid soils, with a pH of less than 6.2, it's advisable to use an inoculant," he says. "Soil micro-organisms prefer a soil pH of 6.5 to 7.5 for maximum activity."

A well-nodulated root system should have many nodule clusters around the taproot and the interior of the nodules should be bright pink to red, he says. If the nodules are green or white, they're not functioning properly and may benefit from 30 to 40 lbs/acre of nitrogen fertilizer, applied before full bloom.

Land coming out of the CRP program is also a good candidate for soybean inoculants.

"Anytime land has been out of soybeans more than three years, we recommend using an inoculant on soybeans," says Vitosh "Inoculants are inexpensive and good insurance where you know you need them."

Vitosh also believes that inoculant manufacturers produce better products now.

"I think they're much better than they used to be," he says. "The companies are paying more attention to product viability."

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