TIME OF NITROGEN APPLICATION - EFFECTS ON WINTER-WHEAT AND RESIDUAL SOIL NITRATE

Rate and time of N fertilizer application can affect winter wheat (Triticum aestivum L.) forage and grain yield and quality, and may influence residual NO3-N concentration and distribution in the soil profile. Objectives of this experiment were to determine the effects of N fertilizer rate and time of application on forage and grain yield, N uptake, and distribution of residual soil NO3-N. Field experiments were conducted for four consecutive years on a Teller sandy loam (fine-loamy, mixed, thermic Udic Argiustoll) cropped to winter wheat under conventional tillage, Urea-ammonium nitrate (UAN) was broadcast and incorporated before planting (PPI or topdressed in December (Feekes 3), January Gate Feekes 3), February (Feekes 4), or March (Feekes 5-6) at rates of 34, 67, 101, and 134 kg N ha(-1), Soil samples were taken from each plot after each grain harvest to 1.2 m and analyzed for NH4-N and NO3-N, Forage yield response (measured in late March to mid-April) to January and December N application was comparable to PPI when adequate precipitation was received after N was applied. Applying N in February and March resulted in plant tissue damage and lower forage yields due to less early-season growth than with other applications. Apparent fertilizer N recovery in forage was highest when N was applied at or before mid-January, Date of N application had minimal influence on grain yields, Quadratic response surface models suggest the optimum time of N application for maximum forage and grain yields to be mid-November and early January, respectively. Timing of N application had no effect on residual NH4-N and little influence on residual NO3-N concentration and distribution in the soil profile at the end of each cropping cycle.