Metal Behavior in Acid Sulfate Soils of Thailand.

Abstract

Metal availability and uptake by rice in flooded acid sulfate soils from the Central Plains region of Thailand were evaluated under field, growth chamber and greenhouse conditions. The results found in this study indicated that Fe$\sp{2+}$ activities are seldom in equilibrium with pure Fe solid phases in these soils. This was believed to be due to (1) transient redox conditions, (2) the presence of ill-defined ferric oxides, and/or (3) cation exchange reactions. Equilibria calculations also indicated that these soils were undersaturated with respect to most Ca, Mg, and Mn solid phases, and their activities were believed to be controlled by cation exchange. The Al$\sp{3+}$ activities were very pH dependent and appeared to be controlled by jurbanite at low pH and amorphous Al(OH)$\sb 3$ at high pH. Zinc activities were also negatively correlated with pH, with many of the soil solutions supersaturated with respect to franklinite. Cupric copper (Cu$\sp{2+}$) activities were positively correlated with pe, with cuprous ferrite indicated as the stable solid phase. Molybdate (MoO$\sb 4\sp {2-}$) activities were positively correlated with pH and appeared to be controlled by wulfenite. Borate (B(OH)$\sb 3\sp\circ$) activities were highly correlated with ionic strength. Leaf analysis indicated that Fe uptake was correlated with Fe$\sp{2+}$ activity, however, a better relationship was found between uptake and E$\sp\prime$-Fe (the divalent charge fraction in solution due to Fe$\sp{2+}$). Calcium, Mg and Mn uptake was also found to be more closely related with E$\sp\prime$-Ca, E$\sp\prime$-Mg, and E$\sp\prime$-Mn than with the activities of these ions. Although Al uptake was not correlated with Al$\sp{3+}$ activity, P uptake was, possibly due to variscite precipitation. Copper and Mo uptake was significantly correlated with the activities of Cu$\sp{2+}$ and MoO$\sb 4\sp{2-}$, however, Zn uptake was not related with Zn$\sp{2+}$ activity. The results of this study also indicated that Fe and Al toxicities and Ca, Mg, and Si deficiencies may occur on these soils. Multiple correlation between soil parameters and growth indices indicated that the two most important variables affecting rice growth on these soils were E$\sp\prime$-Fe and pH. While E$\sp\prime$-Fe was considered to be a measure of Fe stress (Fe toxicity combined with basic cation deficiencies), pH measurements reflected the availability of many nutrients and toxins, such as P and Al.