Calcium and Magnesium Availability and Uptake by Rice in Acid Sulfate Soils

Abstract

AbstractDeficiencies of Ca and Mg have been suggested to be constraints on rice (Oryza sativa L.) growth in acid sulfate soils. This study was conducted to determine the solubility of Ca and Mg under natural conditions in these soils, to determine what governs the solubilities of these metals, and to investigate uptake of these metals by rice. Metal availability and uptake by rice were evaluated in 134 flooded acid sulfate soils in the Central Plains region of Thailand and in a growth chamber study utilizing 50 of the same soils. Metal activities in soil solutions were determined using GEOCHEM. The results of this study indicate that the soils studied were generally undersaturated with respect to Ca and Mg minerals, suggesting that mineral equilibria probably does not govern the solubility of these metals in acid sulfate soils. Exchange isotherms indicated that Ca2+ and Mg2+ in soil solutions were in equilibrium with Ca and Mg on the cation exchange capacity. Therefore, cation exchange reactions probably govern the solubility of Ca2+ and Mg2+ in acid sulfate soils. The results also showed that although Ca and Mg uptake by rice is correlated with Ca2+ and Mg2+ activity, a more pronounced relationship exists between uptake and the divalent charge fraction in soil solution attributable to these cations. This was believed to be due to competitive inhibition for uptake sites. This study also demonstrated that Ca and Mg deficiencies in rice may be occurring on these soils.