DEGRADATION OF A SULFONAMIDE HERBICIDE AS A FUNCTION OF SOIL SORPTION

The degradation of DE-498 (proposed common name flumetsulam) was studied in 21 US soils as a first step in developing a management plan for this new herbicide. Degradation half-lives were shorter in soils that adsorbed the compound less. Adsorption was lower in soils with higher pH and lower organic carbon content. Degradation half-lives were thus influenced by both pH and organic carbon: they were in the range 2-4 weeks in higher pH soils (pH greater-than-or-equal-to 7.0) unless the organic carbon content was above 2.5%, at which half-lives were in the range 1-3 months. In medium pH soils (6.4 less-than-or-equal-to pH less-than-or-equal-to 6.9) half-lives were 1-2 months, while in lower pH soils (5.9 less-than-or-equal-to pH less-than-or-equal-to 6.3) they were 1-4 months. The laboratory data were supported by the response of sunflowers (Helianthus annuus L.) planted 1 year after application of flumetsulam to different soils. A quantitative model relating half-life to sorption K(d) (r2 = 0.85) was coupled with an additional equation relating sorption K(d) to the proportion of neutral and anionic forms of the compound at different pH values. The study allows estimates of this herbicide's degradation in soil to be made if its pH and organic carbon content are known.