Persistence and mobility of simazine in some New Zealand soils

Abstract

The residual activity and movement of the herbicide simazine was measured in 4 soils varying widely in their physical and chemical characteristics. The levels of simazine remaining in the soil collected from field plots were measured at regular intervals by bioassay with oats (Avena sativa L.), as well as by gas liquid chromatography. Simazine persisted at the 2 kg/ha application rate in phytotoxic amounts in the 0–10 cm depth up to 160 days and > 168 days at the 4 kg/ha rate. Samples collected from the 10–20 cm depth showed that simazine had leached in phytotoxic amounts in only one site and that much of the herbicidal activity remained in the top 10 cm of all soils throughout the study. The chemical residues of simazine disappeared in accordance with first order kinetics. At 2 sites with pH 5.4–5.5, and organic carbon levels of 4.6 and 9.4%, the half-lives were 25 and 32 days respectively. Bioassays suggested that the residual activity was higher at higher pH levels and at lower organic matter levels. At the rates used for selective weed control, simazine was dissipated within 4–8 months and thus a progressive build up of residues should not occur in most New Zealand soils.

Keywords:

• simazine
• persistence
• leaching
• mobility
• herbicide residues
• residual toxicity
• bioassay
• gas liquid chromatography