• Nitrogen (N) fertilizers are used to supplement the soil’s natural N supply to increase corn grain yield and maximize economic profit.
• However, the use of N fertilizer has been associated with increased nitrate levels of ground and surface waters. High nitrate levels can cause negative environmental, human, and animal health effects.
• Splitting N fertilizer across multiple application timings during the growing season is a suggested method to lower the risk of N loss by applying N fertilizer in amounts and at timings that more closely match corn N uptake.
• The objectives of this study were:
  • Determine the effects of split N application on soil nitrate (NO₃-N) at VT, N uptake at maturity, and corn grain yield.
  • Classify environmental conditions in which split N application is likely to increase corn yield.
  • Compare the effect of split N application when a low or high N rate is applied near planting on soil N concentrations, corn N uptake, and corn yield.

• Research was targeted to two sites (representing higher- and lower-yielding environments) in each of eight Midwestern states over three years (2014-2016). A total of 49 site-years were included in the study (Figure 1).

• Nitrogen fertilizer application timing did not affect soil NO3-N at VT (65-77%) in the majority of the sites (Figure 2).
• In the sites where application timing affected soil NO3-N at VT, single N applications were always less than split N applications. Thus, splitting up the application of N only increased the amount of soil NO3-N available at VT in a small percentage of sites.
• The amount of N fertilizer applied at planting (40 vs. 80 lbs/acre within the split N applications did not alter the amount of NO3-N available for the corn crop at the VT development stage in 98% of the sites.
• These results demonstrate that applying a low or high rate of N at planting with a split-N application did not significantly alter the amount of soil NO3-N available for the corn at VT.

• Nitrogen timing had no effect on corn N uptake at physiological maturity 77-84% of the time regardless of N rate (Figure 3).
• When differences occurred, N uptake was greater 9-10% of the time using split N applications and 6-15% of the time using a single N application.
• For split N applications, there were no differences in N uptake at physiological maturity when applying 40 or 80 lbs/acre at planting 98% of the time regardless of the N rate.
• When there was a difference, applying less N at planting and more at sidedress (40+SD) led to greater N uptake at physiological maturity.

• Corn yields ranged widely across the 49 site-years of the study, although yield ranges were similar among nitrogen rates and application timings (Table 1).

Table 1. Corn yield ranges across all site-years of the study associated with N rates and application timings.

• No differences in corn yield among 40+SD, 80+SD, and single N applications were found in the majority of site-years (77-84%) regardless of N rate.
  
• Overall, split N applications had greater corn yield in areas with consistent rainfall around the time of sidedress application that incorporated the fertilizer, and in soils with greater potential for N loss early in the growing season (i.e. sandy soils that have greater leaching potential).
• In general, single N applications had greater corn yield in soils with greater potential for mineralization throughout the season (i.e. greater total N content) and better nutrient and water retention as indicated by greater CEC and silt and clay content.
• No differences between 40+SD and 80+SD were found in 97-98% of the sites, indicating that applying 40 lbs/acre near planting is all that may be needed when using split N applications.