Calcium (Ca)-loaded biochar was produced from rice husks by loading the Ca source with subsequent carbonization and was examined from the evaluation of P adsorption capability about the loading method, the availability of oyster shells as the alternative, and the effect of carbonization temperature. Ca-loaded biochar prepared at 650°C had the CaCO₃ form, and that at 400°C had unconverted Ca(OH)₂ with less CO₂ generation. Moreover, silicon in the rice husks might have existed as an amorphous structure of calcium silicate hydrate by reacting with Ca. As the loading method of the Ca source, the powder mixing method was as effective as the conventional impregnation method. It was clarified that oyster shells can be used by decarboxylation and hydration in the powder mixing method, and that the chemical form of the Ca source is important for improving P adsorption capability. Furthermore, the Ca-loaded biochar prepared at 400°C as the condition that could be performed at the paddy field location was higher P adsorption capability than that at 650°C, even though the pores were not developed. The formation of Ca(OH)₂ by low-temperature carbonization of rice husks with Ca affected the improvement of P adsorption capability.