The impact of tree edificators and forest management on topsoil pH shift and trophism was studied in various natural and changed forest geobiocenoses. A drop of topsoil (10 cm) pH(H2O) values by 0.61-0.96 and pH(KCl) values by 0.56-0.78 was found in a mesotrophic beech geobiocenosis (Fagetum pauper inferiora) during 2-3 years after an intensive felling (stocking 0.3) and clear felling. The soil layer between 10-25 cm was temporarily acidified only on a clear-felled area. The topsoil pH(H2O) values of the spruce monocultures planted on mesotrophic pastures 27 years ago have also strongly decreased (to 0.61-1.05), however, the pH(KCl) values were not substantially changed probably because of a high parent rock mineral power and low forest age. On the other hand the pH(KCl) value (3.22) of 10 cm topsoil developed from flysh rock (also in secondary spruce stand but affected by immissions) is substantially lower in comparison with the rest part of soil solum (pH(KCl) > 4.45). Similar trend was found in old mixed forest below spruce. Ameliorating tree species, such as ash, can increase the pH(H2O) value of topsoil to more than 1 (in Fagetum pauper inferiora from 4.93 to 6.02). However, the ecologically important changes of topsoil pH values only are usually accompanied by a transformation of phytocenose. According to preliminary results the decrease of topsoil pH(H2O) value below 4.9 (form mesotrophic order to hemioligotrophic interorder of geobiocens) is accompanied by a decrease of plant available K+ (by Schachtschabel) below 100 ppm (on territories not influenced substantially by immissions).