The active acidity (pH) of forested mineral land topsoil, its relationships with alternative soil acidity characteristics and the influence of soil acidity on forest soil profile fabric and functioning are analysed in the pedo-ecological conditions of northeastern Europe. The active acidity of the forest floor (O-horizon) was taken as the baseline for this analysis. The acidity of the forest floor was studied in accordance with the underlying soil horizons, which are humus and/or raw-humus horizons in the more fertile soils, whereas in less fertile soils, they are podzolized horizons. The data on soil acidity are presented by main soil types, forest site types, forest humus cover types (pro humus forms) and forest stand groups. In this work, the influences of forest floor acidity on mineral topsoil fabric, soil processes and peculiarities of carbon sequestration are explained. The active acidity of the forest floor provides a great indicative value in elucidating the regularities of the forest soil cover, as a whole, formation and functioning. For evaluating the normality of soil functioning, or the absence of disturbances in ecosystem functioning, the intervals of soil type-specific reference pH levels have been established.

Keywords: forest floor, active acidity, humus cover, humus cover type (pro humus form), soil cover