Thresholds of Leaf Nitrogen for Optimum Fruit Production and Quality in Grapefruit

Fertilization is critical for sustainable production of citrus on sandy soils. However, information on nutritional diagnosis standards for grapefruit (Citrus × paradisi Macfad.) is lacking and this information is needed for implementation of best management practices (BMPs). A field experiment was conducted from 1997 to 2000 on a Riviera fine sand (loamy, siliceous, hyperthermic, Arenic Glossaqualf) with 30-yr-old+ white Marsh grapefruit trees on sour orange (Citrus aurantium L.) rootstock to evaluate irrigation and fertilization effects on fruit yield and quality and to validate leaf nutrient concentration standards for guiding fertilization of grapefruit. Fertilizers were applied as water soluble granular (WSG, 3 applications yr⁻¹), by fertigation (FRT, 15 applications yr⁻¹), or as controlled-release fertilizers (CRF, 1 application yr⁻¹) and at five rates (0, 56, 112, 168, or 224 kg N ha⁻¹ yr⁻¹) with an N:P:K blend (1.0:0.17:1.02). Fruit yield and quality were not affected by irrigation treatments or fertilizer sources. There was a significantly positive correlation between leaf N concentrations and N rates (r = 0.98**). Fruit yield was linearly related to N rates or leaf N concentrations. At 90% of maximum yield, leaf N concentrations (dry weight basis) were 22 to 23 g kg⁻¹ Fruit quality parameters such as soluble solid concentration (SSC), juice, and total soluble solids (TSS) were positively correlated with leaf N concentrations, whereas fruit titratable acidity (TA) was negatively related to leaf N concentrations or N rates. The effect of N rate on TA outweighed that on SSC and consequently, the SSC/TA ratio decreased with increasing N rates or leaf N concentration. Fruit size was quadratically related to N rate or leaf N concentration. Overall, fruit sizes and SSC/TA ratios were acceptable for fresh marketing or processing at leaf N concentrations of 22 to 23 g kg⁻¹ Therefore, this leaf N concentration of 22 to 23 g kg⁻¹ can be considered the optimal concentration guideline for fertilization of grapefruit provided that other nutrients are sufficient.