Adsorption, Volatility, and Migration of Thiocarbamate Herbicides in Soil

Abstract

The relative adsorption and migration of four thiocarbamate herbicides were studied in five soil types. Pebulate (S-propylbutylethylthiocarbamate) was adsorbed to the greatest degree by any one of the soils or adsorbents used. EPTC (ethyl N-N-dipropylthiocarbamate) showed the least adsorption while ethyl N-ethyl-N-cyclohexylthiocarbamate (hereinafter referred to as R-2063) was intermediate. Charcoal and bentonite clay were very effective adsorbents while the soils varied in their ability to adsorb the herbicides. A correlation was found between the rate of adsorption and the amount of organic matter in the soil. Such correlation was not found with the clay content, but a multiple regression test showed that the clay content is also closely associated with adsorption. The leaching of EPTC, pebulate, R-2063, and S-2,3-dichloroallyl N,N-diisopropylthiolcarbamate (diallate) in soil columns was directly related to their solubilities in water and inversely related to the organic matter content of the soil. Incorporation of the herbicides into the upper 5 cm of the soil did not alter the pattern or depth of their leaching. The lateral diffusion of thiocarbamates in the soil was much more restricted than the leaching downward. Again, this movement was closely related to the water solubilities of the herbicides, to soil organic matter, and apparently to the vapor pressures of the herbicides. EPTC vapor loss was the greatest followed in order by pebulate and R-2063. The rate of vapor loss from soils of all of the herbicides tested was reduced when the soil properties enabled rapid infiltration and drying of the spray drops.