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Time of Nitrogen Application: Effects on Winter Wheat and Residual Soil Nitrate

Department of Agronomy, Oklahoma State Univ., Stillwater, OK 74078

*Corresponding author (r-boman{at}tamu.edu).

ABSTRACT

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 (late 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.

Approved for publication by the director, Oklahoma Agric. Exp. Stn.

Received for publication September 8, 1994.

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