What Plant and Soil Testing from 16 Sites in Eight Midwestern States Tells Us about Split Nitrogen Applications.

It is hypothesized that split-nitrogen (N) relative to single near-planting applications improve corn (Zea mays L.) production, N recovery efficiency, and lessen environmental impacts of fertilization. However, these hypotheses have not been fully tested. A 16-site study across eight US Midwestern states was conducted to compare near-planting and split-N applications. Sixteen treatments were applied: eight rates from 0 to 314 kg-N ha^-1 as a single or split-N [45 kg-N at planting + V9-sidedress (45SD)], and 90 kg-N at planting + V9-sidedress (90SD) to achieve a total of 179 and 269 kg-N ha^-1. Before sidedress application, soil NO₃^--N (0-60 cm depth) at the V5 development stage was 22, 31, 49 and 64 mg kg^-1 for the 45SD, 90SD, single 179 kg-N ha^-1 and single 269 kg-N ha^-1 applications, respectively.  At tasseling plant biomass and N content were similar for single and 90SD, but the single application was approximately 8% higher than 45SD at both rates.  At tasseling soil (0-60 cm depth) NO₃^--N for the 179 kg-N ha^-1 rate had 10, 25, and 18 mg NO₃^--N kg^-1 for the single, 45SD, and 90SD applications, respectively.  No differences in plant N content, biomass, and grain yield was found at maturity between N application timings.  Post-harvest (0-90 cm depth) soil NO₃^--N for the 179 kg-N ha^-1 rate was 4, 6, and 8 mg kg^-1 for the single, 90SD, and 45SD applications, respectively.  At the 269 kg-N ha^-1 rate soil NO₃^--N was similar for the 45SD and 90SD (18 and 20 mg kg^-1) with the single application being lower (11 mg kg^-1).  Inconsistent results were observed for in-season measurements and between individual locations. Overall, relative to single N applications, split-N applications resulted in similar end of season plant N content and yield and similar or higher post-harvest soil NO₃^--N.