Submitted to: Weed Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/1999
Publication Date: N/A
Citation: Interpretive Summary: Many herbicides are soil-applied, and their soil behavior impacts both weed control efficiency and fate in environment. Sorption is one of the main processes controlling herbicide movement, and this process has been extensively characterized in soils developed in non-tropical climates. However, there is very little information on soil-herbicide interactions in ntropical soils, which are chemically and physically distinct from non- tropical soils. The objectives of this study were to determine sorption of nine herbicides (alachlor, atrazine, dicamba, hexazinone, imazethapyr, nicosulfuron, metsulfuron, simazine and sulfometuron) on six tropical soils (Brazilian soils), and to estimate their leaching potential using the sorption coefficients obtained and published degradation half-lives. The herbicides represent a wide range of properties and chemical groups and are intensively used in agricultural activities of relevant importance in Brazil. Very few studies concerning to the behavior of these herbicides in tropical soils are available in literature. Using sorption values obtained on these Brazilian soils and average published degradation half-lives in temperate soils and climates, it was determined that all the herbicides would have the possibility to leach to ground water. While the use of leaching indices is a valuable tool to screen potential herbicide leaching, data developed under tropical conditions is needed to better assess the potential. Results from this research provides the background to prioritize herbicides or chemical groups that should be evaluated in field conditions with regard to their leaching potential to ground water in tropical soils; and provides needed information for their safe and effective use in tropical areas, including Brazil.
Technical Abstract: Sorption of the herbicides alachlor, atrazine, dicamba, hexazinone, imazethapyr, metsulfuron-methyl, nicosulfuron, simazine, and sulfometuron- methyl was characterized on six Brazilian soils, using the batch equilibration method. In general, weak acid herbicides (dicamba, imazethapyr, metsulfuron-methyl, nicosulfuron and sulfometuron-methyl) were ethe least sorbed, whereas weak bases such as triazines and non-polar, non- ionic herbicides (alachlor) were the most sorbed. The Kd values found showed a significant correlation with soil organic carbon content (OC) for all herbicides except imazethapyr and nicosulfuron. Koc values showed a smaller variation among soils than Kd. To estimate the leaching potential, Koc and the Groundwater Ubiquity Score (GUS) were used to calculate half-lives (t1/2) that would rank these herbicides as leachers. Comparison of calculated values to published values for t1/2 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 that should be evaluated in field conditions with regard to their leaching potential to ground water in tropical soils.