Metal Availability and Rice Growth Under Controlled Redox Potential and pH in Acid Sulfate Soils of Thailand.

The effects of controlled pH and redox potential (Eh) conditions on transformations of several metals and their effects on rice growth were studied in laboratory microcosms using acid sulfate (Sulfic Tropaquept) and non-acid sulfate (Typic Tropaquept) soil materials from Thailand. Some microcosms were incubated at selected controlled Eh conditions (500, 250, 50, and $-150$ mV). Others were incubated at controlled pH and Eh levels (3.5, 4.5, and 5.5 and redox potential levels $\geq$500, 250, and 50 mV respectively). Acid sulfate soil-tolerant and sensitive rice varieties (IR 46 and IR 26, respectively) were used in this study. Results indicated water-soluble Fe and exchangeable Fe were inversely related to both pH and Eh, and reducible Fe was positively related. Redox potential and pH had the same effect on water-soluble Mn as Fe. However, Eh had less of an effect on Mn than on Fe. Water-soluble Al and percent Al saturation of the CEC was negatively related to pH in both soil types under controlled pH and Eh conditions, but was negatively related to pH in only acid sulfate soils under controlled Eh conditions. Aluminum activity was negatively correlated with pH in both soil types and over all controlled conditions. Rice uptake of Fe increased with decreasing Eh and pH. Iron uptake was significantly correlated with water-soluble Fe, Fe$\sp{2+}$ activity, and E$\sp\prime$-Fe. The IR 26 accumulated Fe more than the IR 46. The Mn content in shoot tissue was positively correlated with Eh and pH. Iron possibly had an antagonist effect on Mn uptake. Aluminum uptake of both rice varieties correlated best with Al$\sp{3+}$ activity in both soil types under controlled Eh conditions. Under controlled pH and Eh conditions, IR 26 uptake of Al was positively related to percent Al saturation of the CEC in only acid sulfate soils whereas no relationship was observed for IR 46 uptake of Al in both soil types. In general, growth of rice was negatively related to the Fe:Mn ratio in shoot tissue and Al$\sp{3+}$ activity but positively related to pe + pH and Zn$\sp{2+}$ activity. Iron solubility was probably controlled by amorphous Fe(OH)$\sb3$ at high pe + pH and goethite at low pe + pH levels. Manganese solubility was regulated by cation exchange processes. Jurbanite and amorphous Al(OH)$\sb3$ may control Al solubility at low and high pH conditions, respectively