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Evaluating N Fertilizer Sources and Timing for Winter Wheat¹

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.

Key Words: Triticum aestivum L. • N fertilizer efficiency • Urea • Ammonium nitrate • ¹⁵N • NH₃ volatilization • Field ¹⁵N technique

² Associate professor of soil science, Oregon State Univ., and assistant professor of soil science, Michigan State Univ., respectively. Research was initiated while the authors were extension soil scientist and assistant professor of soil science, respectively, at Montana State Univ., Bozeman, MT 59715.

Received for publication January 15, 1982.

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