THE INFLUENCE OF REDUCED RATE OF APPLICATION OF PHOSPHORUS AND POTASSIUM FERTILIZERS ON FRUIT QUALITY AND SOIL IN A SATSUMA MANDARIN ORCHARD

The influence of reducing the application rates of phosphorus and potassium fertilizers on fruit quality and soil status was examined in a Satsuma mandarin (Citrus unshiu Marc.) orchard. Thirteen-year-old trees grown on gray upland soil in Japan were used. Treatments were defined by a factorial combination of three levels of phosphorus and potassium fertilizers (PK0%; 0, 0 or PK50%; 60, 120 or PK100%; P₂O₅=120 kg·ha^-1, K₂O=240 kg·ha^-1) , and two fruit load treatments (FL1; 25-30 in 2010 and 50-100 in 2011 or FL2; leaf/fruit ratio of 50-70 in 2010 and 25-30 in 2011). There was no influence on fruit quality by reducing the application rates of phosphate and potassium fertilizers. Furthermore, reducing the rate of phosphate application did not alter available phosphoric acid levels in the soil. The phosphorus concentration of leaves from PK0%-treated trees was lower than that of leaves from PK50%- and PK100%-treated trees; however, the concentration of phosphorus in leaves remained within the standard range for citrus leaves. The exchangeable potassium content in the soil of PK0%-treated trees was lower than that in the soil of PK100%-treated trees. These results show that lower levels of phosphorus fertilizer may be applied to Satsuma mandarin orchards to prevent the accumulation of available phosphoric acid in the soil. In contrast, exchangeable potassium levels in the soil declined. Hence, further soil analysis is required to determine the extent to which this fertilizer may be reduced. Fruit bearing volume was influenced by potassium in the leaves, peel, and pulp, as well as fruit quality.