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Diffusion of Herbicides in the Adsorbed Phase¹

ABSTRACT

Equations were presented which describe the diffusion of the soil-adsorbed herbicide phase. Assuming (i) no precipitation and volatilization, (ii) a linear adsorption isotherm, and (iii) no net change in herbicide concentration in the adsorbed phase, the data needed to make such a calculation can be determined experimentally and are as follows: soil bulk density, distribution coefficient, apparent self-diffusion coefficient of the herbicide in soil, self-diffusion coefficient of the herbicide in water and the transmission factor. The results indicated that the magnitude of the diffusion coefficient of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H) one] in Dubbs soil depends primarily upon the amount in the adsorbed phase and on the moisture content. The magnitude of the adsorbed phase diffusion coefficients of ¹⁴C-metribuzin was less than the apparent self-diffusion coefficients and decreased with decreasing soil water content. Since negative adsorbed phase diffiusion coefficients were observed at the lower soil water contents, the data suggested that the amount of metribuzin in the adsorbed phase increased as soil water content decreased.

¹ A joint contribution from the Agr. Exp. Sta. of the University of Arkansas and the University of Kentucky. The research was supported in part by the Office of Water Resources Research, USDI Project A-021-Ark. Published with the approval of the director of Arkansas Agr. Exp. Sta.

² Assistant Professor, Department of Agronomy, University of Arkansas, Fayetteville, Ark. 72701; Professor of Soil Science, University of Kentucky, Lexington, Ky. 40506; and Graduate Assistant, University of Arkansas, respectively.

Received for publication July 9, 1973. Accepted for publication March 4, 1974.