Estimation of Changes in Available Soil Phosphate under Submerged Conditions Associated with Temperature during the Tillering Stage of Rice Plant in the Cool Climate Region of Japan

This study was performed to investigate changes in available soil phosphate associated with temperature under submerged conditions and to explore the possibility for estimating those under submerged conditions during the early growth (tillering) stage of rice plants (Oryza sativa L.). An incubation experiment was conducted under submerged conditions at three temperatures (10°C, 17.5°C and 25°C) using paddy soils collected over a widespread area in Japan. In most soils, significant positive correlations were observed between available soil phosphate and cumulative temperature to 650°C, which corresponded to the tillering stage in Japan. Relationships between the regression formulae of the available phosphate against cumulative temperature to 650°C and soil chemical properties measured in air-dried soil were investigated. The results indicate that the available phosphate of paddy soil against cumulative temperature during tillering stage under submerged conditions can be estimated from the results of air-dried soil analyses which can be conducted before a crop season.