In the early 1980s, the problem of acidification was acknowledged on a governmental level in the Netherlands as posing a major environmental threat. The concern for damage to forests and ecosystems, and materials and monuments, as well as for acidification of soils and moorland pools stimulated research and policy development. Acid deposition was used as a characterisation of the atmospheric input leading to these effects. As such, wet and dry deposition of acids, and dry deposition of acid precursors as sulphur dioxide and nitrogen oxides, were considered. At the same time, the important role of ammonia in the acidification of soils and moorland pools was acknowledged. Ammonia, the most important acid neutralising component in the atmosphere, was shown to be nitrified to an important extent in soil into nitric acid, thereby contributing to acidification. This led to the concept of potential acid deposition, in which the deposition of actual acid (H +) was added to that of potential acid (ammonia + ammonium). Moreover, the role of atmospheric nitrogen input was recognised as a disturbing factor for natural ecosystems and forests growing on poor soils.