Sorption and leaching potential of herbicides on Brazilian soils

Sorption of the herbicides alachlor, atrazine, dicamba, hexazinone, imazeth apyr, metsulfuron-methyl, nicosulfuron, simazine and sulfometuron-methyl wa s characterized on six Brazilian soils, using the batch equilibration metho d. In general, weak acid herbicides (dicamba, imazethapyr, metsulfuron-meth yl, nicosulfuron and sulfometuron-methyl) were the least sorbed, whereas we ak bases such as triazines and nonionic herbicides (alachlor) were the most sorbed. The Kd values found showed a significant correlation with soil org anic carbon content (OC) for all herbicides except imazethapyr and nicosulf uron. Koc values showed a smaller variation among soils than Kd. To estimat e the leaching potential, Koc and the ground-water ubiquity score (GUS) wer e used to calculate half-lives (t(1/2)) that would rank these herbicides as leachers or non-leachers. Comparison of calculated values to published val ues for t(1/2) demonstrated that sulfonylureas and hexazinone are leachers in all soils, alachlor is transitional, and atrazine, simazine and dicamba are leachers or transitional, depending on soil type. Results discussed in this paper provide background to prioritize herbicides or chemical groups t hat should be evaluated in field conditions with regard to their leaching p otential to ground-water in tropical soils.