Flooding across parts of the Mid-South has caused or will cause many farmers to miss the planting window for corn. Reviewing some recommendations offered two years ago from Jeremy Ross, Extension agronomist, University of Arkansas, will, even today, provide some relevant insight related to questions about nitrogen and inoculants for anyone going with soybeans on ground they were previously preparing for corn.

“Many farmers asked me if the 70 to 100 pounds of nitrogen they applied for corn would hurt a soybean crop,” says Ross. “The short answer is no. We can assume some portion of this nitrogen will be lost from runoff as flood waters recede or lost from leaching down into the soil profile. The remaining nitrogen will be used by the soybean plant.”

A soybean plant requires around 5 pounds of nitrogen to produce one bushel of grain. It does not matter if the nitrogen comes from a native soil nitrogen, fertilizer nitrogen, or nitrogen supplied by the bacteria associated within the nodules on the soybean roots. “A 50-bushel-per-acre soybean yield requires about 250 pounds of nitrogen per acre,” says Ross. “Between 50 to 75 percent of this nitrogen comes from ‘nitrogen fixation’ that occurs in the nodules on the plant’s root system.”

High levels of residual nitrogen in the soil may inhibit nodule formations and activity, but, Ross says, “The remaining nitrogen, intended for the corn that wasn’t planted, should be used by the soybean plants quickly and proper nodulation should occur.”

Once the residual soil nitrogen is reduced to a low level, the process of nitrogen fixation in the nodules will begin. “It will, however, take about two weeks for the nodules to produce nitrogen the soybean plant can use,” says Ross.

Ross has been conducting soybean inoculant research for several years. Some of his earlier field trial research he got out in an early-season planting window (mid-April to mid-May) showed no yield response from the use of inoculants — compared to the untreated check.

For the last three years though, Ross has been evaluating effects of various planting dates and inoculants on soybean yields, and results have shown a significant yield increase using inoculants when planting late. “Mid-June planting dates are showing an average of 6 bushels an acre yield increase and mid-July planting dates showed an 11 bushels an acre yield increase when compared to our untreated check,” says Ross. “All commercially available inoculants and seed treatments we used performed equally as well.”

Ross did see a slightly lower yield response with dry, hopper box products compared to seed treatments, which may be attributable to better coverage with the seed treatments — possibly because of more bacteria per-seed with seed treatments.

“Our new recommendation is the use of soybean inoculants for any soybeans planted after May 16,” says Ross. “We would prefer the use of a seed treatment, but using the dry, hopper box product is fine if that is a grower’s only option. Our previous recommendations pertaining to soybean inoculants have not changed.”