MOVEMENT OF METRIBUZIN IN A LOAMY SAND SOIL UNDER IRRIGATED POTATO PRODUCTION

Movement and persistence of the herbicide metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] was assessed following two annual applications to a loamy sand soil under irrigated potato production in east-central Minnesota. Russet Burbank potato (Solanum tuberosum) was grown in plots in 1989 and 1990. Metribuzin was applied prior to potato emergence at rates of 0.56 and 1.1 kg ha-1. Water samples were collected weekly from suction samplers 90 and 150 cm deep. Bromide tracer and water balance data confirmed the movement of water to a depth of 150 cm. Metribuzin was not detected in soil samples below 45 cm during either year. Metribuzin was detected (mean concentration, 0.6 mug L-1) during 1990 in 17 of 473 (3.6%) water samples collected at the 90- and 150-cm depths in 8 of 16 plots. Averaged across rates and years, time for 50% dissipation of metribuzin was 30 d. At the end of the cropping seasons, an average of 17% of initial soil metribuzin remained in the 0- to 45-cm soil layer. Surface soil (0 to 15 cm) metribuzin adsorption coefficients, K(d) and K(oc), were 1.7 and 120, respectively. Metribuzin appeared to remain in the surface 15 cm of soil where it then degrades.