DETERMINATION OF THE ORIGINALLY ADSORBED SOIL-PHOSPHORUS BY MODIFIED FREUNDLICH ISOTHERM

Phosphorus adsorption was studied on four different Hungarian soil sam ples previously treated with phosphorus. Six different isotherm models were fitted to the P adsorption data. All the models took into accoun t the amount of phosphate originally adsorbed (Q). The models applied were the Langmuir isotherm, a Freundlich isotherm with a calculated ex ponent 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 wh ose parameters gave the best characterisation of the phosphorus adsorb ing ability of the soil and the amount of phosphorus originally adsorb ed (Q). It should be possible to fit the model to the measured data wi th low residual variance and to evaluate the parameters fairly indepen dently of each other. In all respects a Freundlich isotherm function w ith a constant 1/3 exponent was found the most suitable for the descri ption of phosphorus adsorption. This Freundlich isotherm can be expres sed as: P-ads. = k . c(1/3)-Q A linear correlation was found between t he phosphorus treatments and the originally adsorbed phosphorus quanti ties (Q) calculated using the model. This indicates that the ratio of phosphorus - in the sorbed and bound states - does not depend on the s ize of the phosphorus dose. The value of the k factor characteristic o f 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 cl early capable of characterising the labile P value of the soil.