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Adding nitrogen to soybeans may boost yields

Research published last year in the Journal of Production Agriculture showing a yield response to mid-season nitrogen applications on irrigated soybeans has some Midwest growers wondering whether the same response is possible on dryland acres.

The study found that 20 lbs. of N applied with center-pivot irrigators at the pod-fill stage increase yields on high-yielding ground by as much as 9 bu./acre and boost net return by $35/acre. The research was based on replicated plots on farmer fields in Kansas and Missouri.

"The increase seems to be consistent on irrigated acres," says Dr. Ray Lamond, agronomist with Kansas State University. "Dryland has potential, but there are a lot more ifs."

One "if" is finding a way, other than irrigation, to apply N mid-season without disturbing the crop. Dry-land options include highboy applicators with drop tubes that dribble urea ammonium nitrate between the rows or aerial applications of dry urea.

A second complication is the weather. Unlike with irrigation, rain is needed on dryland after application of N to move it into the soil.

Finally, in the study, yield increases were significant only on ground with a high yield potential, typically silt loam with 1 to 2% organic matter. Lamond advises growers to limit N applications to those acres with a yield potential of 60 bu./acre or more. At that level, the crop may not be able to fix enough N from the air to satisfy protein requirements and may benefit from added N.

Because of these variables, the economic returns found on irrigated ground will not be as consistent on dryland, Lamond says. However, such applications should not hurt yields at the 20 lbs./acre rate. And even the lowest yield response found in the study was still enough to cover costs of the application. -Jodie Wehrspann

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