NPK Fertilization Application Guideline for Low-chill Peach Production in Florida’s Subtropical Climate: A 3-Year Assessment of Seasonal Soil and Leaf Nutrient Dynamics and Fruit Physicochemical Quality

The effects of different NPK fertilization rates (0, 0.45, 0.9, and 1.36 kg/tree), applied at three different times, on soil nutrient dynamics, plant nutrient status, yield, fruit quality, and bioactive compounds in peach trees were evaluated over three growing seasons (2023–25) on sandy soils under the Florida subtropical climate. Fertilization rate, depth, and sampling time significantly influenced soil nutrient availability, with NO and NH concentrations increasing under higher NPK levels and predominantly accumulating in the surface layer (0 to 30 cm). Macronutrients such as P and K exhibited clear stratification in the soil profile, while S and some micronutrients showed partial mobility and temporal variability. Leaf nutrient concentrations displayed pronounced seasonal dynamics, with most macronutrients peaking early in the growing season and declining thereafter, whereas Ca and Mg increased at later stages. Fertilization enhanced leaf N and P status but showed an antagonistic effect on Ca. In fruit tissues, N and K concentrations increased with higher fertilization rates, while Ca decreased, and micronutrients such as B and Zn showed moderate increases depending on season and year. Moderate to high NPK rates (0.9 and 1.36 kg/tree) significantly improved total yield, fruit weight, and fruit size, with the highest yield observed at 0.9 kg/tree in 2025. In contrast, fruit quality attributes, including titratable acidity and color parameters, were unaffected by fertilization. However, bioactive compounds responded positively, with total phenolic content consistently increasing under higher NPK rates in later seasons, while total anthocyanin content and antioxidant activity also showed favorable trends. Although the 0.9- and 1.36-kg/tree treatments significantly increased yield, fruit weight, and bioactive compound concentrations relative to the control, no statistically significant differences were detected between these two fertilization rates. Accordingly, the 0.9 kg/tree rate may be considered the preferred agronomic recommendation, as it achieved comparable benefits with a lower fertilizer input.