Predicting change in residual extractable P for site specific management in paddy soils of Indonesia

Paddy-soils in Java, Indonesia, have been intensively fertilized with nitrogen (N), phosphate (P), and potassium (K) for at least three decades through centralized government subsidy programs, such as BIMAS, INMAS, INSUS, and SUPRAINSUS. One side effect of these programs has been an accumulation of soil phosphorus (P) to excessive levels. The N, P, and K nutrients were added to all fields in rice production without modification for soil P status or for soil properties. The goals of this study were to: 1. ensure sustainable rice production; 2. improve nutrient efficiency; and 3. improve phosphate management in paddy soils to reduce both fertilization cost and environmental hazards. The objective of this study was to develop a mathematical model to predict how long P fertilization could be suspended and how to tailor P fertilization to soil criteria. A mathematical model was developed to predict extractable P as a function of frequently measured soil properties, namely initial extractable P, pH, clay content, organic carbon content, and clay physical activity. The model for HCl-extractable P status in paddy rice soil was: EP, = {0.67 EPi - 11.63} + (22.26 OC) + (0.006 CLAY P) + (59.13 OC EXP(-(-0.14 + 0.32 CAT) Time)} The resulting model was combined with a geographical information system (GIS) software, ARC/INFO, to map P status and to predict the change in extractable P of paddy soils of Java with successive cropping. The model was also used to estimate site specific P recommendations which then mapped and analyzed by GIS. Two scenarios of the P fertilization strategies were considered: 1. A) Fifty kg TSP ha"' would be applied to soils with medium and high extractable P, applied in every 4 crops; B) Fifty kg TSP ha"' for soils low in extractable P, applied in every crop. And, 2. A) Fifty kg TSP ha’^ would be applied to soils high in extractable P, applied once every 8 crops; B) Fifty kg TSP ha'' for soils medium in extractable P, applied once every 4 crops; and C) Fifty kg TSP ha"' for soils low in extractable P, applied to each crop. Approximately 85 000 to 94 000 tonnes of TSP fertilizer per crop would be saved compared to the Centre for Soil and Agroclimate Research (CSAR)/ government recommendations if the first and second scenarios were followed, respectively. The CSAR/government recommendations are: 1. Fifty kg TSP ha-1 applied to soils high in extractable P, once every four crops; 2. Seventy-five kg TSP ha-1 applied to soils medium in extractable P, two times every four crops; and 3. One hundred twenty-five kg TSP ha-1 applied to soils low in extractable P, every crop