Mechanism of the Water Quality Change in Paddy Fields

Abstract

To clarify the mechanism of the water quality change in paddy fields, the water quality of irrigation water, ponding water of the paddy field and drainage water have been investigated for six years in the Kameda District.
The results obtained are as follows.
1) Concentrations of COD increase and those of N, PO₄-P and SiO₂ drop in the paddy field in the first half of the stage of the growing period of rice plants.
2) In the second half of the stage of the growing period of rice plants, concentrations of N and PO₄-P still drop, but those of COD and SiO₂ vary only to a very small extent.
3) Water quality of irrigation water changes mostly in ponding of the paddy field in process from inlet to drainage water.
4) When the concentration of nutrient salt is at a high level, it drops the paddy field, and when that is at a low level, it increases inversely.
5) In first half of the stage of the growing period of rice plants, nutrient salts are purified by immobilization to organic matter and those in second half of the stage of the growing period of rice plants are purified by assimilation of rice plants and absorption of soil.