Determination of the originally adsorbed soil phosphorus by modified Freundlich isotherm

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Abstract
Phosphorus adsorption was studied on four different Hungarian soil samples previously treated with phosphorus. Six different isotherm models were fitted to the P adsorption data. All the models took into account the amount of phosphate originally adsorbed (Q). The models applied were the Langmuir isotherm, a Freundlich isotherm with a calculated exponent and 3 Freundlich isotherms with different constant exponents (1/1, 1/2 and 1/3). The aim of the comparison was to select the model whose parameters gave the best characterisation of the phosphorus adsorbing ability of the soil and the amount of phosphorus originally adsorbed (Q). It should be possible to fit the model to the measured data with low residual variance and to evaluate the parameters fairly independently of each other. In all respects a Freundlich isotherm function with a constant 1/3 exponent was found the most suitable for the description of phosphorus adsorption. This Freundlich isotherm can be expressed as: Pads.=k#lbc?‐Q A linear correlation was found between the phosphorus treatments and the originally adsorbed phosphorus quantities (Q) calculated using the model. This indicates that the ratio of phosphorus ‐ in the sorbed and bound states ‐ does not depend on the size of the phosphorus dose. The value of the k factor characteristic of the shape of the sorption curve is near to constant within each soil, so it is characteristic of the given soil. The value of Q is thus clearly capable of characterising the labile P value of the soil.