Surface Charge Characteristics of Alfisols and Inceptisol and Their Changes on Phosphatisation

Surface soil samples (0–0.15 m) were collected from Jhargram (Altisol), Bolpur (Altisol); and Mohitnagar (Inceptisol) representing three different locations of West Bengal, having contrasting physiochemical properties. High values of zero point of charge (ZPC) noted for the Jhargram and the Bolpur soils, are attributed to presence of relatively large amount of free (variable charge) sesquioxides (1.51–1.58%) in these soils, while lower ZPC values were noted in the Mohitnagar soil which is relatively low in free sesquioxides (0.44%). Furthermore, the negative magntidues of ΔpH [pH(KCI)-pH(H₂O)] values, noted for the present soils, suggested that the Mohitnagar soils had less developed characteristics, whereas the Jhargram and the Bolpur soils were in more advanced stages of pedogenic development. The ZPC values of the phosphate treated soil samples registered a decrease, i.e. shift to lower pH values. This is attributable to an increase in negative charge on soil colloids, fallowing ligand exchange with the added phosphate. This is evident from increase in pH values of phosphate treated samples. Due to the same reason the surface potential, and the surface charge density of each soil were found to increase in negative magnitude on phosphatisation. The implications of the above changes on phosphate treatment of Soils, have been related to the tendency of the given soils to immobilise the fertilizer or soil derived phosphate.