An understanding of P sorption by soils is important for fertilizer recommendation. Little is known about P sorption characteristics of the calcareous soils of Hamadan province. In this study 15 soil samples of calcareous soils were collected from Hamadan province. The objectives of the study were to evaluate P sorption by using adsorption isotherm models as well as the correlation of these equation parameters with soil properties. Phosphorus adsorption data were adequately described by Freundlich and the linear adsorption Isotherms, wherease Langmuir adsorption isotherm did not describe P adsorption in all soils. Phosphorus adsorption maxima (b) ranged from 69.93 to 204.08 mg kg-1. The constant related to binding energy(k) ranged from 0.1 to 2.0. Maximum buffering capacity(MBC) ranged from 12.9 to 238.08 mg kg-1. In Freundlich equation n parameter ranged from 0.26 to 0.99, and kd from 7.77 to 232.75 L kg-1. In the linear model, equilibrium buffering capacity (EBC) ranged from 4.88 to 126.61 L kg-1 and Labile P ranged from 0.71 to 29.37 mg kg-1. Standard phosphorus requirement, calculated by Langmuir, Freundlich and linear model, ranged from 3.21 to 34.01, 2.7 to 65.1, and 1.01 to 62.81 mg kg-1 respectively. The results imdicated that adsorption isotherm parameters were significantly correlated with equivalent calcium carbonate, wherease not significantly correlated with other soil properties. The results of correlation studies among adsorption isotherm parameters revealed that K was significantly correlated with Freundlich equation parameters as well as Labile P, however b was not significantly correlated with Freundlich and linear model parameters. Parameter Kd was significantly correlated with EBC and Labile P, wherease n parameter was significantly correlated with Labile P. MBC was significantly correlated with EBC. The results finally indicated that Langmuir, Freundlich and linear models can be employed to describe P sorption characteristics, and calcium carbonate equivalent may affect P sorption characteristics.
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