Post-Harvest Soil Nitrate Following Corn Production in Eight Midwestern States.

Applying nitrogen (N) at economically optimal rates (EONR) and at times of rapid crop uptake are practices that are thought to minimize the amount of residual soil nitrate (RSN) in the profile that may be susceptible to loss. The objective of this study was to evaluate the effects of rate (0 to 314 kg N ha^-1 in 45 kg increments) and timing (pre-plant (PP) or PP plus V9 sidedress (PP+SD)) of N application on corn grain yield and RSN in the top 0.90 m of soil relative to the calculated EONR. Thirty-two site years were established between 2014 and 2015 in Illinois, Indiana, Iowa, Minnesota, Missouri, Nebraska, North Dakota, and Wisconsin. At 20 of the 32 locations, the EONR for PP was greater than for PP+SD by an average of 37 kg N ha^-1. Average grain yields in 2014 at the EONR were not different between N application timings, but were slightly greater for PP+SD than for PP in 2015. Nitrogen application rate significantly (P<0.10) affected RSN at 30 of 32 sites. Nitrogen application timing was significant at 19 of 32 sites; 18 of those 19 sites had average RSN levels that were greater for PP+SD than PP by an average of 56 kg N ha^-1. Average residual soil nitrate values at the EONR were significantly different (P<0.05) across sites in both years with values of 34 and 55 kg N ha^-1 for PP and PP+SD, respectively, in 2014; and 55 and 72 kg N ha^-1 for PP and PP+SD, respectively, in 2015. While applying N as a split application tended to lower the EONR, it also resulted in greater amounts of RSN remaining in the soil profile that could be subsequently lost to the environment.