Water collects to lowland. It carries fine particles of weathered materials from the catchment, and deposits them where the flow slows down on the flat terrain. The soils formed on these sediments are called alluvial soil. Table 1 shows the distribution of alluvial soil in the world. We can see that though tropical Asia accounts for only 7.6 per cent in total land area of the world, it does for 28.6 per cent in terms of the alluvial soil area, and as much as 36 per cent in the potentially arable alluvial area. Concentration of alluvial plains in Asia, particularly in tropical Asia is evident.