Developing a nitrogen fertilizer management model for wheat in calcareous soils using the critical nitrogen dilution curve

The critical nitrogen (N) dilution curve, which expresses whole-plant critical N concentration as a function of shoot biomass, can be used as a N management diagnostic tool for cereals. The objec-tives of this research were to develop a critical N dilu-tion curve for wheat grown in calcareous soils and to formulate a model for estimating N fertilizer require-ment of wheat crop at different growth stages. Six N fertilization rates (0-250 kg N ha(-1)) were used to induce variability in plant growth throughout six site-years (three locations at West Delta of Egypt and two seasons [2020/21-2021/22]). Aboveground shoot bio-mass (W; Mg DM [dry matter] ha(-1)) and N concen-tration (Nc; g kg(-1) DM) were determined on five sam-pling dates during the growing season. A critical N dilution curve was developed as: N-c = 50.141W(-0.424). The Nc dilution curve was then used to develop a N fertilizer topdressing strategy. The study relied on N nutrition index inferred from the Nc based on N uptake, and instead of relying on a single N recovery