Impact of soil physical and chemical properties on residual diuron toxicity against canola seedlings

Context Herbicide residues in soil can harm non-target crops, yet limited data exist on critical toxicity thresholds across herbicides and crop species. These thresholds are influenced by soil properties that affect herbicide bioavailability. Aims This study aimed to determine how soil organic carbon (OC), clay content, and pH influence the phytotoxicity of soilborne diuron residues to canola, and to establish predictive models for toxicity thresholds. Methods Greenhouse experiments were conducted using Western Australian cropping soils with varying OC (3.8–26 g kg−1) and clay content (40–195 g kg−1). Soil pH was adjusted to four levels (4.4–7.3) using agricultural limestone. Six diuron doses were applied, and canola was grown for 3 weeks. Shoot and root growth responses were used to estimate effective doses (ED10, ED20, and ED50) for toxicity. Key results Toxicity thresholds (ED values) were strongly and positively correlated with OC, cation exchange capacity, and exchangeable calcium. Clay content and pH had no significant effect within the tested ranges. The ED20 was ~7 times higher at 26 g OC kg−1 than at 4.2 g OC kg−1. For soils with <25 g OC kg−1, ED20 values ranged within 25–50% of the label rate. Conclusions Soil OC is a key determinant of diuron toxicity to canola. Clay and pH were less influential under the conditions tested. Implications Measuring residual diuron and assessing soil OC can help predict phytotoxic risk to canola, supporting safer herbicide use and crop management decisions.