Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2007
Publication Date: January 1, 2008
Citation: Shaner, D.L., Farahani, H.J., Buchleiter, G.W. 2008. Predicting and Mapping Herbicide-Soil Partition Coefficients for EPTC, Metribuzin and Metolachlor on Three Colorado Fields. Weed Science. 56:133-139 Interpretive Summary: One of the needs in site-specific weed management is to map soil heterogeneity quickly and inexpensively. Under non-saline conditions, bulk soil electrical conductivity (EC) is strongly dependent on soil organic matter (OM) and clay content, which are also major determinants of herbicide binding and soil EC can be mapped quickly and inexpensively. A study was done to determine the relationship between bulk soil electrical conductivity (EC) and herbicide binding and to evaluate if this relationship could be used to develop herbicide application maps for three irrigated fields in Colorado. There was a good correlation between the soil binding of three herbicides (EPTC, metribuzin and metolachlor) and soil EC in all three fields. Subsequent resampling the three fields in areas that had not been previously sampled revealed that we could classify these fields in to low binding and high binding zones based on EC maps. It appears to be possible to create maps to vary herbicide application rates based on soil EC maps.
Technical Abstract: 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 macro-scale, 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 (0-30 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 Kds for EPTC, metribuzin and metolachlor were determined for each sample. There were significant correlations between ECs and foc (R=0.75), and between foc and Kd for EPTC, metribuzin and metolachlor (R=0.66, 0.61,0.71, 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.