Equilibrium isotherms for the adsorption of herbicides, diuron, and 2,4,5‐T on whole soil and various‐size separates of Webster soil (Typic Haplaquolls) were measured. The Freundlich adsorption coefficient, K , for diuron varied by a factor of about 7 among the different particle‐size fractions. However, the sorption coefficient, K , normalized with respect to soil organic carbon (OC) content, denoted as K oc = ( K /OC), varied only within a factor of 1.5. For 2,4,5‐T herbicide, the K oc values for the silt and clay‐size fractions, as well as the whole soil, were essentially identical and about 3 times larger than that for the sand‐size fraction. The results suggested that the soil particle‐size separates may be grouped into two size classes: “fine” (≤ 50 µ m) and “coarse” (2,000‐50 µ m). For a given pesticide, a specific K oc value may be assigned to each class.
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Nkedi‐Kizza et al. (1983) studied this question.