To Inoculate or Not

Do Your Beans Need a Boost?

Getting a yield response from inoculants is far from guaranteed, especially if you plant soybeans in a regular rotation. (DTN photo by Pamela Smith)

ROCKVILLE, MD. (DTN) -- The ability of soybeans to fix nitrogen from the air might seem like magic, but sometimes the plants need a little help.

Nitrogen fixation relies on soybean nodules, which require plentiful populations of a helpful soil bacteria species known as Bradyrhizobium. Inoculants can supplement those bacteria populations and boost soybean yields. However, an actual yield response from inoculants is most likely when fields have been out of beans for several years, have experienced sustained flooding or drought or have a low soil pH. All these factors can lower soil bacteria populations, according to a University of Nebraska CropWatch article.

Seed-applied inoculants cost between $1.50 and $2.75 per acre, according to Purdue University reports. That might have seemed like cheap insurance in the past, but this isn't the year to pile on inputs without assurance of a solid yield gain, University of Nebraska Extension agronomist Roger Elmore, soil fertility specialist Charles Shapiro and Extension educator Nathan Mueller warned in the CropWatch article.

"Using products that may increase yields 1-2 bushels per acre might have been worth the gamble when soybeans were $14.40 per bushel in 2012, but become a poor roll of the dice at the $8.85 per bushel set in February for some crop insurance policies," they wrote.

To simplify your inoculation decision, the researchers dug into the three culprits that contribute to poor nodulation: field history, environment, and soil fertility.


Fields that have been without soybeans for three to five years are likely to have lower populations of the necessary soil bacteria. Land coming out of pasture or continuous corn could benefit from an inoculant, the researchers noted. One 2009 Iowa State study documented a yield boost of 2 bushels per acre in a soybean field that had been grass pasture for many years.

Fields that have been planted to alfalfa for a long time might also show yield gains from an inoculant. Alfalfa uses (and thus encourages) a different bacteria species from soybeans, the Nebraska researchers noted.

If your field history doesn't fall into these categories, inoculation is not a sure bet. "The probability of a yield response in fields with a recent history of soybeans is extremely low," the researchers wrote. A study conducted in five Midwestern states from 2000 to 2008 tested 51 inoculant products in 73 experiments in fields that regularly grew soybeans and found an average yield response of zero bushels per acre.


Soil bacteria need a healthy environment, just like crops. A full week in flooded soils can starve the bacteria of oxygen and lower populations.

Droughts can also take a toll on these bacteria, but only if the field is in a crop other than soybeans at the time.

Even then, a yield response to inoculants isn't guaranteed, the Nebraska researchers said. After extensive and long-term flooding that occurred in Wisconsin fields in 2008, researchers found that soil bacteria populations were still adequate and the addition of an inoculant did not produce a yield response. (See the study here:…).

A low soil pH (under 6.0) can lower nodulation and kill certain bacteria species in the soil. Likewise, when soil pH is too high, nodulation and bacteria also suffer. Use lime applications to keep soil pH around 6.8, the Nebraska researchers advised.

See the University of Nebraska Crop Watch article here:….

See the Purdue guide on soybean inoculants here:….

Find the 2000-2008, 5-state study referenced in this story here:….