Predicting and Mapping Herbicide–Soil Partition Coefficients for EPTC, Metribuzin, and Metolachlor on Three Colorado Fields

Understanding the spatial variability of herbicide sorption to soil is important in determining the bioavailability as well as leaching potential of the chemical across a field. Multiple methods have been used to estimate herbicide sorption variability at the macroscale, but it has been difficult to measure soil heterogeneity or herbicide sorption at the individual field level. One method to determine soil heterogeneity is to create zones within a field based on maps of the apparent bulk soil electrical conductivity (ECa). These zones can be used to direct soil sampling to determine the fraction of organic carbon (foc) of each zone. The foc, in turn, can be used to predict the variability of herbicide binding among zones. Surface (0 to 30 cm) bulk-soil electrical conductivity (ECs) maps were made for three sandy fields in eastern Colorado, and soil samples were taken from the ECs zones within each field. The foc, and the soil–water partition coefficient (Kd) for EPTC, metribuzin, and metolachlor were determined for each sample. There were significant correlations between ECs and foc (R 5 0.75) and between foc and Kd for EPTC, metribuzin, and metolachlor (R 5 0.66, 0.61, and 0.71, respectively) across all three fields. Additional soil samples taken from the ECs zones located in previously unsampled areas of the three fields showed that one could reasonably predict Kd values for metribuzin, metolachlor, and possibly, EPTC based on the foc zones derived from ECs maps. Nomenclature: EPTC; metolachlor; metribuzin.