IONIZATION AND SORPTION OF FOMESAFEN AND ATRAZINE BY SOILS AND SOIL CONSTITUENTS

Fomesafen exhibited weakly acidic properties and was titrated spectrop hotometrically and determined to have a pK(A) of 3.0. [C-14]fomesafen and [C-14]atrazine, as references, were used in slurry-type sorption s tudies with H+- and Ca2+-saturated soil organic matter, Ca2+-saturated montmorillonite clay, Norfolk sandy loan (Typic Paleudult) and Drumme r silt loam (Typic Haplaquoll) at suspension pH values of 2.0 to 6.3. Sorption of atrazine was greater than that of fomesafen for the organi c matter sorbents but similar for montmorillonite clay and the two soi ls. Sorption of both herbicides on the Drummer soil was much greater t han on the Norfolk soil, due to the higher organic matter content of t he former. Decreasing the suspension pH greatly increased sorption of both herbicides by all sorbents. Atrazine sorption was attributed to i onic bonding at low pH and physical bonding at neutral pH. Fomesafen s orption occurred by way of physical forces at near neutral pH and by w ay of hydrophobic bonding and/or precipitation at low pH. Mobility and bioavailability of the herbicides in soils is expected to be lower at low pH than at high or neutral pH.