NUTRITIONAL PHYSIOLOGY OF THE RICE PLANTS AND PRODUCTIVITY DECLINE OFIRRIGATED RICE SYSTEMS IN THE TROPICS (REPRINTED FROM PLANT NUTRITIONFOR SUSTAINABLE FOOD-PRODUCTION AND ENVIRONMENT, 1997)

Rice yield declines observed in long-term experiments are associated w ith N and K deficiencies despite the use of recommended fertilizer rat es. Modern rice varieties have an extensive root system, a large N upt ake capacity, and produce about 50 kg grain per kg of N uptake. These traits have not changed over the past 30 years. Instead, reduction of soil N and K supply contribute to the observed yield declines, and the se deficiencies may result from generic processes associated with mana gement practices common to irrigated lowland rice systems. Most other limiting factors are of more local importance. Negative yield trends c an be reversed by increasing the rates of applied nutrients. Maintaini ng high yields requires nutritional balance and attention to disease-n utrient interactions. Evidence from on-farm studies suggest that these same constraints occur in a several rice production domains in the lo wland tropics of Asia. Achieving continued increases in rice yields wi ll require field-specific soil and fertilizer management to improve th e congruence between nutrient supply and crop demand.