Determination of a critical dilution curve for nitrogen concentration in cotton

Several nitrogen (N)‐rate field experiments were carried out in cotton to define dilution curves for critical N concentrations in individual plants (i.e., the minimum N concentration required for maximum growth at any growth stage). Nitrogen application rate had a significant effect on aboveground dry matter, N accumulation, and N concentration. As expected, shoot N concentration in plants decreased during the growing period. These results support the concept of critical N concentration in shoot biomass of single plants as described by Lemaire et al. (2007) and reveal that a dilution curve for critical N concentrations in cotton plants can be described by a power equation. The pattern of critical–N concentration dilution curves was consistent across the two sites. Nitrogen concentration for a given biomass varied greatly with the supply of N. After initial flowering, the N‐nutrition index (NNI) for aboveground biomass of individual plants increased with increasing N rates. Relationships between plant total N uptake and accumulated dry matter in the aboveground biomass can be described by the allometric‐relation equations for each dose of N. Nitrogen‐dilution curves can be used as a tool for diagnosing the status of N in cotton from initial flowering to boll opening. The relationship can also be used in the parameterization and validation of growth models for predicting the N response and/or N requirement of cotton.