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In order to replace the rather complicated physico-chemical method, a mathematical technique related to the binding of phosphate ions was elaborated to determine the labile, and thus plant-available phosphate content of the soil. The technique is based on the analysis of the quantity of phosphate bound to the soil in the equilibrium state when soil samples are suspended in solutions containing various concentrations of phosphate. The majority of soils contain a certain amount of adsorbed phosphate, so negative adsorption is observed if the analysis is carried out with solutions containing little or no phosphate. The isotherm models generally used to describe adsorption processes do not assume the presence of any material originally bound on the adsorbent surface, so their starting point is a state with a zero quantity of adsorbed material on the adsorbent in a solution with an initial equilibrium concentration of zero. In the case of soil phosphate adsorption, however, different results are obtained, as the soil already contains adsorbed phosphate in its original state. For this reason the isotherm model was modified. The modified isotherm model was fitted to the measurement data using non-linear regression. The quantity of phosphate originally adsorbed on the soil can be calculated using a model parameter. The phosphate quantity calculated in this way exhibits a close correlation both with the isotopically exchangeable phosphate content, and with the results obtained with the best chemical extraction methods.

DETERMINATION OF THE PHOSPHATE CONTENT ORIGINALLY ADSORBED ON THE SOIL BY FITTING AN ADSORPTION ISOTHERM MODEL