Most of Indonesian agricultural soils are acidic and low in both total and available phosphorus particularly. The major nutrient constraint in these soils is P and responses to both N and K are poor unless P deficiency has first been corrected. Correction of P deficiency therefore constitutes a major part of improving fertility of acid soils. Phosphate rock is recommended for application to acid soils where phosphorus is an important limiting nutrient on plant growth. This problem faces most acid soils in Sumatra and Kalimantan.