SOIL ADSORPTION OF NEUTRAL AND ANIONIC FORMS OF A SULFONAMIDE HERBICIDE, FLUMETSULAM

The binding of a weakly acidic herbicide (N-2, 6-difluorophenyl-5-methyl-1,2,4-triazolo(1,5a)pyrimidine-2-sulfonamide) to soil is presented as an ionization of the weak acid in solution, and the partitioning of both the anionic and neutral forms of the compound into the soil organic matter. This theoretical description is combined with experimentally determined bulk adsorption constants to give the organic C adsorption constants (K(oc) for neutral (650 +/- 40 L kg-1) and anionic (12 +/- 1 L kg-1) forms of the molecule. Additional experiments with four soils adjusted to different pH levels gave an average K(oc) of 750 L kg-1 for the neutral form. Describing the soil sorption as a combination of anionic and neutral forms provided an adequate description of the measured sorption.