Physicochemical property effects of three herbicides and three soils on herbicide mobility in field lysimeters

Understanding herbicide mobility in soils is necessary to prevent ground water contamination. We studied the mass balance distribution of three 14C-labeled herbicides (atrazine, metolachlor, and primisulfuron-methyl) in three soils (Blanton, Norfolk, and Rains) 128 days after treatment (DAT) to fallow soil column field lysimeters. Analyses were made of surface soil, subsoil, and leachate samples. Volatilization losses were calculated by difference. Our objectives were to examine, measure, and correlate the leaching patterns of the chemicals and correlate their leaching characteristics with physicochemical and biological properties of the herbicides and the physicochemical properties of the soils. Metolachlor was the most mobile herbicide, as indicated by the retardation factor (Rf) (Rf=0.48 in 1992 and 0.19 in 1993), followed by primisulfuron-methyl (Rf=0.41 in 1992 and 0.12 in 1993), and atrazine (Rf=0.38 in 1992 and 0.15 in 1993), where mobility was greatly affected by water input (637 mm in 1992 and 509 mm in 1993). Herbicide mobility (Rf) was related to 14C-parent compound in leachate (0.02-6.9% of 14C applied), 14C in the subsoil (9-24%), and the pesticide leaching potential (PLP) index of each herbicide, as computed using a simple decision-aid model. The herbicides were most mobile through Blanton, followed by Norfolk and Rains soils and mobility (Rf) was inversely related to mean % organic matter (OM) content of the soil profiles and directly related to soil pH and soil leaching potential (SLP) indices of the soils. Physicochemical and biological properties of the herbicides and soils were related to many of the measured herbicide distribution parameters.