Determining optimum macro- and micronutrient leaf concentrations for lowbush blueberry (Vaccinium angustifolium Ait.) using the boundary line approach and quantile regression

Maine wild blueberry fertilizer recommendations have been based on a set of prune year leaf nutrient standard concentrations proposed by Trevett in 1972 to maximize berry yield. Researchers at the University of Maine have field tested the standards and found some nutrients (B, Fe, Zn) appear to be too high. This paper compares two mathematical approaches to determine optimal leaf nutrient standard concentrations, the boundary line approach (BLA) and quantile regression (QR), when applied to leaf nutrient and yield data from nutritional studies conducted by the University of Maine between 1991 and 2009. Both procedures were effective in generating regression models from which optimum, minimum, and maximum leaf nutrient concentrations could be derived. The statistical soundness of the BLA may be less desirable, however, due to the arbitrary decisions made prior to the fitting of the regression models to the data. The optimum nutrient concentration levels calculated by QR for N (1.76%), P (0.136%), K (.469%), Ca 0.378 %), and Mg (0.137%) were higher than Trevett's standards, while those calculated for Fe (35 ppm) and Zn (15 ppm) were lower. Values were not obtainable for B, Mn, or Cu. These concentrations should be considered for future recommendations.