Experiments were conducted to determine adsorption of methanearsonate by soils, soil separates, and reference clay minerals from solutions of disodium methanearsonate (DSMA), effects of soil type and pH on the leaching of surface-applied DSMA, and effects of soil type and added organic matter on the oxidation of the methyl carbon of methanearsonate.
Adsorption of methanearsonate by kaolinite and vermiculite increased with increasing concentration of DSMA in the equilibrium solution. Kaolinite and limonite adsorbed much more methanearsonate than vermiculite and montmorillonite. The clay fraction of Augusta silt loam adsorbed more methanearsonate than did the sand and silt fractions. Adsorption by whole soils was related generally to their clay contents.
Leaching with 20 successive 1-in increments of water removed 52% of surface-applied methanearsonate from Norfolk loamy sand. Rates of movement of DSMA in Norfolk loamy sand did not differ at three pH levels. About one-half of the applied methanearsonate remained in the surface inch of Decatur clay loam and none was leached below 6 in.
Oxidation of the methyl carbon of methanearsonate was associated with oxidation of soil organic matter in each soil. In three of the soils, there was no evidence of microbiological adaptation to methanearsonate. In Norfolk loamy sand, however, increasing decomposition of methanearsonate relative to soil organic matter occurred with time of incubation. Decomposition of added plant material in this soil enhanced the oxidation of methanearsonate.