Metolachlor Distribution in a Sandy Soil Under Irrigated Potato Production

Irrigated sandy soils have a high potential for leaching of agricultural chemicals to ground water. Objectives of this study were to determine the movement and persistence of metolachlor applied annually to a Hubbard loamy sand soil (Udorthentic Haploboroll) under irrigated potato production. Metolachlor was applied to the plots at rates of 1.7 and 3.4 kg ai ha-1. A KBr tracer was applied to the same plots at the rate of 130 kg ha-1. Intact soil cores were collected before and after herbicide application during the cropping season. Water samples were collected weekly from suction samplers, 90 and 150 cm deep in each plot, during the growing season. Potato yields of 60 and 59 Mg ha-1in 1989 and 1990, respectively, were above Minnesota state averages. Bromide tracer data confirmed the movement of water through the soil profile to at least a 150-cm depth. No detectable quantities of metolachlor were found below 30 cm in soil during either year. Time for dissipation of 50% of metolachlor ranged from 85 to > 106 d. Dissipation in 1990 appeared slower than in 1989 and residues remaining at the end of the season, averaged across application rates, were 44 and 64% in 1989 and 1990, respectively. No water samples contained metolachlor in 1989. Metolachlor was detected in 18 of 409 water samples collected during and after the growing season in 1990. Median detectable concentration of metolachlor was 0.4 μg L-1in water samples from suction samplers at 90- and 150-cm depths in eight of 16 plots. Metolachlor Kdand Kocvalues, 3 and 220, respectively, were similar to reported values. Metolachlor sorption in the soil profile was correlated, in order of contribution, to organic carbon content, cation exchange capacity, percent sand, and percent clay.