Effect of Calcium Carbonate and Clay on Phosphate Sorption by Soils

Laboratory investigation was carried out to study the effect of CaCO₃ and clay on phosphate sorption by calcareous soils. Three surface soil samples (0–0.15m) representing; Typic Camborthid, Typic Haplustepts and varying in CaCO₃ and clay were collected. The samples of each soil were equilibrated for 48 hrs at 25±2°C with aqueous solution of NaH₂PO₄ containing (0–465 µg P mL⁻¹) in a 1:20 soil: solution ratio. The amount of phosphate sorbed (x/m), per cent of added phosphate sorbed (X_Ad) and distribution coefficient (K_d) increased with increasing CaCO₃ content of the soils. The x/m increased whereas, X_Ad and K_d decreased with increasing concentrations of P in solution phase. The phosphate sorption was satisfactorily (r²=0.98 to 0.99, P=0.01) described by Freundlich equation. The Freundlich ‘k’ and ‘n’ increased with increasing CaCO₃ content of soils. Removal of CaCO₃ from soil decreased the x/m, X_Ad, K_d, ‘k’ and ‘n’. The magnitude of decrease in P sorption parameters varied with CaCO₃ content of the soils. In different soils, CaCO₃ contributed 11.3 to 59.7% and clay 17.9 to 53.8% to the total P sorption at different solution P concentrations. However, the effect of clay content of soils and solution P concentration on contribution of clay to phosphate sorption was not consistent.