Can Fertilizer Nitrogen Applications Increase Productivity of Soybean?.

For optimal productivity, soybean [Glycine max (L.) Merr.] may require fertilizer nitrogen (N) to supplement biological N fixation. Our objective was to identify the best N fertilizer source and the best time of application to increase soybean yield. Field experiments were conducted at three locations in Illinois to evaluate seven N sources, along with an unfertilized control. The fertilizer sources included: ammonium nitrate (AN); ammonium sulfate (AMS), liquid urea ammonium nitrate (UAN), urea, ESN (controlled release urea), Limus Urea (contains the urease inhibitor Limus), and a mixture of ammonium nitrate + potassium nitrate + ammonium sulfate. Each source was applied at four stages during soybean growth (preplant, V3, R1, or R3) at an application rate of 112 kg N ha^-1 (100 lbs N acre^-1). When averaged over the three locations, AN resulted in the greatest yield increases (4.0 to 5.7 bushels acre^-1) followed by UAN (3.1 to 4.1 bushels acre^-1), where both sources significantly increased yield regardless of application time. Based on the yield results from the 2015 growing season, applications of N during the early reproductive period (R1 or R3) produced the greatest yield response. When examining the individual locations, variation in N source and the application time that gave the greatest yield increase was evident. Dependency of substantial yield increases upon a given location, N source, and/or application time suggests that certain N sources may work better with certain soil types and environmental conditions, and offers an opportunity to fine-tune the N applications to increase soybean yield.