Dissipation of Acetanilide Herbicides from Soils1

Three commercial α‐chloroacetanilide herbicides, alachlor, butachlor, and propachlor, were surface‐applied in aqueous solution on two soil types in the field to determine their soil half‐lives. Laboratory studies were conducted to assess the major routes of dissipation from soils. While much is known about the activity and mode of action of these herbicides, little quantitative information is available concerning the modes of dissipation from soils. This research was conducted to determine dissipation rates under field conditions and to quantitate the contributions of microbial decomposition, chemical breakdown, volatilization, and leaching. Dissipation followed first order kinetics with short field half‐lives ranging from 2 to 14 days depending upon the herbicide and soil type. Leaching did not contribute to dissipation since no residues were found below the upper 4 cm layer. Dissipation from sterilized soil was 50 times slower than dissipation from viable soil, indicating that microbial decomposition played a major role in herbicide degradation. Rapid herbicide volatilization occurred from continuously moist soil exposed to a constant 21C, 3.2 kph air stream impinging upon the treated soil surface but little loss occurred from dry soils. Atrazine was also rapidly lost from moist soil under these conditions, with volatilization rates falling in the order propachlor > atrazine > alachlor > butachlor. The major avenue of dissipation of these acetanilide herbicides from soil is by microbial decomposition. Significant volatilization losses occur only under windy conditions during the interval that exposed soil surfaces remain moist.