Leaching of silicon (Si) has been recognized as an important mechanism for diminution of soil plant-available Si budgets. However, loss of dissolved Si via transport in flooded paddy fields is poorly studied. Here Si transport in a 100-? deep soil profile of a typical rice-based cropping system of the Red River Delta(Vietnam) is modelled to provide estimates of fluxes of Si under waterlogged conditions and to suggest a staregy for Si fertilization. By measuring Si content of the rice straw and rice yield, it was predicted that 1 ton ha-1 of Si can be annually cycled and contribute to amorphous Si (ASi) pools. A low ratio of the soil ASi and total Si content(1/90) provides evidence for Si leaching under waterlogged condition in the paddy soil. Infiltration rates of Si between top- and bottom layers were found to decrease from 2.5 to 0.6 cm day-1, depending on soil hydraulic properties. Arrival of Si at lower boundary of a 100-cm soil domain can be observed after 75 days. Approximately 10 ton ha-1, which is about one order-of-magnitude higher than that potentially taken up by rice was predicted to be annually lost by leaching. It can be concluded that a strong deficiency of Si might occur near the end of the flooding period, and a peak application of Si at the same time is necessary to insure optimum supply of Si for rice.