New Leaf Nitrogen Sufficiency Ranges for Southern Highbush Blueberry

Leaf analysis is a key tool for determining blueberry plant nutritional status. For leaf analysis to be useful, optimal concentration thresholds must be established. Existing thresholds, which were developed for northern highbush blueberry (NHB; Vaccinium corymbosum ), recommend optimal blueberry leaf nitrogen (N) concentrations between 1.76% and 2.00%. It is unclear, however, whether these values accurately represent southern highbush blueberry (SHB; V. corymbosum interspecific hybrids) because SHB and NHB exhibit contrasting physiological responses and cultivation ranges. This study aimed to refine N sufficiency ranges for SHB, accounting for variations across production systems (deciduous and evergreen) and phenological stages (vegetative growth, flower bud initiation, bloom, and fruit development). Data from five previous experiments were analyzed by fitting Normal, Gamma, and Weibull distributions. A sufficiency range approach was then used to identify optimum leaf N thresholds for SHB. Finally, the accuracy of these thresholds was assessed with a random forest classification model that included plant, management, and environmental data. The Gamma distribution provided the best fit in three of the seven system-by-stage groups, with normal and Weibull distributions each selected in two. Refined leaf N sufficiency thresholds, derived from the best-fitting distribution model for each system by stage group, were proposed. The random forest classification supported the agronomic and physiological relevance of the proposed leaf N concentration thresholds. These results will improve nutritional status diagnostics in SHB production.