When submerged in water, the soil of a ricefield experiences slow breakdown of organic matter, thorough decomposition of vegetation, stable fertility, and less loss of nutrients. If submerged for a long time, the soil has intensive reduction and anaerobic decomposition, which produce large amounts of organic acid. These acids are unfavourable for rice roots. When soil reduction is intense, methane and hydrogen sulphide are produced and damage rice roots. Therefore, the field must be tilled and dried during the middle growth stage to intensify soil oxidation and control the formation of reducing substances.
Fish move about in the ricefield looking for food.
Consequently, they enhance contact between the water and air and increase the dissolved oxygen level. As the fish move, they stir the anaerobic layer of the soil. This accelerates the breakdown of organic matter and favours root growth. When rice roots draw nutrients from the soil, they decrease the concentration of nutrients around the root. The plants depend on infiltration of soil water that contains nutrients.