Asian Journal of Soil Science and Plant Nutrition

The environmental behaviour of herbicides is largely determined by sorption–desorption and dissipation processes that regulate their persistence, bioavailability, and mobility in soil. Herbicides that are weakly adsorbed are more prone to leaching and runoff losses, leading to possible groundwater contamination, while strongly adsorbed compounds may persist longer and cause carryover injury to succeeding crops. Thus, understanding the sorption–desorption and dissipation behaviour of herbicides under specific soil and climatic conditions is critical for devising safe and efficient weed management strategies. The present study was conducted to examine the adsorption and dissipation behaviour of pyrithiobac-sodium in representative red and black soils of Telangana State, India, which differ in texture, organic matter, and cation exchange capacity (CEC). The adsorption of pyrithiobac-sodium was studied using the batch equilibrium method as per OECD guidelines. Soil samples were equilibrated with pyrithiobac-sodium solutions of varying concentrations (0–50 µg mL⁻¹) at 27 ± 1°C. Adsorption data were fitted to the Freundlich equation to describe the isotherm characteristics. Results revealed that adsorption increased with equilibrium concentration in both soils. At the lowest concentration (5 µg mL⁻¹), the amount adsorbed was 1.06 µg g⁻¹ in red soil and 1.62 µg g⁻¹ in black soil, whereas at the highest concentration (50 µg mL⁻¹), it increased to 7.41 and 14.41 µg g⁻¹, respectively. The Freundlich model provided an excellent fit (R² > 0.95). The K_f values were 0.26 and 0.36, while K_d values were 0.19 and 0.39 for red and black soils, respectively. The black soil, with higher clay and CEC, exhibited greater sorption capacity, whereas red soil showed higher mobility potential due to its coarse texture and low organic carbon. Pyrithiobac-sodium displayed weak overall sorption, suggesting higher leaching potential in coarse-textured soils. These results highlight the significance of soil properties in predicting herbicide fate and guiding site-specific management in Telangana’s cotton-growing regions.