Surface Charge Characteristics of Alfisols and Inceptisol and the Changes on Phosphatization

Three surface soil samples (0-1.15 m) were collected from different locations of West Bengal, India, namely, Jhargram (Alfisol), Bolpur (Alfisol), and Mohitnagar (Inceptisol) having contrasting physicochemical properties. The charge characteristics of the clay fraction of these soils were probed in terms of the zero-point of charge (ZPC) of the soils. The high values of ZPC noted for the Jhargram and the Bolpur soils may be due to the presence of a large amount of free (variable-charge) sesquioxides in these soils; lower ZPC values were noted in the Mohitnagar soil. Furthermore, the negative magnitudes of ΔpH [pH (KCl) - pH (H₂0)] values, noted for the present soils, suggested that the Mohitnagar soil had less developed characteristics, whereas the Jhargram and the Bolpur soils were in more advanced stages of pedogenic development. The soil samples studied for ZPC were also treated with aqueous KH₂PO₄ solution, and the ZPC values of these phosphate-treated soil samples registered shifts to lower pH values. This may have arisen from an increase in negative charge on soil colloids, following ligand exchange with the added phosphate. The surface potential and the surface charge density of each soil were found to increase in negative magnitude on phosphate application, possibly due to concomitant increases of the negative charges of the clay fraction of the given soils on phosphate fixation through ligand exchange mechanism. The implication of the above parameters, especially their changes on phosphate treatment of the soils, have been related to the tendency of the given soils to immobilize the added phosphate.