Submitted to: International Weed Control Congress Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: April 1, 2004
Publication Date: June 21, 2004
Citation: Koskinen, W.C., Locke, M.A. 2004. Characterization of herbicide availability in soils. International Weed Control Congress Abstracts. Abstract No. 164. Technical Abstract: Herbicide availability is the result of integration of various processes in soil, and it controls herbicide transport, uptake, and degradation. Sorption-desorption is arguably the most important soil process that determines herbicide availability. It is suggested that only herbicide in solution, or readily desorbable from soil, is available for transport, degradation, or uptake. Herbicides sorbed to soil are not instantaneously available, but must first be desorbed from the soil, therefore availability is directly related to desorption characteristics. Various methods have been used to characterize availability. Batch methods have traditionally been used to generate sorption coefficients, i.e. Kd, Koc, with lower sorption coefficients indicating lower affinity for soil, thus greater availability. Other commonly used techniques include determination of aqueous-extractable herbicide, which is considered "readily available pesticide", and various solvent extraction techniques, which determine different degrees of availability. These methods have limitations, 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 typical field moisture contents; and determination of nonavailable herbicides by sequential solvent extraction; methods that also have limitations. Precaution must be taken in method selection. For instance, depending on the method of characterization of sorption-desorption, herbicide mobility classification can be changed. After 40 years of research, there is still no universally accepted method to characterize availability, and the search continues for innovative, practical, and realistic methods.