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Sorption isotherms were estimated for two model organophosphorus pesticides - methamidophos and paraoxon-ethyl - and four typical Central European soils endangered by these pesticides: haplic Chernozems, Cambisols, Luvisols, and Fluvisols. A photometric microplate assay based on the recognition capability of the enzyme acetyicholinesterase toward organophosphorous pesticides was used for the construction of sorption isotherms. The sorption capacity of each soil was then determined and it was found that the soils with the highest content of humic acid (haplic Chemozems and haplic Luvisols) sorbed pesticides most. Pesticides in concentrations over the sorption capacity were easily removed from soil by water. [ABSTRACT FROM AUTHOR]