The Langmuir and Freundlich isotherms were fitted to P sorption data for 10 highly calcareous soils of Fars province, ranging widely in relevant properties. In this study, 2-g soil samples were equilibrated for 24 hours with 40 mL 0.O1M CaC12 solution contating 5 to 100 mg P/L. The P-treated soils were then extracted with 0.01 M CaC12 solution to study the relationship between P adsorption and desorption characteristics of soils. Plotting the sorption data to the linear form of Langmuir isotherm demonstrated two intersected linear portions. This indicates probably the presence of two adsorption sites which have a widely different affinity for P. The sites in part 1 (corresponding to lower equilibrium P concentration ) had a much higher binding energy constant (K1) than those in part 2 (corresponding to higher equilibrium P concentration). The adsorption data were also observed to follow closely the Freundlich isotherm. Adsorption maxima (b1) calcuated from Langmuir isotherms were positively correlated with .pH (saturated paste), clay, and CEC, and negatively correlated with calcium carbonate equivalent (CCE) in a multiple-regression analysis. Linear relationships were observed between P adsorption and desorption, the slope of lines being different for different soils.
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