Study of Phosphorus Adsorption and Its Relationship with Soil Properties, Analyzed with Langmuir and Freundlich Models

Abstract

Phosphorus (P) adsorption capacity and its relationship with some properties of five different soil types (Fluvisols, Cambisols, Vertisols, Arenosols and Nitosols) of Tigray Region, northern Ethiopia were studied during the main cropping season in 2012. Besides, fractionation of the total soil P in to Ca-P, Fe-P and Al-P was carried out and their relationships with P adsorption characteristics of the soil type under study were also studied. Adsorption data for the different soil types were obtained by equilibrating the respective soil samples for 24 hours at room temperature with 25 ml of 0.01 M CaCl2, containing 0, 60, 120, 180, 240, 300, 360, 420, 480 and 540 mg kg-1 of applied external P as KH2PO4. The results of this study showed that Ca-P was highest followed by Fe-P in all the soils except in the Arenosols. Testing the Langmuir and Freundlich adsorption isotherms both showed well fitted with the correlation coefficients (r) that ranged from 0.951 to 0.999 in all the soils. Comparing the two adsorption models, the Freundlich model was slightly better in Vertisols, Arenosols and Nitosols while the Langmuir model was better in the Fluvisols and Cambisols in predicting P adsorption. The adsorption maximum (b) obtained from the Langmuir isotherm ranged from 296.6 mg P kg-1 soil in Fluvisols to 820.4 mg P kg-1 soil in Vertisols. The constant related to bonding energy calculated from the Langmuir adsorption model varied from 0.0151 to 0.0447 L mg-1. The Freundlich adsorption parameters also ranged from 7.28 to 30.15 mg kg-1 for the Freundlich isotherm constant (Kf) and from 0.4874 to 1.012 for slope of the Freundlich isotherm (1/n). Among the soil properties (CaCO3, CEC, organic matter, EC, pH, and sand, silt, and clay contents), clay content, CEC and CaCO3 were positively correlated with the Langmuir adsorption maximum. The adsorption maximum significantly (P ≤ 0.05) correlated (r = -0.902 but decreased with increasing sand content. The constant (Kf) positively and significantly (P ≤ 0.05) correlated (r = 0.953) with EC of the soil. The remaining soil properties studied were not significantly correlated with the adsorption parameters.