**Determination
of the originally adsorbed soil phosphorus by modified Freundlich isotherm**

**L. Tolner, and Gy. Füleky**

Department of Soil Science and Agricultural Chemistry, University of Agricultural Sciences Gödöllõ, H-2103 Gödöllõ, Hungary

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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 (

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:

_{}

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.