Estimation of the Critical Nitrogen Dilution Curve for Spinach for Processing Use

Accurate diagnosis of nitrogen status is essential to prevent excessive nitrogen fertilization and maintain productivity. Critical nitrogen dilution curves (CNDCs) serve as nitrogen diagnostic tools for numerous crop species. The critical nitrogen concentration (Nc) represents the minimum nitrogen concentration (%N) required to achieve maximum yield. Despite the substantial nitrogen requirements of spinach, the diagnostic criteria for assessing its nitrogen status have rarely been investigated. Here, the applicability of two frameworks, sequential (a method that has been used over the years) and hierarchical (a method that has recently garnered interest), was examined to estimate the CNDC for spinach for processing. The CNDC for spinach for processing was estimated at Nc = 5.03W−0.099 using a sequential framework and Nc = 4.67W−0.030 using a hierarchical framework. The power exponent estimated in this study was smaller than that of other crops, revealing that spinach maintained a high %N until the late growth stage. The nitrogen nutritional status of a crop can be determined using the nitrogen nutrition index (NNI), which is calculated by dividing the observed %N by the Nc. The response of crop productivity to nitrogen can be represented by variations in daily intercepted radiation (DIR) and radiation use efficiency (RUE). Spinach for processing remains under cultivation even after the saturation of radiation interception, with productivity in the later stages of the growing season reliant on RUE. At harvest time for spinach for processing, the RUE was regressed using a quadratic model with NNI as the explanatory variable, indicating that the response of RUE to NNI reached saturation or even slightly declined at high NNI levels. NNI = 1.11 (sequential framework) or NNI = 1.29 (hierarchical framework) may serve as criteria for maintaining RUE at elevated levels and preventing overfertilization. The findings of this study provide essential insights into nitrogen management strategies for optimizing spinach productivity, providing a foundation for sustainable fertilization practices that balance yield efficiency with environmental responsibility.