The effects of pH and Ca concentration in the soil solution on the Si dissolution from two soil samples (Eutric Gleysols, local name ; Fujimi soil and Kounosu soil) were investigated under flooded condition. The amount of Si dissolved from the soils decreased with increasing pH when the pH was lower than 9. At higher pH, the amount of Si dissolved increased with increasing pH and the maximum value was obtained at around pH 10 (Kounosu soil) or pH 11 (Fujimi soil), and then decreased again. These results suggested that the amount of the dissolved Si is mainly controlled by adsorption-desorption equilibrium. The Si dissolution from the soils was not affected by the Ca concentration in the soil solution. The slags were applied to the submerged Fujimi soil and CO_2 gas was supplied into the soil to study the effect of CO_2 gas on the Si dissolution from the slags and the soil. The slag application increased the pH and the Ca concentration in the soil solution. On the other hand, the Si concentration in the soil sulution did not necessarily increase because the ability of the soil to adsorb Si was enhanced by the increased soil pH. In almost all slag application treatments, the CO_2 gas supply decreased the soil solution pH and increased the Si concentration in the soil solution, while no change was observed in the treatment without slag. These results indicated that the decrease in the soil solution pH, which was caused by the neutralization effect of CO_2 gas, enhanced the Si dissolution from both the soil solid phase and the slags. It was also indicated that the solubilities of a phosphorus slag, stainless steel slag, magnesium slag and convertor slag in the soil were high compared with those of a water-cooled blast furnace slag and a ferronickel slag.
