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Research Highlights
Exploring Soybean Yield Response to Nitrogen Applications

Photo: United Soybean Board

By Barb Baylor Anderson

Regardless of where soybeans are grown in the United States, they have large nutrient requirements throughout the growing season. And soybean nitrogen (N) requirements are especially high given protein content that averages 40 percent based on seed dry weight.

So, what are the best options for adding nitrogen to a growing crop to enhance yield?

That is the question Shawn Conley, University of Wisconsin soybean specialist, set out to answer as principal investigator of research funded by the Wisconsin Soybean Marketing Board.

“Soybean N requirements peak during the R3 to R5 growth stages. That N requirement is generally fulfilled by biological nitrogen fixation (BNF) plus N uptake from soil,” Conley explains. “However, BNF activity can be limited by a number of environmental conditions nationwide, such as low soil moisture, soil pH and temperature extremes and soil compaction, any of which can result in insufficient N supply to soybean plants.”

Conley says research has previously documented the impacts of N fertilizer sources, application rates and methods and seasonal timing on soybean yields. Many of these studies found inconsistent yield responses within and across states. Such variation may be due to differences in soybean cultivars, soil properties, growing conditions, topography and management practices.

“We combined data from multiple soybean N fertilization studies across multiple locations and years to examine the effects of N fertilizer in terms of single or split applications using soil surface, incorporated or foliar methods or a combination of them. We looked at timing that included pre-plant, at-planting, Vn or Rn growth stages or a combination, and N rates,” he says.

Conley’s analysis suggests N management can only be optimized when considering the cropping system. That’s because non-N management practices such as irrigation and seeding rates interact with N timing and N rate. Conley notes yields from combined N application methods were less than two bushels per acre greater than yields from non-treated plots and soil incorporated.

In addition, only small differences were observed by Conley among different N application timings. Yield response to N rate was more variable at lower application rates.

“Large yield differences among individual experiments where a similar N rate was applied were attributed to in-season weather variability among diverse growing environments and to major management practice differences,” he says. “These results suggest that other, non-N practices might affect soybean yield alone or in interaction with N decisions.”

Bottomline, Conley found application decisions had a minimal effect on soybean yield. Such limited nitrogen responses suggest that positive economic returns from them are unlikely.

For more information, download the document: Soybean Response to Nitrogen Application Across the U.S.