Phosphorus Movement and Retention by Two Calcareous Soils

To predict P sorption and leaching behavior in calcareous soils, we examined the adsorption and movement of applied P in columns of two calcareous soils. Phosphorus and various other ions were monitored in the leachate of the soil column by passing a 100 mg P l-1 solution through the soil column. Concentrations of P, K+, Ca2+, Mg2+, Na+, HCO- 3, Cl-, EC and pH were determined in the leachates. Movement of K+ and P ions was retarded due to K+ ion-exchange and P adsorption and precipitation, respectively. Phosphorus leaching was affected by supersaturation with respect to P-Ca minerals, but undersaturated with respect to Mg-P minerals. Phosphorus retention based on batch and miscible displacement experiments revealed profound discrepancies that can be attributed to the short residence time of P in the miscible displacement. Breakthrough curves of P and K+ were analyzed by a CXTFIT program. The equilibrium model provides good results to the transport process of P and K+. Results indicated that the mobility of P in these calcareous soils reflects that a high downward movement of water-soluble P in soils may occur and much attention should be paid to leaching of P and potential contamination of P to surface and ground waters.