Desorption kinetics of picloram as affected by residence time in the soil

Desorption kinetics of picloram (4‐amino‐3,5,6‐trichloropicolinic acid) was studied as a function of incubation time in seven soils incubated for up to 300 d in the presence of picloram. The amount of picloram readily released from the soil decreased significantly with incubation time. A two‐step sorption‐desorption mechanism was used to model the observed behavior that can be described in terms of external and internal sorption sites. Desorption from external sites is relatively fast, taking place in less than 5 h, whereas desorption from internal sites can take up to 300 h to reach equilibrium. The amount of picloram sorbed to internal sites increases with residence time in the soil. Implications of this study for the transport of picloram under field conditions and for models describing transport are discussed.