Acidification or alkalinization of soils occurs through H+ transfer processes involving vegetation, soil solution and soil minerals. A permanent change in the acid neutralizing capacity of the inorganic soil fraction (ANC(s)),i.e. soil acidification (ΔANC 0), results from an irreversible H+ flux. This irreversible H+ flux can be caused either by direct proton addition or depletion, by different mobility of components of the ANC(s) or by a permanent change in redox conditions. The contributions of (a) acidic atmospheric deposition, (b) nitrogen transformations, (c) deprotonation of CO2 and of organic acids and protonation of their conjugate bases, (d) assimilation of cations and anions by the vegetation, (e) weathering or reverse weathering of minerals and (f) stream output to changes in the ANC(s) are illustrated by means of H+ budgets for actual soils and watersheds.