Effect of Nature of Exchangeable Cations on Phosphate Sorption by Soils

Effect of nature of exchangeable cations on phosphate sorption was studied on surface (0–0.15 m) soil samples of Camborthids and Haplustepts of semi-arid region of Haryana. To study the effect of different cations on phosphate sorption, the original soil samples were made homoionic with respect to Ca²⁺, Mg²⁺, NH4⁺, K⁺ and Na⁺ using their chloride salts. The samples of each soil except Na-soil were equilibrated for 48 hrs at 25±2°C with aqueous solution of NaH₂PO₄ containing 0 to 465 μg P mL⁻¹. The Na-soil was equilibrated with solution of KH₂PO₄. Phosphate sorption was satisfactorily (r² = 0.980 to 0.996, P= 0.01) described by Freundlich equation in the entire range of P concentration in all the soils. The x/m, XAd, Kd, ‘k’ and ’n’ for P sorption of K-, NH4- and Na-soils were lower than Ca- and Mg-soils. The magnitude of phosphate sorption of different homoionic soils was in the order: Ca-soils > Mg-soils> K-soils> NH4-soils> Na-soils whereas the order of equilibrium pH was reverse of this order. The Ca-soils and Mg-soils sorbed more phosphate than K-, NH4- and Na-soils due to valence effect. The Ca-soils sorbed more P than Mg-soils; K-soils and NH4-soils sorbed more P than Na-soils due to their hydrated ions size effect. These results showed that the phosphate sorption of different homoionic soils was influenced by equilibrium pH, valence and size of hydrated ions, thickness of electrical double layer and negative potential of the surface.