INFLUENCE OF SOIL AGGREGATE SIZE ON ATRAZINE SORPTION KINETICS

Soil aggregate size may be an important physical property regulating herbicide behavior; however, little information exists to evaluate this effect. The influence of aggregate size on atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] sorption coefficients (K-d) and sorption kinetics is investigated by batch equilibration using a suite of whole (0-5 mm) and crushed (<0.4 mm) aggregates isolated from two Iowa soils. Atrazine K-d values for both whole and crushed aggregates equilibrated for 72 h ranged between 3.02 and 4.09. Although significant differences occurred when size classes within soil type were compared, the magnitude of the aggregate effects on atrazine K-d values appears to be minor. Time-course experiments for atrazine sorption by whole aggregates analyzed using a modified first-order regression equation indicated that sorption was initially rapid (within 24 h), followed by a period of slower sorption. Aggregate size influenced the rate constants somewhat, but the effects also were minor.