Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 12/31/2003
Publication Date: 2/10/2004
Citation: Koskinen, W.C., Locke, M.A. 2004. Characterization of herbicide availability in soils: implications to environmental fate. Weed Science Society of America Meeting Abstracts. p. 47.
Technical Abstract: Herbicide availability is the result of the integration of various processes in soil, and it controls herbicide transport to water and air, exposure to and uptake by target and nontarget organisms, and degradation. Herbicide sorption-desorption are arguably the most important processes in soil that determine herbicide availability. It is often suggested that only herbicides in solution, or that are readily desorbable from soil, are available for transport, degradation, or uptake. Herbicides sorbed to soil are not instantaneously available, but must first be desorbed from the soil. Availability is thus directly related to desorption characteristics. Various methods have been used to characterize availability. Batch sorption methods have traditionally been the most commonly used to characterize the soil/water partitioning from which sorption coefficients, i.e. Kd, Koc, are calculated, with lower sorption coefficients indicating lower affinity for soil, thus greater availability. Other commonly used techniques include determination of aqueous CaCl2-extractable herbicide, which has been considered as "readily available pesticide" and mild to harsh solvent extraction techniques and sequential solvent extractions, which determine different degrees of availability. There are limitations with these methods, e.g., a slurry does not represent field conditions and does not account for desorption hystersis or changes in availability over time. Alternative methods for availability characterization have included isotopic exchange, which determines the non-readily available herbicides; use of supercritical fluids to determine desorption at field typical field moisture contents; and determination of nonavailable herbicides by sequential solvent extraction. All of these methods have limitations, but most importantly, it has been shown that depending on the method of characterization of sorption-desorption, the herbicide mobility classification can be changed. After 40 years of research, there is still not a universally accepted method to characterize availability. The topic is wide open and the search continues for innovative, practical, and realistic methods.