Evaluating N Fertilizer Sources and Timing for Winter Wheat

Ammonia volatilization can reduce the effectiveness of surface applied N fertilizers. Our objectives were to evaluate N source by timing interaction under field conditions conducive to NH₃ volatilization and to compare the sensitivity of three approaches to assessing N source by timing interaction. Ammonium nitrate and urea fertilizers were topdressed at rates of 0, 34, 67, and 101 kg N/ha on 19 November or 23 March to ‘Winalta’ winter wheat (Triticum aestivum L.) grown on a nonirrigated Scobey silt loam (Aridic Argiboroll, fine, montmorillonitic). Plots receiving topdressed N contained sub-plots fertilized at the appropriate rate and time with ¹⁵NH₄ ¹⁵NO₃ or ¹⁵NH₂CO¹⁵NH₂ labeled fertilizers containing 1.31 atom percent ¹⁵N. Nitrogen fertilization increased straw yield, grain yield, grain protein, test weight, and N content of straw and grain. Uptake of soil N and fertilizer N in straw and grain were determined. Grain yield was fitted to a concurrent Mitscherlich regression model while total N uptake and fertilizer N uptake were fitted to multiple linear regression models to evaluate the relative effectiveness of each N source—timing combination. Comparison of regression coefficients for either grain yield or total N uptake revealed no significant differences between N source—timing combinations. Fertilizer N uptake regressed on rate of applied N was a better parameter for evaluating N source by timing interaction and showed that fall topdressed urea was only 69% as effective as fall topdressed ammonium nitrate. Spring topdressed urea, spring topdressed ammonium nitrate, and fall topdressed ammonium nitrate were equally effective in supplying N to wheat plants. Volatilization of NH₃ from surface applied fertilizer probably accounts for the lower relative effectiveness of urea topdressed on 19 November.