Influence of soil moisture on sorption and degradation of hexazinone and simazine in soil

Sorption and degradation rates of hexazinone and simazine on soil were dete rmined in a sandy loam soil incubated, during 44 days, at 25 degrees C with moisture contents ranging from 4% to 18%. Herbicide levels in soil solutio n were also measured, after extraction of this solution by a centrifugation method. All experiments were conducted with treated soil in plastic column s, and the results showed that this method is suitable for the simultaneous study of pesticide sorption and degradation in soil at different environme ntal conditions. In general, sorption of both herbicides was higher for age d herbicide residues compared to recently applied herbicides, and soil subj ected to drying and rewetting cycles had the highest sorption values. K-f v alues ranged from 0.5 to 1.2 for simazine and from 0.2 to 0.4 for hexazinon e. Degradation rates increased with soil moisture content for both herbicid es, and drying-rewetting of soil yielded degradation rates slower than that obtained at 10% soil moisture content. Hexazinone concentration in soil so lution decreased with incubation time faster; than simazine.