INITIAL ACTIVITY OF ATRAZINE AND CHLORSULFURON RESIDUES IS GREATER INTHE GRAY CLAYS THAN IN THE BLACK EARTHS OF SOUTHERN QUEENSLAND

Commercial experience is that soil type appears to be a major factor i nfluencing initial weed control from soil applications of atrazine and chlorsulfuron. We tested this by collecting 26 soils from the major c ropping areas in southern Queensland. Initial activity was determined by measuring the dose required for 50% inhibition (ID50) of seedling g rowth 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 blac k earths was probably due to higher clay and cation contents and, cons equently, greater adsorption of the herbicides. These results suggest that application rates of atrazine and chlorsulfuron can be adjusted f or differences in initial activity between soil types.