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The degradation rate of quinclorac was investigated to afford scientific basis for its rational application with paddy soil of South China at different temperature,humidity and pH value. Quinclorac was sprayed into soil at the concentration of 1 mg·kg-1 . The soils were placed in corresponding temperature condition. Soil samples were taken for analysis on 1 h and 1,3,7,14,21,30,45,60, 120 d and extracted with potassium hydroxide solution. The supernatant was cleaned up with methylene dichloride and was evaporated on a rotary evaporator. Quinclorac was analyzed by high performance liquid chromatography. The results showed that pH value was the most obvious impact factor,the degradation rate of quinclorac was the fastest at pH = 7. 0 and the half-life is 10. 58 d. The degradation rate of quinclorac at pH=7. 0 was followed by alkaline(pH=8. 0)and acidic soil(pH=6. 0)and the half-lives are 18. 53 and 30. 81 d,respectively. Quinclorac degraded faster with higher temperature at 5-25 °C. The half-lives at (25±0. 5) °C and (35±0. 5) °C were 15. 04 d and 17. 33 d,respectively. Humidity had little effect to promote the degradation of quinclorac and the half-lives were more than 20 d at 30%,60%,90%. Therefore,adjusting the pH value of paddy soil to 7 or applying quinclorac in warm seasons 25-35 °C are beneficial to its degradation,which causes less residue and phytotoxicity. [ABSTRACT FROM AUTHOR]