SULFONYLUREA HERBICIDES APPLIED TO ACIDIC SANDY SOILS - MOVEMENT, PERSISTENCE AND ACTIVITY WITHIN THE GROWING-SEASON

Chlorsulfuron and triasulfuron were applied to the surface of acidic, sandy loams at a low rainfall site in 1989 (129 mm June-October) and a high rainfall site in 1990 (217 mm July-August). Four environments were obtained by early and late application times and lime addition in 1989 and by, a wetter site in 1990. The pH of the surface 10 cm was 4.9 in 1989, 5.8 in 1990 and 6.5 after the addition of limestone in 1989. The plots were left fallow or sown, prior to herbicide applications, to wheat (cv. Kulin). Hourly averages of rainfall, soil and air temperature were recorded. The temperature range was 2.7-degrees-C to 23.2-degrees-C. The soil profiles were sampled on 5 to 7 occasions and herbicide residues were determined by a laboratory bioassay (sensitivity >0.4-mu-g kg-1 soil). Chlorsulfuron and triasulfuron were detected to 300 mm in the wetter environment but neither herbicide was as mobile in the profile as water. More herbicide moved to the lower layers of the profile in the higher pH environment. The half-lives for residues ranged from 12 to 28 days. Shoot biomass of wheat seedlings was suppressed by both herbicides but grain yields were unaffected. The residues failed to prevent reinvasion of the wheat plots by weeds, notably Arctotheca calendula.