Initial activity of atrazine and chlorsulfuron residues is greater in the grey clays than in the black earths of southern Queensland

Commercial experience is that soil type appears to be a major factor influencing initial weed control from soil applications of atrazine and chlorsulfuron. We tested this by collecting 26 soils from the major cropping areas in southern Queensland. Initial activity was determined by measuring the dose required for 50% inhibition (ID50) of seedling growth in Wimmera ryegrass in controlled environment cabinets. The mean ID50 for atrazine and chlorsulfuron in black earths was 3 and 4 times greater, respectively, than in grey clays. Lower activity in the black earths was probably due to higher clay and cation contents and, consequently, greater adsorption of the herbicides. These results suggest that application rates of atrazine and chlorsulfuron can be adjusted for differences in initial activity between soil types.