The study aims at assessing the accuracy of determining the fertility of mountain forest soils by using the Forest Soil Trophism Index (FSTI) [Bożek 2001]. To establish the magnitude of the diagnosis error, eighty soils representing different site conditions of the Western Beskid Mts were analysed for physicochemical properties. It was found that using FSTI for diagnosing site type resulted in a major error (40%) in the case of the sites located at lower elevations of the lower mountain forest zone; the error was much smaller (8%) for the sites situated at higher elevations of that zone. It is suggested that the method for calculating FSTI should be modified to reflect the actual variation observed in some properties of mountain forest soils such as the sum of exchangeable basic cations and the levels of the fractions < 0.02 mm and 0.1-0.02 mm. Modification consists in changing the ranges of the values of those properties, assumed in the original method for successive soil horizons. In addition, the fraction > 1.0 mm is omitted from the calculations. The modified method improved the accuracy of soil classification by FSTI: the error of diagnosis decreased to 25 and 3%, respectively, for the sites located at lower and higher altitudes in the lower mountain forest zone.

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