Agronomists recommend that farmers reduce nitrogen by 40 lbs/acre on corn following soybeans vs. corn after corn. Although some producers are hesitant to do so, recent University of Illinois research should reassure them.

“Some people just don't want to risk being short on N,” says Dave Ricke, an independent crop consultant in Greensburg, IN. “They are concerned about the looks of the corn and about getting lower yields than their neighbors. But most of my clients, especially the larger-acreage farmers, are bottom-line thinkers and do reduce N on corn following beans. They are more interested in profits than in looks.”

The Illinois study, conducted by researchers Lowell Gentry, Fred Below and Mark David, confirms that growers can safely reduce N on corn after soybeans. It also explains why.

The researchers compared unfertilized corn grown after nodulated soybeans, corn grown after non-nodulated soybeans and corn following corn.

“We used nodulated and non-nodulated soybeans to separate out the effect of nodules on nitrogen mineralization,” Below says. (Mineralization is the microbial breakdown of N from organic matter into plant-available forms such as nitrate and ammonium.)

The research team measured soil N mineralization, yield and plant nitrogen accumulation.

To determine soil mineralization, they used a buried bag technique. Soil cores were placed in polyethylene bags and buried in the field to incubate at ambient (surrounding) temperatures. Approximately every month, soil in the bags was analyzed for net changes in nitrate and ammonium.

They found that the previous crop significantly influenced soil N mineralization. Ninety-five lbs/acre of N were available to corn following nodulated soybeans. That dropped to 77 lbs/acre for corn after non-nodulated soybeans and 58 lbs/acre on corn after corn.

Corn (unfertilized) after nodulated soybeans yielded 120 bu/acre vs. 45 bu/acre for corn following corn (0% moisture basis). Plant N accumulation was 81 lbs/acre on corn following nodulated soybeans; 62 lbs/acre on corn after corn.

“Part of the reason for the disparity in mineralization between corn and soybeans as the preceding crop,” says Gentry, “is the fact that corn, with its greater residue, requires more N for decomposition. So N that would normally be available to plants is now absorbed (or immobilized) by soil microbes as they decompose the corn residue.”

In view of this; the term “soybean N credit” seems somewhat misleading. That's because the difference in N availability between corn after soybeans compared with corn after corn is mostly due to greater immobilization of soil N following corn production.

Another factor in the greater N availability following nodulated soybeans is an increase in soil N from the breakdown of soybean nodules. “We did find more N near the old soybean row following harvest of the nodulated soybean plots,” says Below. “So, our overall conclusion is that soybean N credits result from a combination of a decrease in net soil N mineralization following corn production and an increase in residual soil N from the breakdown of soybean nodules.”