Effect of Soil pH on Microbial Degradation, Adsorption, and Mobility of Metribuzin

Abstract

Metribuzin [4-amino-6-tert-butyl-3-(methylthio-as-triazin-5(4H)one] degradation by soil microorganisms decreased as the soil pH decreased. Metribuzin and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] mobility increased with increasing concentration and soil pH.14C-metribuzin had greater mobility on soil thin-layer plates than14C-atrazine because of differences in water solubility and basicity. Adsorption increased with decreasing soil pH. The pKa for metribuzin was 0.99 ± 0.08. Adsorption increased gradually as the soil pH decreased toward the pKa of metribuzin. Protonation of the amine group with subsequent adsorption to soil colloids was the postulated reason for the reduction in phytotoxicity, microbial degradation, and mobility of metribuzin at low soil pH levels.