Processes of acidification or alkalization of soils are treated, taking the qualitative changes in soil chemistry into consideration. Following a theoretical background of ecosystem proton budgets, the application for assessing external and internal acid loads are demonstrated. The chemistry of organic matter and the oxides of aluminum, iron, and manganese are treated in the context of being sources and sinks for acid loads in soils. Special attention is payed to the assessment of solubility and reaction kinetics of aluminous minerals. The formation of toxic elements in soil solution resulting from the solubilization of inorganic oxides as well as aspects of changes in the nutrient status of soils, changes of fertility and processes leading to a transfer of acidity from soils to surface are discussed.