Three mathematical models were proposed to describe the effects of sorption of both bacteria and the herbicide (2,4-dichlorophenoxy)acetic acid (2,4-D) on the biological degradation rates of 2,4-D in soils. Model 1 assumed that sorbed 2,4-D is not degraded, that only bacteria in solution are capable of degrading 2,4-D in solution, and that sorbed bacteria are not capable of degrading either sorbed or solution 2,4-D. Model 2 stated that only bacteria in the solution phase degrade 2,4-D in solution and that only sorbed bacteria degrade sorbed 2,4-D. Model 3 proposed that sorbed 2,4-D is completely protected from degradation and that both sorbed and solution bacteria are capable of degrading 2,4-D in solution. These models were tested by a series of controlled laboratory experiments. Models 1 and 2 did not describe the data satisfactorily and were rejected. Model 3 described the experimental results quite well, indicating that sorbed 2,4-D was completely protected from biological degradation and that sorbed- and solution-phase bacteria degraded solution-phase 2,4-D with almost equal efficiencies.

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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 Koc = (K/OC), varied only within a factor of 1.5. For 2,4,5‐T herbicide, the Koc 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 µm) and “coarse” (2,000‐50 µm). For a given pesticide, a specific Koc value may be assigned to each class.

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