The uptake of nitrogen by wheat, its agronomic efficiency and their relationship to soil and fertilizer nitrogen

Nitrogen uptake by wheat from both soil and fertilizer, and the efficiencies of fertilizer N (up to 116 kg/ha) for increasing yield and protein, were measured in 53 wheat fertilizer experiments during 1985-89 on the north-western slopes and plains of New South Wales. There was a highly significant (r2> 0.70) and common relationship between N uptake in unfertilized wheat (tops and grain) and soil nitrate to 90 cm depth for 4 of the 5 years of the study. A different but significant relationship occurred in 1988 when heavy rainfall before sampling leached some of the soil N beyond the sampled depth but within the rooting zone. The uptake and recovery of fertilizer N were lower in 1989, when in-crop rainfall was much lower than in the other 4 years. However, there was greater transfer of N from the herbage to the grain than in the wetter years. With increasing increments of fertilizer N. there was a much larger average decline in agronomic efficiency than in the recovery of fertilizer N or in physiological efficiency. Consistent with this, the average protein efficiency of fertilizer N tended to increase with increasing increments in every year except 1989. Although the highest increment of fertilizer N was always the least efficient for increasing grain yield, it exceeded the level required for profitability (8 kg grain/kg fertilizer N) in 20% of experiments. In experiments in which agronomic efficiency of the highest fertilizer increment was too low for profitability, there were at least 10 experiments in which the protein response was probably sufficient to make the highest increment profitable. The agronomic, protein and physiological efficiencies of fertilizer N in at least 10% of these experiments were higher than previously recorded in Australia and are comparable with the highest values recorded for wheat in other regions of the world.