between bloom and the end of Stage I growth. A similar strategy for four unequal splits is shown in Figure 1C. Growers may need to experiment with timing to find the best schedule for their varieties and harvest windows. For early-season varieties, the final nitrogen application should occur in mid-summer, while mid- and lateseason varieties can receive their last split in late summer or early fall. Keep in mind that fertilizer applied during the rainy season carries a higher leaching risk. For fresh-fruit production, leaf sampling can help maintain nitrogen in the proper range (Table 1): • High end of the optimum range (≈2.7%) during bloom, fruit set, cell division and cell enlargement stage • Low end of the optimum range (≈2.5%) or the low range (2.2–2.4%) during fruit maturation in the fall and winter The optimal timing for potassium differs slightly from nitrogen. Citrus requires less K during bloom and fruit set, but more K than N during the cell division, enlargement, and maturation stages. Potassium is especially important for proper fruit sizing. Example N:P₂O₅:K₂O fertilizer blends for different growth stages include: 1. 15:3:11–prebloom stage 2. 9:3:18–during cell division stage 3. 12.5:3:18–during cell enlargement stage HLB and Micronutrient Requirements Huanglongbing (HLB) causes leaf chlorosis and blotchy mottling similar to Mn and deficiencies. The disease also damages feeder roots, the main site of nutrient uptake, leading to deficiencies of K, P, Mg, Ca, Mn, Zn, and Fe. Research shows that supplemental foliar and soil applications of Ca, Mg, Mn, and Zn can help mitigate HLB symptoms. This has led to the widespread adoption of enhanced foliar nutrition programs across Florida. However, the effectiveness of these programs can vary by grove tree health and nutrient formulation. Because no single program fits all situations, UF/IFAS recommends at least two leaf tissue analyses per year to accurately assess nutrient status and adjust fertilizer applications accordingly. Element Unit of Measure Deficient Low Optimum High Excess N % < 2.2 2.2–2.4 2.5–2.7 2.8–3.0 > 3.0 P % < 0.09 0.09–0.11 0.12–0.16 0.17–0.30 > 0.30 K % < 0.7 0.7–1.1 1.2–1.7 1.8–2.4 > 2.4 Ca % < 1.5 1.5–2.9 3.0–4.9 5.0–7.0 > 7.0 Mg % < 0.20 0.20–0.29 0.30–0.49 0.50–0.70 > 0.70 Cl % --- --- < 0.2 0.20–0.70 > 0.701 Na % --- --- --- 0.15–0.25 > 0.25 Mn mg/kg or ppm2 < 18 18–24 25–100 101–300 > 300 Zn mg/kg or ppm < 18 18–24 25–100 101–300 > 300 Cu mg/kg or ppm <3 3–4 5–16 17–20 > 20 Fe mg/kg or ppm < 35 35–59 60–120 121–200 > 200 B mg/kg or ppm < 20 20–35 36–100 101–200 > 200 Mo mg/kg or ppm < 0.05 0.06–0.09 0.10–2.0 2.0–5.0 > 5.0 1 Leaf burn and defoliation can occur at Cl concentration >1.0%. : 2 ppm = parts per million. Table 1. Guidelines for interpretation of leaf analysis based on 4-to-6-month-old spring-flush leaves from nonfruiting twigs of healthy trees. Source: Florida Citrus Nutrition Guide UF/IFAS Citrus Magazine December 2025 11