Phosphorus Sorption Characteristics in Relation to Soil Properties in Some Calcareous Soils of Western Azarbaijan Province

Phosphorus (P) sorption in soils is a key process governing its availability to crops. There has been scanty information available on the P sorption characteristics of calcare-ous soils of Western Azarbaijan Province, Iran. In this study, P sorption characteristics and their relationship to soil properties were determined in ۳۶ soil samples belonging to ۱۵ agricultural soil series located in the south of Urmia, Western Azarbaijan Province. Total CaCO۳ contents ranged from ۱۰ to ۴۸.۵%. Active CaCO۳ contents ranged from ۱.۷ to ۲۰% and were significantly related (r = ۰.۶۱, P<۰.۰۰۱) to the total CaCO۳ of the soils. NaHCO۳-extractable P contents ranged from ۴ to ۳۸ mg P/kg soil. The amount of P sorbed by the soils differed among various soil series. The results showed that equilibrium solution P concentration (EPC) was lower (<۰.۲ mg/L) than the requirement for most crops. The amount of P adsorbed by the soils at ۰.۲ mg/L EPC ranged from ۵ to ۱۱۴ mg/kg soil. The phosphate adsorption was well described by the Freundlich (r۲ = ۰.۹۶) and Langmuir (r۲ = ۰.۸۸) isotherms. The Langmuir maximum adsorption (Xm) and Freundlich coefficient (aF) estimated from Langmuir and Freundlich equations ranged from ۱۲۷ to ۲۳۸ mg P /kg soil and from ۴۳ to ۲۱۱ mg P/kg, respectively. Soil clay content was significantly related to the soil P sorption indices, P۰.۴ (P sorbed at ۰.۴ mg P/L, r = ۰.۴۰, P<۰.۰۱), PBC (P buffering capacity, r = ۰.۵۴, P<۰.۰۰۱), aF (r = ۰.۴۸, P<۰.۰۱), and Xm (r = ۰.۴۰, P<۰.۰۱). Total CaCO۳ and active CaCO۳ were found to be less important factors affecting P adsorption.