DISSIPATION AND DISTRIBUTION OF HERBICIDES IN THE SOIL-PROFILE

The distribution and dissipation of alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl) acetanilide], atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5 triazine), and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] in soil were studied in 1990, 1991, and 1992. Crop management practices included four tillage methods-chisel plow, moldboard plow, no-till, and ridge-till-and two crop rotations-continuous corn (Zea mays L.) and a corn-soybean [Glycine max (L.) Merr.] rotation. All herbicides were broadcast-spray applied with no incorporation. No-till plots had the smallest amounts of alachlor and metribuzin, whereas ridge-till plots had the smallest amounts of atrazine. Moldboard-plow plots usually contained the highest amounts of all three herbicides, although ridge-till plots had the highest metribuzin levels in 1992. These differences were seldom significant at the 0.05 level of probability, however. Throughout the growing season, 50 to 84% of the alachlor and metribuzin were retained in the top 10-cm layer of soil, and at least 68% of the atrazine was retained in the top 20 cm. From 84 to 98% of the herbicide applied was lost each year, probably by microbial degradation and, for alachlor, by volatilization after application. First-order half-lives were 36 d for alachlor, 55 d for atrazine, and 32 d for metribuzin. A two-compartment model better fitting the alachlor data returned a half-life of 24 d for that herbicide.