A proposal to standardize soil/solution herbicide distribution coefficients

Herbicide soil/solution distribution coefficients (K-d) are used in mathema tical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The uni versal soil colloid that binds most herbicides is organic matter (OM), howe ver clay minerals (CM) and metallic hydrous oxides are more retentive for c ationic, phosphoric, and arsenic acid compounds. Weakly basic herbicides bi nd to both organic and inorganic soil colloids. The soil organic carbon (OC ) affinity coefficient (K-oc) has become a common parameter for comparing h erbicide binding in soil; however, because OM and OC determinations vary gr eatly between methods and laboratories, K-oc values may vary greatly. This proposal discusses this issue and offers suggestions for obtaining the most accurate K-d, Freundlich constant (K-f), and K-oc, values for herbicides l isted in the WSSA Herbicide Handbook and Supplement.