Adsorption and Desorption of the Herbicide Thiazafluron as A Function of Soil Properties

The effect of soil composition on the adsorption and desorption of the herbicide thiazafluron [1.3-dimethyl-1-(5-trifluoromethyl-1,2,3-thiadiazol-2-yl) urea] by 20 soil samples of 13 selected soil profiles of southern Spain has been studied. The adsorption curves conformed the Freundlich equation and the values of the constants, Kf and nf, ranged from 0.13 to 4.64 and from 0.14 to 1.30, respectively. The simple and multiple regression analysis between Kf and soil properties revealed soil clay content, illite content and CEC as fundamental factors determining thiazafluron adsorption by soils. Unlike other substituted ureas, non significant correlation was found with soil organic matter. Desorption of thiazafluron was hysteretic in all cases, showing and values much lower than those for adsorption. Desorption kinetic indicated that this hysteresis is essentially due to irreversible adsorption, although some degradation seems also to occur. The Freundlich desorption Kfd values were closely related to the same factors as Kf: clay and illite contents and also montmorillonite content, suggesting that most of the hysteresis was due to thiazafluron irreversibly bound to soil clay mineral components.