Soybeans fix nitrogen, everyone knows that, but did you know soil bacteria are key partners in the process? Rhizobia, the soil bacteria in question, form a symbiotic relationship with the soybeans to create nodules and fix nitrogen all season long.

But sometimes soil might not have enough rhizobia for soybeans. In that case, a soybean inoculant could help add beneficial bacteria back to the soil.

Rhizobia, the soil bacteria important to  nitrogen fixation in soybeans, sometimes  need help from inoculant.

“If you use inoculant in a field that hasn’t had soybeans for three to four years, you’ll have a greater response [than one that had them more recently],” says Jim Pullins, marketing manager for Verdesian Life Sciences seed treatments and inoculants. 

In years without soybeans, rhizobia cannot feed or reproduce as well. In this case, levels are often below what soybeans prefer. Adding them back through inoculant can produce more nodulation.

When soil pH is greater than 8 or less than 5.8, inoculants can be added, says Justin Clark, technical market specialist with BASF. Bacteria naturally in the soil struggle to survive in those pH extremes and you can use inoculant to replenish their populations.

Fields with poor drainage or large pockets that held standing water the previous year are great candidates for inoculant. Renew rhizobia populations in especially damaged fields or spot treat if you don’t want the expense of inoculating the entire field.

On the other hand, extremely dry areas or sandy soils have low rhizobia survival rates, Clark says. Fields with organic matter of 1% or less have low bacterial activity and might be worth treating.

“Inoculant needs to be used anywhere a fumigant has been applied,” Clark adds. The fumigant kills whatever is living in the soil—the bad with the good.

“Using inoculant generally provides a 2% to 3% yield advantage,” Pullins says. In some cases, where soil is poor or bacterial levels are low, he has even seen yields double from inoculant use.

Testing averages a 4:1 return on investment for inoculant, or at least a 2 bu. per acre advantage. “The technology is usually less than $5 per acre,” Pullins says. “It can change depending on geography and seeding rate.”

Careful Considerations When Handling Inoculants

Since inoculants use living organisms (bacteria)they’re more susceptible to death during storage and application. Ensure whoever applies your seed treatment inoculant or stores your in-furrow treatment doesn’t kill the bacteria before you can use them.

“One thing that’s important to realize is not only the benefits of inoculants, but also the watch outs,” says Justin Clark, technical market specialist with BASF. Here are a few reminders he says are important this year and every year:

  • Make sure to follow mixing directions. Don’t use chlorinated water with application. The chlorine will kill the bacteria.
  • Watch storage temperatures. Aim for between 44°F and 77°F, not too hot or too cold.
  • Store the product and its packaging away from direct sunlight.
  • Make sure the product doesn’t freeze, as that kills rhizobia bacteria.
  • Don’t treat frozen seed.