Rice is the staple food of over half the world's population. It is the predominant dietary energy source for 17 countries in Asia and the Pacific, 9 countries in North and South America and 8 countries in Africa. Rice provides 20 % of the world's dietary energy supply, while wheat supplies 19 % and maize 5 %. The majority of the rice in Asia is grown during the wet season, thus the dependence on rainfall is the most limiting production constraint for rainfed cultivation. Rice areas in South and South East Asia can, in general, be classified into irrigated, rainfed upland, rainfed shallow water lowland and rainfed deep water lowland areas.
A clear understanding of the different stages of growth and development of the crop as well as the nutritional requirements during the important stages is a pre-requisite for nutrient management.
During the initial growth stage the accumulation of N in the vegetative body is high and declines with age towards the later growth stages. Translocation of N from the vegetative organs to the grains becomes significant only after flowering. There is some translocation of carbohydrates from the vegetative plant parts to the grains after flowering and a large amount of carbohydrates accumulates in the grains. Protein synthesis is active during the vegetative stages, and during the reproductive stage the synthesis of cell wall substances (cellulose, lignin, etc.) is activated, while the protein synthesis still continues at the same pace. The starch synthesis becomes only active in the ripening stage. The sequence of nutrient mobility in the rice plant follows P > N > S > Mg > K > Ca. The elements that form immediate components of proteins have a high rate of mobility, whereas those that are continuously absorbed until senescence have a relatively low mobility. Thus, N, P and S, which are essential constituents of proteins, are absorbed rapidly during the active vegetative growth stage and are subsequently translocated to the grain after flowering. Other nutrients such as Ca and K on the other hand, are absorbed at a rate matching the rate of dry matter production over the growth period.