Redox, Ph, and Ion Chemistry of Acid Sulfate Rice Soils in Thailand.

Five Thailand soils belonging to the Rangsit Very Acid (Rsa), Rangsit (Rs), Mahaphot (Ma), Bang Pakong (Bg), and Bangkok (Bk) series were employed in redox-pH-ion chemistry studies. Decreases in soil redox potential upon submergence were relatively slower and less intense in the acid sulfate soils than in the non-acid marine soil. Among the acid sulfate soils, the Rsa soil was most sluggish with respect to decreases in redox potential. Addition of organic matter or liming in combination with submergence accelerated decreases in redox potential in the flooded Rsa soil as well as in the other acid sulfate soils to the point of sulfide formation. Soluble and extractable Fe in all soils was inversely related to soil redox potential and pH. The ratio of soluble Fe$\sp{+}$ to total soluble Fe or the ratio of extractable Fe3$\sp{+}$ to total extractable Fe observed in the Rsa and Bg soils was positively related to soil redox potential and pH. At intermediate redox range, the Eh/pH slopes observed in all soils were close to $-177$ mV, the theoretical value of Fe2$\sp{+}$-Fe(III) oxide redox system (governing Fe2$\sp{+}$ activities and redox equilibria). The critical redox potentials of the Rs, Rsa, Bg, Ma, and Bk were observed at pe + pH of 11.23, 12.57, 12.92, 7.69, and 10.13, respectively. Added organic matter enhanced iron reduction and accelerated the formation of insoluble reduced iron. Liming caused a decrease in soluble Fe. Soluble Mn in all soils was inversely related to soil redox potential and pH. Soluble Mn was relatively low in the Rsa, Rs, and Bk and relatively high in the Ma and Bg soils ($\sim$300 mg kg$\sp{-1}$). Organic matter added did not significantly affect soluble Mn in the soils. Liming decreased Mn availability in all soils. Redox potential affected soluble Mn, Al, and Si similarly, that is, soluble Mn, Al, and Si increased upon soil oxidation throughout the Red $\to$ Ox half cycle. Upon soil re-reduction (Ox $\to$ Red), soluble Mn, Al, and Si further increased and attained a peak at an intermediate redox potential prior to declining thereafter. (Abstract shortened with permission of author.)