Behavior and Fate of Metribuzin in Eight Ontario Soils

The adsorption, desorption, and mobility of metri- buzin ( 4-amino-6-tert-butyl-3-(methylthio)-as-triazine- 5(4H)onel in eight Ontario soils were investigated. The distri- bution coefficients varied from 0.56 for the least adsorptive Fox sandy loam soil to 31.67 for the most highly adsorptive Leamington muck soil. Most of the adsorbed metribuzin was desorbed from the mineral soils after six aqueous extractions, however substantial amounts of the herbicide remained ad- sorbed on the muck soils after 10 aqueous extractions. The herbicide was relatively mobile in mineral soils but was im- mobile in the muck soils. Mobility was inversely correlated with the amount of soil organic matter. Bioassay studies indi- cated that phytotoxicity of metribuzin also decreased with increasing organic matter content of the soil. Persistence of metribuzin was dependent on microbial degradation. In a field study with Guelph loam, the half-life of metribuzin was ap- proximately 3 months.