Fertilizer inputs, nutrient balance and soil nutrient supplying power in intensive, irrigated rice systems .3. Phosphorus

Data from long-term experiments at 11 sites in Asia with a wide range of nutrient input treatments and yield levels were used to quantify crop P requirements of rice (Oryza sativa L.) and the P balance in intensive, irrigated rice systems. Uptake of 1.8-4.2 kg P was required to produce one ton of grain yield. Physiological P use efficiency varied between 220 to 900 kg grain kg P-1. Without added P there was a net loss of 7 to 8 kg P ha(-1) per crop; with added P there was a net gain of 4 to 5 kg P ha(-1) per crop. Phosphorus adsorption kinetics on mixed-bed ion-exchange resin capsules provided an integrative measure of soil P status, P diffusion, and acid-induced P solubilization. The resin capsule was a sensitive tool to characterize buildup or depletion of soil P as a result of different P balances. Both Olsen-P and the resin capsule were suitable methods to predict P uptake of tropical lowland rice. It is hypothesized that both methods measure a similar soil P pool which is soluble under alkaline, aerobic conditions but transformed into acid-soluble P forms as a result of submergence and reduction. Present recommendations for P fertilizer use on rice of 20-25 kg P ha(-1) are adequate to maintain yields of 5-6 t ha(-1), but sustaining higher yields of 7-8 t ha(-1) will require farm-specific management strategies based on knowledge of the long-term P balance and soil P-supplying capacity.