Nitrogen response characteristics of wheat protein in relation to yield responses and their interactions with phosphorus

Abstract

Wheat fertilizer experiments at 58 sites on the north-western slopes and plains of New South Wales clearly demonstrated a widespread and severe deficiency of nitrogen on many soils. The frequency (70%) and magnitude of responses to nitrogen were much greater than previously recorded. Nitrogen fertilizer required to achieve near-maximum yields was also much greater, with more than half the experiments requiring more than 30 kg N/ha and 23 experiments requiring more than 60 kg/ha. Deficiency of nitrogen for grain protein was almost universal with only two experiments failing to respond to nitrogen fertilizer. The yield response curves for all except three experiments were well fitted by the exponential (Mitscherlich) equation, but the majority of protein response curves were convex to the X axis, or linear, so that maximum protein concentrations could not be estimated. There were four distinct types of protein response curves, and their occurrence seemed to be related to the degree of nitrogen deficiency. Where nitrogen was most deficient (mean protein <10.5%), response curves were convex or linear; at intermediate deficiency (mean protein 11.7%), response curves were sigmoid, and at low deficiency (mean protein 13.4%), curves were exponential. Yield response rarely occurred where grain protein was greater than 12%. In 10 experiments with convex or sigmoid curves, the first increment of fertilizer depressed protein levels, due to the dilution effect of a large yield response. Increasing amounts of phosphorus fertilizer increased the response to nitrogen in nine experiments and in most of these the response curvature was correspondingly decreased, especially at the highest rate of phosphate. These interactions showed that nitrogen was the primary limiting factor in most of these experiments. P fertilizer tended to depress protein concentrations, especially in the absence of N fertilizer, but it had no consistent effect on protein response to N. Because of the dominance of convex protein response curves, much higher levels of fertilizer N were required to give maximum protein responses than were required to give maximum incremental yield responses. It was usually uneconomic therefore to use fertilizer solely to maximize protein increases.