ADSORPTION AND DESORPTION OF THE HERBICIDE THIAZAFLURON AS A FUNCTIONOF SOIL PROPERTIES

The effect of soil composition on the adsorption and desorption of the herbicide thiazafluron methyl-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 Freundlic h equation and the values of the constants, Kf and nf, ranged from 0.1 3 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 th iazafluron adsorption by soils. Unlike other substituted ureas, non si gnificant correlation was found with soil organic matter. Desorption o f thiazafluron was hysteretic in all cases, showing nfd values much lo wer than those for adsorption. Desorption kinetic indicated that this hysteresis is essentially due to irreversible adsorption, although som e degradation seems also to occur. The Freundlich desorption Kfd value s were closely related to the same factors as Kf: clay and illite cont ents and also montmorillonite content, suggesting that most of the hys teresis was due to thiazafluron irreversibly bound to soil clay minera l components.