Abstract
The effects of water percolation under a submerged condition on the rice growth and an nutrient and toxic substnces in various kind of soils were studied by means of frame and glass lysimeter experiments. The results obtained were as follows. 1) The water percolation decreased the amount of NH3-N in soil when ammonium sulphate was applied in a large amount, or when the percolation rate was high. In cases of re-irrigation or of circulation where water being drained through the bottom of the glass lysimeter, was poured back to the soil surface, the arnount of NH3-N in soil decreased significantly. It was interpreted to be due to the denitrification and/or immobilization of the compound by microorganisums in the surface soil. 2) It was found that the higher the percolation rate, the less was the amount of free-C02 in soil water. 3) Swampy soils rich in organic matetrs and poor in iron content lead to the production of large amounts of H2S in soil water when temperatur was high. H2S in soil water was decreased remarkably by the treatment of water percolation only when larger amounts of H2S were evolved, and was increased slightly when smaller amounts of H2S were evolved. 4) The water percolation increased the grain yield of rice only when larger amounts of toxic substances were evolved on swampy soils, while no changes were observed on soils poor in organic matters. On swampy soils, relatively large amounts of water percolation were favourable for the rice yield. 5) Helminthosporium leaf spot disease occured on swampy soils less in silica contents and water percolation was effective to sorne extent for reducing the occurence.
