No abstract

Wheat in pots and in the field was subjected plied at the boot stage of growth, N increased to different watering regimes and fertilizer-N, dry matter much less than when applied at either as ammonium-N with a nitrification sowing, but increased the percent N in the inhibitor or as nitrate-N, was applied at sow-grain. Divided dressings, half at sowing and ing or during growth. More weight of leaves half during growth, increased yield and N and stems was produoed during early growth uptake by the same amount as applying all with ammonium-N than with nitrate-N, but the N at sowing. Drought during spike forthe two forms gave similar yields of grain. mation or floret development led to smaller The grain:straw ratio with ammonium-N was spikes and yields of grain, and also to a smaller than with nitrate-N, and more N was smaller grain:straw ratio; up to three-quarters retained in the straw. Both forms of N in-of the extra N taken up from fertilizer-N creased the weight per spike but not the num-remained in the straw.

ABSTRACT

kg P/ha applied. The NaHCO3-extractable phosphorus level of the i00 kg P/ha treatment after 8 yr of cropping was reduced to about 8 kg P/ha which is considered to be inadequate for crop production. Soils treated with 200 and 400 kg P/ha contained high levels of NaHCO3-extractable P (20-54 kg P/ha) and little response in yield to additional P would be expected in the near future. Annual variation in yield and phosphorus uptake by the crops were affected by water supply. In years of high water supply, yield and phosphorus uptake were generally higher than in years of low water supply.

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.