Yield Response and Calibration of Critical Potassium Levels in Soil, Leaves, and Fruit Pulp of “Royal Gala” and “Fuji Suprema” Apples

The yield of apple trees as a function of potassium fertilization and the critical levels (CLs) and sufficiency ranges (SRs) of K in the soil, leaves, and fruits were determined in two experiments (two orchards) in four crop seasons. Plants of “Royal Gala” and “Fuji Suprema” cultivars were treated with 0, 50, 100, 150, or 200 kg K2O ha−1 year−1. Potassium was applied annually during the bud swelling phase and onto the soil surface in the projection of the plant canopy, without incorporation. Critical levels and SR were estimated by Bayesian segmented quantile regression models. The cultivar factor was the main source of variation in fruit yield, K concentration in leaves and pulp, and K exported by apples. The crop season was the second factor with the greatest contribution to apple yield and K concentrations in leaves. When data from all crop seasons and orchards were pooled, yield did not vary by K treatments. The concentration of K in the leaf and fruit pulp also did not change as a function of the K dose with grouped data. For fruit production, the CL of K in the soil was 170 mg dm−3 for both cultivars; 17.8 g kg−1 and 15.8 g kg−1 in leaf for “Fuji Suprema” and “Royal Gala”, respectively; 1150 mg kg−1 and 1080 mg kg−1 in fruit pulp for “Fuji Suprema” and “Royal Gala”, respectively. The lack of response to K fertilization indicates that the trees were operating within a nutritional plateau. Consequently, we recommend that K fertilization in subtropical apple orchards be guided strictly by soil and plant analysis. For orchards exceeding the soil critical level of 170 mg dm−3 and leaf concentrations of 17.8 g kg−1 and 15.8 g kg−1 in leaf for “Fuji Suprema” and “Royal Gala”, respectively, and 1150 mg kg−1 and 1080 mg kg−1 in fruit pulp for “Fuji Suprema” and “Royal Gala”, respectively, K applications may be reduced or temporarily withheld under similar high-K soil conditions, provided that soil and plant nutritional status are regularly monitored. This management strategy ensures high yields and more efficient and sustainable nutrient management.