Diffusion coefficients of atrazine, simazine, prometone, prometryne, 2,4-D, chlorpropham, diphenamid, fluometuron, trifluralin, and benefin, were measured in Lanton silty clay loam. The effects of soil moisture content and adsorption upon the diffusion coefficients were evaluated.
Differences were observed in the magnitude of diffusion coefficients of the 14C-labeled herbicides in the soil. The self-diffusion coefficients of 2,4-D, chlorpropham, diphenamid, and prometone decreased as diffusion time increased, which suggest that changes in the structure of these herbicides might have occurred. The self-diffusion coefficients of fluometuron, atrazine, simazine, prometryne, trifluralin, and benefin were found not to be significantly changed as diffusion time increased.
The rate of diffusion increased as the soil water content increased for all herbicides except trifluralin. Diffusion of trifluralin was higher at the lower moisture content and this was attributed to a greater amount of trifluralin moving in the vapor phase.
Linear adsorption isotherms were obtained with all herbicides. The order of adsorption was 2,4-D < fluometuron ≃ prometone < simazine ≃ atrazine ≃ diphenamid < prometryne ≪ chlorpropham ≪ trifluralin ≪ benefin. Correlation analysis showed that the diffusion coefficients of the nonvolatile herbicides were inversely related to the amount of adsorption indicating that most of the diffusion occurred in the solution phase. For those herbicides that are considered to be volatile, diffusion occurred in the vapor phase and the solution phase.