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-isopr opylamino-1,3,5 triazine), and metribuzin thylethyl)-3-(methylthio)-1, 2,4-triazin-5(4H)-one] in soil were studied in 1990, 1991, and 1992. C rop management practices included four tillage methods-chisel plow, mo ldboard plow, no-till, and ridge-till-and two crop rotations-continuou s corn (Zea mays L.) and a corn-soybean [Glycine max (L.) Merr.] rotat ion. All herbicides were broadcast-spray applied with no incorporation . No-till plots had the smallest amounts of alachlor and metribuzin, w hereas ridge-till plots had the smallest amounts of atrazine. Moldboar d-plow plots usually contained the highest amounts of all three herbic ides, although ridge-till plots had the highest metribuzin levels in 1 992. These differences were seldom significant at the 0.05 level of pr obability, however. Throughout the growing season, 50 to 84% of the al achlor and metribuzin were retained in the top 10-cm layer of soil, an d 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 microb ial degradation and, for alachlor, by volatilization after application . First-order half-lives were 36 d for alachlor, 55 d for atrazine, an d 32 d for metribuzin. A two-compartment model better fitting the alac hlor data returned a half-life of 24 d for that herbicide.