FIELD LEACHING AND DEGRADATION OF SOIL-APPLIED HERBICIDES IN A GRADATIONALLY TEXTURED ALKALINE SOIL - CHLORSULFURON AND TRIASULFURON

A bioassay method was used to monitor changes in the leaching and degradation of chlorsulfuron and triasulfuron in a field experiment on a gradationally textured alkaline cropping soil. Following application in May 1991, it was estimated that approximately 46% of chlorsulfuron and 21% of triasulfuron were leached beyond 50 cm by late winter 1991 with 103 mm of cumulative rainfall. No leaching was detected from mid spring 1991 to early autumn 1992. During this time the sulfonylureas were rapidly degraded, with a 50% reduction in residues complete by 80 days. The degradation was increased by increasing soil temperatures and not constrained by lowering soil water contents. Leaching of the sulfonylureas took place again in the following growing season, between late autumn and mid spring 1992, where trace quantities of the herbicides moved from shallower depths to reaccumulate at higher levels, deeper in the soil profile. A part of these residues still remained within the 20-30 cm, 30-40 cm and 40-50 cm soil layers at mid spring 1992, after 344 mm of growing season rainfall. The residues were in the range 0.1 to 0.4 ng g(-1) soil of chlorsulfuron and triasulfuron, for this last sampling taken approximately 18 months after the commencement of the experiment. These residue levels are known to be phytotoxic to leguminous crop and pasture species, and oilseed crop species.