Polyhalite Compound Fertilizer Improves Apple Yield and Fruit Quality by Enhancing Leaf Photosynthesis and Alleviating Soil Acidification: A Three-Year Field Study

Apple cultivation faces soil acidification and pollution due to excessive fertilization, compounded by a scarcity of potassium (K) fertilizers. Polyhalite, a natural multi-nutrient mineral, offers a potential sustainable alternative. Therefore, a three-year field experiment was conducted, comprising a no-potassium control (CK), two conventional potassium fertilizers (sulfate of potash-based and muriate of potash-based), and six polyhalite compound fertilizer treatments (with different basal and topdressing strategies), to evaluate their effects on apple growth and soil fertility. Results showed that the single topdressing application of potassium chloride-type polyhalite compound fertilizer (T6) achieved the highest yield in the final year, which was 10.11–28.03% higher than the other potassium-applied treatments. It also achieved the highest fruit vitamin C and soluble solids content (9.53 mg 100 g−1 and 13.27%, respectively). The T6 treatment demonstrated the best performance in terms of agronomic efficiency and partial factor productivity of potassium fertilizer, reducing fertilizer waste and loss. Furthermore, the T6 treatment effectively increased soil pH, available potassium, and exchangeable calcium levels, thereby improving soil fertility. Thus, polyhalite proves effective in replacing conventional K fertilizers, with the single topdressing of MOP-type polyhalite compound fertilizer (T6) offering the most comprehensive agronomic and environmental benefits.