Effect of Ammonium and Nitrate Forms of Fertilizer‐N and their Time of Application on Utilization of N by Wheat1

Abstract A hypothesis was tested: that NH4+‐N applied early and NO3−‐N applied later would be a beneficial cultural practice for wheat production. It is generally agreed that cereals prefer NH4+‐N during early growth. One growth chamber and three field experiments were conducted with wheat (Triticum aestivum var. Manitou) using ammonium (NH4+‐N) and nitrate (NO3−‐N) sources of fertilizer‐N applied either at sowing or as the heads emerged or at both times. A nitrification inhibitor was used with ammonium sulphate to supply NH4+‐N and untreated calcium nitrate and ammonium nitrate were used as sources of NO3−‐N. Levels of nonassimilated mineral‐N (NH4+‐N and NO3−‐N) in the green leaves and stems of wheat varied widely according to the source of N applied and its time of application. High levels of NO3−‐N in the plant at flowering led to high percentages of N in the grain. Consequently, fertilizer‐N applied as NO3−‐N as the heads emerged (‘boot’ stage) increased the percent N (protein) in the grain more than NH4+‐N. Split applications of fertilizer‐N, with NH4+‐N applied at sowing (with a nitrification inhibitor) to give maximum leaf and stem growth and NO3−‐N applied at the ‘boot’ stage to increase grain protein levels, may be worth considering as an agronomic practice for wheat production, particularly if new cultivars could be developed to use effectively the increased leaf area to produce more grain. Also, manipulating the nitrogen metabolism of the wheat plants affected the uptake of other nutrients, for example, applying NH4+‐N increased P and Zn uptake.