Journal of the Indian Society of Soil Science

Effect of ionic strength and cationic species of contacting solution on phosphate sorption was studied on surface (0–0.15 m) soil samples of Camborthids and Haplusteps of semi-arid region of Haryana. The samples of each soil were equilibrated for 48 hrs at 25±2°C with aqueous solution of KH₂PO₄ containing 0 to 465 mg P mL⁻¹. The amount of phosphate sorbed (x/m) by soils at different solution P concentrations varied from 18.3 to 2585.8 mg g⁻¹ at 0.001 M, 18.8 to 2666.2 mg g⁻¹ at 0.01M and 18.9 to 2727.8 mg g⁻¹ at 0.1M ionic strength of CaCl₂. The corresponding values of x/m at these ionic strengths varied from 18.2 to 2346.6, 18.6 to 2446.2 and 18.7 to 2536.2 mg g⁻¹ with MgCl₂ and 17.7 to 2283.6, 18.0 to 2307.5 and 18.0 to 2325.2 mg g⁻¹ with NaCl. Phosphate sorption (x/m), X_Ad and K_d increased with increase in ionic strength. The x/m increased and X_Ad and K_d decreased with increasing concentration of P in solution phase at all the levels of ionic strength. At a particular molarity, phosphate sorption (x/m), X_Ad and K_d were higher in the presence of CaCl₂ as compared to NaCl. Phosphate sorption was satisfactorily (r² = 0.936 to 0.993, P=0.01) described by Freundlich equation in the entire range of P concentrations in all the soils. Constants ‘k’ and ‘n’ increased with increase in ionic strength. These results thus illustrate that in addition to ionic strength, nature of cations supporting electrolyte also played an important role in P sorption.