Charge Characteristics of Soils and its Variation on Phosphatization

Variable charge characteristics of a few Entisols of West Bengal and its variation on phosphatization are reported. Phosphate adsorption study showed that major phosphate absorbent sites are free oxides (CBD extractable), amorphous ferrialumino silicate (AFAS) minerals along with active components (like A1³⁺, Fe³⁺) in the exchange complex. AFAS minerals together with associated free oxides are largely responsible for variable negative charge on colloidal soil surface. Model diagram, shows that for equal amount of phosphate association on different soil components, an unequal development of negative charge occurs due to difference in release of OH ions by corresponding components. On this basis, the proposed mechanism of phosphate intake in soils involves an initial surface attachment of phosphate through active bridging components of exchange complex such as iron and aluminium followed by fixation of phosphate by the free sesquioxides, thereby, releasing negative components.