Optimizing phosphorus and potassium input based on phosphorus and potassium uptake characteristics in young dwarf apple trees

The nutrient requirements of apple trees for phosphorus (P) and potassium (K), as reflected in their uptake characteristics, are crucial for formulating appropriate fertilizer application strategies and improving P and K use efficiency. However, uncertainties persist concerning the uptake of these two elements in apple trees. A two-year field experiment (2019-2021) was conducted in a newly built dwarf 'Fuji' apple orchard, with and without nitrogen (N) fertilizer (75 kg N ha(-1)), to determine the uptake patterns of P and K by trees and the contribution of reserve P and K to total P and K. The results showed that the annual requirement for young dwarf apple trees was 1.4-1.8 kg P ha(-1) and 5.3-8.6 kg K ha(-1) at a P: K ratio of 1.0: (3.3-5.6). N fertilization had no significant effect on fruit yield, quality, nutrient absorption, and partitioning of the young apple trees. There was a significant positive relationship between P and K partitioning and biomass partitioning. Both P and K uptake were directly affected by the tree growth status. In the 2019-2020 season, the peak of P and K uptake occurred from the end of spring shoot growth to nutrient withdrawal, when 76.2 % of P and 76.1 % of K were absorbed by the trees. From nutrient withdrawal to dormancy, the nutrient uptake decreased, accounting for only 16.5 % and 7.4 % of P and K uptake, respectively. In the 2020-2021 season, the nutrient uptake peaked from spring shoot slow growth to the end of spring shoot growth. The proportion of P and K absorbed at this duration was 53.4 % and 57.2 %, respectively. However, P and K uptake was limited from nutrient withdrawal to dormancy and from the end of spring shoot growth to autumn shoot growth. Notably, >85 % of the P and 55 %-67 % of the K in the trees were correspondingly derived from the reserve P and K at spring shoot slow growth in the two seasons. The P and K fertilization can be synchronized with the tree demand by increasing the amount during the peak of tree uptake and reducing the application during stages of inactive uptake. The contribution of reserve P and K in trees must be considered when developing fertilization strategies for fruit trees.