The Acidic Precipitation in Ontario Study (A.P.I.O.S.) was established in 1979 by the Ontario Ministry of the Environment to protect the environment from the detrimental effects of acidic deposition and determine the effects of acidic deposition on terrestrial ecosystems. The effect of acid inputs on the environment depends on the quantity and acidity of atmospheric inputs, and the buffering capacity of the terrestrial and aquatic ecosystems. The effect of acidic precipitation on soil is potentially significant because the soil is the reservoir of plant nutrients and the source of much of the water for aquatic systems. Changes in the soil's chemistry may therefore be reflected in both the terrestrial and aquatic environments. Information on the effects of acidic precipitation on soils is available from effects near point sources of pollution, from experimentation with simulated rain solutions, from natural changes observed in soil over time, and from models and soil forming theories. Anticipated effects of soil acidification include: reduced pH, leaching of basic cations (such as magnesium and calcium) and other exchangeable plant nutrients, reduction in base saturation and cation-exchange capacity, mobilization of soil-bound metals such as aluminum, and changes in biological activity such as decreased nitrification and soil respiration. The principal objective of the baseline program is to establish a uniform data base for soils across the province. This data base * provides current data to identify future trends, * enables the development of laboratory experiments which define soil sensitivity criteria to acidic precipitation, and * provides information required for sensitivity mapping of soils throughout Ontario.