Results from a two-year (mid 1990’s) Kansas State University study at four irrigated sites in Kansas show that late-season application of N to soybeans at the R3 growth stage will increase soybean yields. These results, coupled with similar results in other states, suggested that retail input suppliers, public and private labs and consultants who make fertilizer recommendations should consider N applications on irrigated soybeans with high-yield potential. Six of the eight site-years were responsive and showed an average soybean yield increase of 6.9 bu/A or 11.8 percent with late-season N fertilization. Yields at these six sites ranged from a low of 56 bu/A to a high of 83 bu/A. The researcher concluded that producers of high yielding soybeans (greater than 55 bu/A) would benefit from a late-season application of N at the 20-lb/A rate of N.
Fast forward to 2016 where University of Delaware researchers arrived at very similar conclusions. These researchers found that application of supplemental N may provide a yield benefit for high-yield soybeans, but only in cases where expected yields are 60 to 70 bu/A or higher. In summing up their research, they concluded that Growers are unlikely to see yield increases from late-season (R3) supplemental fertilizer N in situations where yields are not likely to exceed 60 bu/A. In production scenarios where yields are likely to be less than 60 bu/A, application of supplemental N is more likely to result in unnecessary expense and increased environmental impact.
Access Kansas State University Fluid Journal Soybean N article
Access University of Delaware Fluid Journal Soybean N article