Soil acidification is considered the process where protons are neutralized in the soil leading to a decrease in the amount of bases ("decrease in acid neutralization capacity'). Soil acidity is both an intensity term (proton activity in the soil solution) and a capacity term (strong base titratable acidity). Acid neutralization by mineral weathering where the products are removed, leads to soil acidification but does not change soil acidity. Soil acidity may be developed, however, when the acid production rate in the soil system exceed the acid neutralization rate. Exchangeable aluminium, sulphate-aluminium, compounds and organic acids are the main sources for soil acidity. Investigations of acid and base properties in five Danish forest soils shows, that in soils with low base contents soil acidification is small but the increase in soil acidity is high, whereas in soils rich in bases soil acidification is high due to leaching of bases but the development of soil acidity is limited. -from Author