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. 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. 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 three weeks. Shoot and root growth responses were used to estimate effective doses (ED 10 , ED 20 , ED 50 ) for toxicity. Toxicity thresholds (ED values) were strongly and positively correlated with OC, cation exchange capacity (CEC), and exchangeable calcium (Exc.Ca). Clay content and pH had no significant effect within the tested ranges. ED 20 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, ED 20 values ranged from 25-50% of the label rate. Soil OC is a key determinant of diuron toxicity to canola. Clay and pH were less influential under the conditions tested. Measuring residual diuron and assessing soil OC can help predict phytotoxic risk to canola, supporting safer herbicide use and crop management decisions.
Pyone et al. (Thu,) studied this question.