Internal nutrient efficiencies of irrigated lowland rice in tropical and subtropical Asia

This study estimates the nitrogen (N), phosphorus (P) and potassium (K) requirements of irrigated rice (Oryza sativa L.) in South- and Southeast Asia. Grain yield and plant nutrient accumulation in above-ground plant dry matter (DM) were measured at physiological maturity of rice (n similar to 2000) in on-station and on-farm experiments in six Asian countries between 1995 and 1997. These data were used to model the nutrient requirements for yields up to 11 t ha(-1) using the QUEFTS (Quantitative Evaluation of the Fertility of Tropical Soils) approach. The model required the estimation of two borderlines describing the minimum and maximum internal efficiencies (LE, kg grain per kg nutrient in plant DM), which were estimated at 42 and 96 kg grain kg(-1) N, 206 and 622 kg grain kg(-1) P and 36 and 115 kg grain kg(-1) K., respectively. The model predicted a linear increase in grain yield if nutrients are taken up in balanced amounts of 14.7 kg N, 2.6 kg P and 14.5 kg K per 1000 kg of grain until yield targets reached ca. 70-80% of the climate-adjusted potential yield (Y-max). The corresponding IEs were 68 kg grain kg(-1) N, 385 kg grain kg(-1) P and 69 kg grain kg(-1) K for a balanced nutrition. The model predicted a decrease in IEs when yield targets approached Y-max. The derived borderlines are valid for current modern, high-yielding indica cultivars with a harvest index of 0.50 kg kg(-1) and can be used for all methods of crop establishment. Only Y-max is required as site- or season-specific information when estimating nutrient requirements for a yield target making the model applicable for all irrigated lowlands in South- and Southeast Asia. Predicted Ifs were greater than actual IEs measured in more than 200 farmers' fields (n similar to 700), the latter averaging 59 kg grain kg(-1) N (27-100 kg kg(-1)), 354 kg grain kg(-1) P (158-1069 kg kg(-1)) and 64 kg grain kg(-1) K (27-179 kg kg(-1)) with grain yields ranging from 1.5 to 9.9 t ha(-1) mean 5.2 t ha(-1)). Low and varying IEs in the farmers' practice were caused by nutritional imbalances, inadequate irrigation, or problems with pests and weeds. (C) 1999 Elsevier Science B.V. All rights reserved.