A laboratory study was conducted to determine the potassium (K) adsorption characteristics of five soils under enset (Ensete ventricosom cheesman) farming system in Sidama. Potassium adsorption isotherms were constructed by equilibrating 5 g soil samples with 5 levels of K (0 to 78mg/L) as KCl in 10 mmoles/L CaCl2 solutions. Suspensions were shaken for half an hour. Adsorption data were fitted to Freundlich, Langmuir and Temkin adsorption equations. On an average, amount of K adsorbed ranged from 29.2 to 67.82% of added K. Significant positive relationship existed among percent K adsorbed, CEC, percent clay and pH. Langmuir equation gave a better fit of equilibrium K adsorption data for sandy clay loam and silty clay loam soils, while Freundlich equation gave a better fit for clay and loam soils. Furthermore, Temkin isotherm gave a better fit of equilibrium K adsorption data for clay loam soil. Sandy clay loam and loam soils had low 1/n values. The n values on the other hand indicated that the sorption is favorable only for these soils. The Freundlich adsorption capacity, kf were found low as compared to available soil potassium. The negative values of Langmuir isotherm maximum monolayer coverage capacity (a) for clay loam and clay soils agreed with the absence of best fit to the model. Sandy clay loam and silty clay loam which indicated best fit had high monolayer coverage. Value of Temkin isotherm equilibrium binding constant (AT) was high for clay loam soil and this was manifested by high coefficient of determination. Low values of constant related to heat of sorption (B) were determined for sandy clay loam and loam whilst clay loam, silty clay loam and clay soils indicated high binding energy (B).
Key words: Sorption isotherm, Freundlich, Langmuir, Temkin model, monolayer coverage, equilibrium binding constant, heat of sorption.
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