Asian Journal of Soil Science and Plant Nutrition

The adsorption and leaching behaviour of the herbicide topramezone was investigated in two different soils—red sandy loam (Alfisol) and black clay (Vertisol)—from maize-growing regions of Telangana, India. Laboratory batch equilibrium experiments were conducted to quantify the adsorption characteristics of topramezone using the Freundlich isotherm model. The red soil was neutral with medium organic carbon and low CEC (cmol(p⁺) kg⁻¹), whereas the black soil was slightly alkaline with low organic carbon and higher CEC (cmol(p⁺) kg⁻¹). Adsorption increased with equilibrium concentration in both soils, ranging from 3.55 µg g⁻¹ in red soil and 2.68 µg g⁻¹ in black soil at 5 µg mL⁻¹ to 28.98 µg g⁻¹ and 24.98 µg g⁻¹, respectively, at 50 µg mL⁻¹. The adsorption isotherms exhibited an S-shaped, nonlinear pattern that fitted well to the Freundlich equation, confirming heterogeneous sorption sites (1/n < 1). Higher Freundlich coefficients (k_f = 0.733; k_foc = 130.9) in red soil compared to black soil (k_f = 0.481; k_foc = 98.2) indicated stronger sorption under slightly acidic conditions. The reduced adsorption in alkaline black soil was attributed to the predominance of the dissociated anionic form of topramezone (pKa = 4.06) and electrostatic repulsion from negatively charged colloids. The results demonstrate that soil pH and organic carbon are the dominant factors influencing topramezone adsorption, while clay content and CEC play lesser roles. Weaker sorption and higher mobility in black soils suggest a relatively higher leaching potential, emphasizing the need for site-specific management strategies for the safe use of topramezone in maize-based cropping systems under semi-arid tropical conditions.