INFLUENCE OF SURFACE AND SUBSOIL PROPERTIES ON HERBICIDE SORPTION BY ATLANTIC COASTAL-PLAIN SOILS

Information on soil properties that influence herbicide sorption throu ghout the soil profile is needed to identify soil-herbicide combinatio ns with varying degrees of risk for groundwater contamination. In this study we investigated the effects of properties on the sorption of fi ve herbicides commonly used in grain production in the Atlantic Coasta l Plain (atrazine, cyanazine, dicamba, 2,4-D, and metolachlor), using subsoil and surface horizons from six Delaware soils. Our objective wa s to develop simple screening models, based on soil properties availab le from routine soil tests or regional soil databases, to identify soi ls that may have an increased risk of groundwater pollution. Each soil was sampled by horizon to a depth of approximately 2 m to include maj or variations present in the soil profile above the surface water tabl e. Herbicide distribution coefficients (Kd(m)) were determined for eac h horizon and herbicide by sorption studies using a batch equilibrium technique and C-14-labeled herbicides. The ranking of herbicide sorpti on, over all soil horizons, was metolachlor > cyanazine > atrazine > 2 ,4-D > dicamba (Kd(m) = 1.03, 0.94, 0.85, 0.65, and 0.25 L kg-1, respe ctively). Multiple regression analysis between Kd(m) and soil properti es identified two rapid soil tests (KCl exchangeable acidity and organ ic matter content) as important predictive variables for atrazine, cya nazine, dicamba, and 2,4-D sorption (R2 = 0.81**, 0.56***, 0.26*, 0.6 8**); metolachlor retention was best described by oganic matter conte nt and effective cation exchange capacity (R2 = 0.78**), another easi ly measured or estimated soil property.