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.

Influence of ammonium: Nitrate ratios on the growth and nitrogen uptake of pearl millet

“…In treatments containing both NH 4 + and N0 3 , uptake of NH 4 * was higher than N0 3 " in the first 2 wk after transfer to the solution, similar to the results of Spratt (12) working with wheat ( Fig. 1).…”

“…was observed to drop 80-95% at mid-pod fill, a time when a heavy demand is placed on the plant for energy by the developing seeds (10). Some plants show a preference for a particular form of N. It is generally agreed that cereals prefer NH 4 + during early growth (12). Mills and McElhannon (11) found that corn also preferred NH 4 * after tasseling.…”

Ammonium and Nitrate Nutrition of Plants

“…early leaves and stems) than does nitrate nitrogen (Spratt & Gasser, 1970). Nitrate applied to wheat as the heads emerge (boot stage) increases the protein in the grain more than does applied ammonium (Spratt, 1974). Thus the effects of ammonium and nitrate on wheat seem to be quite different from those on the dicotyledonous flowering plants mentioned earlier.…”

Plant tissue tests for predicting nitrogen fertilizer requirements in spring wheat

“…Previously Vaughan et al (1990a) concluded that the observed differences between critical N levels could be due to the differences in the growing conditions in varying geographical regions and/or wheat classes. The study of Spratt (1974) indicated that critical N0 3 -N concentration of 300 ppm in dry-matter (DM) of above-ground parts of plant at anthesis was required for optimum grain protein concentration in grains. Peltonen (1992), in turn, showed that differences in R 2 in tissue N content between different plant parts at anthesis were insufficient to justify any particular choice of plant parts for sampling in the field.…”