The presence of a straw mulch on the soil surface did not significantly affect the persistence of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)-one] in the soil. The half-life of metribuzin was 5.3 days in 1979 and 12.5 days in 1980 in the field when the time the herbicide was exposed on the soil and straw surface prior to rainfall or irrigation was 10 and 0 days, respectively. The half-life of metribuzin was 17.1 days in the greenhouse where soil temperature was lower and leaching, volatilization, and photodegradation losses were less than in the field. Increasing the amount of straw on the soil surface greatly decreased the initial soil reception of metribuzin as well as the amount of metribuzin that was subsequently washed off the straw by water. Less than 45% of the applied metribuzin reached the soil surface when 2250 kg/ha of straw was present and 0.3 cm or more of water was applied.
