Atrazine Mobility in Two Soils Under Conventional Tillage

Atrazine mobility was studied in two different Pennsylvania soils under conventional tillage. The herbicide was applied at four preemergence rates (1.1, 2.2, 4.5, and 9.0 kg ai (active ingredient)/ha to one soil and as a 4.5-kg ai/ha equal split treatment (fall surface-spring preemergence; early spring surface-spring preemergence) to another soil heavily infested with quackgrass. Surface residues from each initial application (2.2 kg ai/ha) were plowed-down in early May. Atrazine mobility, dissipation rate, and residual activity were evaluated by chemical analysis and oat bioassay of soil cores sampled at 15-cm increments to 122 cm and from chemical analysis of suction lysimeter leachates. Chemical recovery levels indicated that atrazine principally reacted and dissipated in the plow-layer in both soils. Atrazine applied at 2.2 and 4.5 kg/ha was detected in each soil to a depth of 76 cm approximately 2 mo after application. At these rates, soil concentrations ranged from 0.01 to 1.12 ppm, recovery levels ranged from 47 to 71%, and 83 to 100% of the residual atrazine was detected in the surface 30 cm of soil. However, some “leakage” of atrazine through the coarse subsoil structural units did occur since very low levels were detected in lysimeter leachates collected at 122 cm. By the end of the growing season, 3 to 21% of the atrazine applied at these same rates was recovered in the surface 46 cm of soil, except where one-half of the 4.5 kg/ha rate was plowed-down in early spring (3% recovered from 46 to 76 cm). The bioassay procedure did not accurately predict atrazine residue concentrations in either soil. It appeared from this study that application of atrazine to fine-textured, conventionally tilled soils at rates ranging from 1.0 to 4.5 kg/ha would not seriously affect ground water supplies through contamination by internal soil damage.