Effect of Humic Acid on Adsorption Behavior of Phosphorus in a Calcareous Soil

In order to investigate the effect of humic acid (HS) on P adsorption behavior, an experiment carried out with three levels of HS (0, 100, 200 mg/L) and various P concentration (0 to 30 mg/L) at two ionic strengths (IS) of 0.1 and 0.01 M. Adsorption data were fitted to Langmuir, Freundlich and Temkin equations, and Langmuir was better fitted than others (R2=0.91 to 0.98). Results indicated that HS application significantly lowered the adsorption isotherm curves and Langmuir maximum mono layer adsorption (qmax) decreased up to %50 in comparison to control. Also sorption parameters including Langmuir bonding energy parameter (KL), Freundlich capacity and intensity factors (KF, n), and Temkin retention parameter (KT) decreased significantly by adding HS. Maximum buffering capacity (MBC), equilibrium buffering capacity (EBC) and standard buffering capacity (SBC) decreased more than %50 in HS200. Application of HS increased equilibrium phosphorus concentration (EPC) in both ionic strength, as EPC increased from 0.28 to 0.40 mg/L (IS=0.01 M) and 0.21 to 0.39 mg/L (IS=0.1 M). Finally, it was concluded that HS reduced P sorption due to competition on sorption sites and resulted in increasing P availability to plants.