Time-dependent adsorption of imazethapyr was studied in the laboratory on a sandy loam soil at 16% moisture for 30 d. Soil pH was adjusted to 4.5 to 6.8. Concentration of imazethapyr in soil water declined rapidly within the first day of incubation for all soil pH levels, indicating rapid initial adsorption. However, the concentration of imazethapyr in the soil water continued to decline slowly over time. Even at pH 6.8, where batch equilibrium studies at high solution to soil ratios indicated low adsorption, only 22% of the applied imazethapyr was in solution 10 d after treatment (DAT), and 10% 30 DAT. Soil column mobility studies also showed a rapid decline in mobility as time after application increased. The amount of imazethapyr remaining in soil columns rapidly increased within 4 to 10 DAT, even at pH 7.8, where low adsorption is predicted. Imazethapyr was stable during the time of these experiments, so degradation to less soluble or more tightly bound products is not the reason for the increased adsorption. These results indicate that batch equilibrium studies that use slurries with high solution to soil ratios (often 2:1 or higher) and short equilibration times may not accurately predict mobility of acidic herbicides in the field, where moisture content is lower.

Nomenclature: Imazethapyr.