Phytotoxicity Response of Tomato to Simulated Clomazone Residues as Affected by Soil Type and Soil Moisture

Soil-applied herbicides can pose a risk to sensitive rotational crops when biologically active residues remain in soil. This study evaluated tomato susceptibility to simulated clomazone residues in two soil types (loam and sand) under three soil moisture levels (20%, 50%, and 70% of field water capacity—FWC) using a laboratory bioassay. Clomazone was applied at concentrations of 17.3–277 μg a.i./kg dry soil. After 28 days of growth, visual injury, shoot dry weight, chlorophyll content index (CCI), and carotenoid content were assessed. Clomazone induced leaf bleaching and reduced pigment content in tomato plants across all soil types and moisture levels, with effects increasing with application rate. Tomato grown in sandy soil exhibited greater visual injury and larger reductions in shoot dry weight, CCI, and carotenoid content than plants grown in loam soil. Pairwise comparisons of the estimated EC50 values for shoot dry weight, CCI, and carotenoid content between loam and sandy soil at the same moisture levels revealed statistically significant differences, indicating that soil type influenced the phytotoxic response to clomazone. Soil moisture did not significantly affect the estimated EC50 values for the evaluated parameters in sandy soil, whereas in loam soil, significant differences were predominantly observed between 20% and 70% FWC and between 50% and 70% FWC. Overall, the results demonstrate that tomato is sensitive to simulated clomazone residues and that the magnitude of phytotoxic responses varies with soil type and, in loam soil, with moisture conditions, supporting the use of the proposed bioassay as a screening tool for assessing clomazone-related phytotoxic risk under controlled conditions.