To monitor changes in the capacities of rice soils to supply NPK over a period of intensive cropping, fertilizer trials were conducted for 10 consecutive cropping seasons from 1968 to 1972 at three experiment stations of the Philippine Bureau of Plant Industry. Each trial included three indicator varieties, with IR8 as the common variety, throughout the 10 crops and in all locations.At all locations, the efficiency of nitrogen use was highest in the first crop and declined with successive croppings; for phosphorus, the trend was opposite. Phosphorus responses were greater with K2O application, especially in the dry seasons. We observed a marked response to potassium, especially at Bicol. The efficiency of K use increased sharply over the years, especially in the dry seasons. Split application of K gave no clear-cut advantage over basal application. Some varieties responded better than others to K, but only in soils which responded well to K.Results indicate that in areas where one crop per year is grown, with adequate N supply, responses to P and/or to K may be marginal. Responses may become marked, however, under intensive rice cultivation.