MANAGEMENT OF PHOSPHORUS, POTASSIUM, AND SULFUR IN INTENSIVE, IRRIGATED LOWLAND RICE

Management of soil phosphorus (P), potassium (K) and sulfur (S) resour ces in intensive, irrigated rice systems has received less attention t han increasing cropping intensity and yields with new cultivars, irrig ation, and fertilizer N. Crop requirements, input-output balance, and soil supplying capacity of P, K and S in irrigated lowland rice are re viewed. Based on projected rice production requirements, we estimate t hat the total annual nutrient demand for irrigated rice will be about 9 to 13 X 10(6) t N, 9 to 15 X 10(6) t K, 1.2 to 2.4 X 10(6) t P and 0 .9 to 1.5 X 10(6) t S in 2025, amounts that represent an increase of 6 5 to 70% above 1990 requirements. At present, negative K balances are widespread and K deficiency has become a constraint to increasing yiel ds, even on heavy-textured lowland soils with high inherent fertility. Because opportunities are limited for breeding cultivars that acquire more P, K or S from soil or have higher internal nutrient-use efficie ncies, long-term management strategies must focus on maintaining adequ ate nutrient balances in the topsoil layer. Interactions among nutrien ts have a large influence on physiological and agronomic efficiency th at result from nutrient applications. Strategies that only aim at incr easing P or K application rates without considering the indigenous sup ply from soil reserves are inefficient; they may not sustain yield inc reases to meet rice demand. Little improvement in fertilizer use effic iency can be expected from the present system of providing blanket rec ommendations for a given production domain. Instead, site-specific nut rient-management approaches will be needed to accommodate the tremendo us variability in indigenous nutrient supply found in the irrigated lo wlands of Asia. (C) 1998 Elsevier Science B.V.