Dissipation of Pyrithiobac-Sodium at Different Soil Moisture Regimes in Cotton Growing Soils of Telangana State, India

Abstract

Pyrithiobac-sodium, a selective post-emergence herbicide used in cotton, is weakly acidic and highly water soluble, making its dissipation strongly dependent on soil type and moisture regime. The present study evaluated the degradation and dissipation of pyrithiobac-sodium under varying soil moisture levels in red and black soils of Telangana State, India. Representative red and black soils were collected, air-dried, and analysed for their physico-chemical properties. The red soil was neutral, sandy loam, low in organic carbon, and had a CEC of 15.91 cmol(p⁺) kg⁻¹, whereas the black soil was slightly alkaline, clayey, medium in organic carbon, and had a CEC of 24.2 cmol(p⁺) kg⁻¹. The dissipation study was conducted under controlled incubation at 27 ± 0.2°C for 135 days at two moisture regimes—field capacity (FC) and 50% field capacity (FC₅₀). Pyrithiobac-sodium was applied at a 10 µg/ml, and residues were quantified at periodic intervals using a validated high-performance liquid chromatography (HPLC–UV) method with analytical recovery of 85.2–90.6%. The initial detected amount (IDA) two hours after application ranged between 90.21 and 93.21 µg 10 g⁻¹ soil. Dissipation followed first-order kinetics with exponential decay (R² > 0.98). Half-lives (DT₅₀) were 46.2 and 53.3 days in red soil and 57.8 and 63.0 days in black soil at FC and FC₅₀, respectively. Dissipation was slower under low moisture and in black soil, indicating higher sorption and reduced bioavailability. Pyrithiobac-sodium exhibited moderate persistence, with soil moisture and texture as the primary controlling factors.