The purpose of this study was to quantify hysteresis during adsorption and desorption of atrazine as a function of incubation time for a Sharkey clay soil. Adsorption was carried out using one day batch equilibration and was followed by incubation periods ranging from 1 to 24 d. Incubation was subsequently followed by six consecutive desorption steps where each step represented 1 d. The Freundlich equation (S = K CN where S is the amount of atrazine retained, μg g-1; C is concentration, μg ml-1; K is the distribution coefficient, cm3g-1; and N is a dimensionless parameter) was used to describe batch results. Both adsorption and desorption isotherms were well described by the Freundlich model. Fitted K parameter values for desorption isotherms were consistently higher than those associated with adsorption. The opposite trend was observed for the exponential parameter N. The results revealed that desorption deviated significantly from adsorption data. The deviation, which is commonly referred to as hysteresis, was more pronounced as incubation time increased. Batch equilibration results also indicated that the extent of hysteresis was not influenced by soil sterilization. Attempts to quantify the extent of hysteresis using a simplified approach are presented. We found that, for a given batch data set, hysteresis can be quantified provided that Freundlich N from adsorption and desorption isotherms is known.

A greenhouse study was conducted to evaluate the efficacy of imazethapyr at 35 and 53 g ai/ha applied preplant incorporated (PPI) or postemergence (POST) under various soil moisture regimes (13, 19, 25, and 50%) on barnyardgrass and red rice. Response of barnyardgrass and red rice to imazethapyr PPI was affected by soil moisture. With imazethapyr PPI, control of barnyardgrass was reduced at 50% soil moisture compared with other soil moisture regimes, and height of barnyardgrass increased as soil moisture increased from 19 to 50% 2 weeks after treatment (WAT). Barnyardgrass control declined and plant dry weight increased with the increase of soil moisture from 19 to 50% at 3 WAT. Imazethapyr PPI activity on red rice was reduced at 50% compared with other soil moisture regimes, as reflected by decreased control ratings as well as increased plant height and dry weight. Imazethapyr activity on barnyardgrass and red rice was increased at 50% soil moisture when applied POST compared with PPI. Imazethapyr POST activity on barnyardgrass and red rice was generally not affected by the soil moisture regimes or application rates. The results suggested that high soil moisture conditions reduced imazethapyr PPI efficacy on barnyardgrass and red rice. Imazethapyr POST activity seems unaffected by soil moisture conditions and thus may be used to control barnyardgrass and red rice if wet soil conditions are a concern.