Background: Although soils in the acidic soils of Tsegede (Skeletic Leptosols, Cambic Leptosols, Leptic Cambisols and Dystric Cambisols) are characterized by low available P contents, study on P adsorption characteristics is limited information. The purpose of this experiment was to evaluate P adsorption characteristics in different acidity ranges of soils.
Material and method: Potassium dihydrogen phosphate (KH2PO4) was used for the adsorption isotherm studies by duplicating 3 g soil from each soil types at different acidity ranges of ten soil samples with 50 ml of P in 0.01 M CaCl2 solution having 0, 20, 40, 60, 80,100 mg P L-1. Langmuir and Freundlich adsorption isotherm models were used to describe adsorption processes and the relationships between P adsorption and soil properties were determined by correlation.
Results: Phosphorus adsorption increased significantly with increasing P levels and it increased as soil pH declines. The maximum and minimum P-adsorptions were obtained from very strongly acidic Skeletic Leptosols and from moderately acidic Dystric Cambisols, respectively. Comparing the two models, Langmuir linear model showed a better fit to the tested soils compared to Freundlich model. The regression coefficients (R2) for the fitted Langmuir P adsorption isotherms were highly significant ranging from (0.955 to 0.999) and the adsorption maxima obtained from the Langmuir isotherm ranged from 357 mg P kg-1 soil in strongly acidic Dystric Cambisols to 2500 mg P kg-1 soil in very strongly acidic Skeletic Leptosols. The bonding energy (k) for Langmuir adsorption model varied from 0.012 to 2.8 L mg-1 and both the highest (2.8 L mg-1) and lowest (0.012 L mg-1) k values were obtained from strongly and moderately acidic Dystric Cambisols. The cation exchange capacity (CEC) and organic carbon (OC) were positively correlated with the Langmuir adsorption maxima (b) and negatively correlated with available P and soil pH.
Conclusion: Increments of soil pH using different amendments and their proper management in soils are important in making productive use and higher dose of P is required by soils with higher fixation. Alternative P management strategies are also needed to reduce P adsorption and enhance P availability in such acidic soils in the study areas.