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 (XAd) and distribution coefficient (Kd) increased with increasing CaCO3 content of the soils. The x/m increased whereas, XAd and Kd 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, XAd, Kd, ‘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.