BENTAZON DEGRADATION IN SOIL - INFLUENCE OF TILLAGE AND HISTORY OF BENTAZON APPLICATION

Laboratory studies determined the fate of bentazon (3-isopropyl-1H-2,1 ,3-benzothiadiazin-4(3H)-one 2,2-dioxide) in soil as affected by tilla ge and history of application. Bentazon degradation in two soils from Mississippi and three soils from Illinois under conventional-tillage ( CT) and no-tillage (NT) (3-18 years) with varying histories of bentazo n application (0-9 applications) was studied. The half-life (DT50) for bentazon degradation ranged from 4.6 to 49.5 d; half-lives for NT of the two soils with the longest history of bentazon application were lo wer than those for CT. Half-lives for soils with no bentazon history w ere 3-11-fold higher than bentazon half-lives of those previously expo sed to bentazon. Dissipation of bentazon was accompanied with increase s in nonextractable material. Methylbentazon was the most consistently observed metabolite (1.7-5.8% applied C-14 after 48 d). Bentazon mine ralization ranged from 12% to 18% applied after 48 d and 2% to 3% appl ied after 22 d for bentazon history and nonhistory soils, respectively . Patterns of mineralization were affected by tillage in the two of th e five soils with the longest bentazon history.