Rainfall is naturally acidic due to carbonic acid forming from carbon dioxide in the atmosphere. This compound causes rainfall pH to be around 5.0–5.5. When rainfall has a lower pH than natural levels, it can cause rapid acidification of soil. Sulfur dioxide and nitrogen oxides are precursors of stronger acids that can lead to acid rain production when they react with water in the atmosphere. These gases may be present in the atmosphere due to natural sources such as lightning and volcanic eruptions, or from anthropogenic emissions. Basic cations like calcium are leached from the soil as acidic rainfall flows, which allows aluminum and proton levels to increase.
Nitric and sulfuric acids in acid rain and snow can have different effects on the acidification of forest soils, particularly seasonally in regions where a snow pack may accumulate during the winter. Snow tends to contain more nitric acid than sulfuric acid, and as a result, a pulse of nitric acid-rich snow meltwater may leach through high elevation forest soils during a short time in the spring. This volume of water may comprise as much as 50% of the annual precipitation. The nitric acid flush of meltwater may cause a sharp, short term, decrease in the drainage water pH entering groundwater and surface waters.  The decrease in pH can solubilize  Al that is toxic to fish, especially newly-hatched fry with immature gill systems through which they pass large volumes of water to obtain O for respiration. As the snow meltwater flush passes, water temperatures rise, and lakes and streams produce more dissolved organic matter; the Al concentration in drainage water decreases and is bound to organic acids, making it less toxic to fish.  In rain, the ratio of nitric-to-sulfuric acids decreases to approximately 1:2.  The higher sulfuric acid content of rain also may not release as much Al from soils as does nitric acid, in part due to the retention (adsorption) of SO by soils. This process releases OH into soil solution and buffers the pH decrease caused by the added H from both acids. The forest floor organic soil horizons (layers) that are high in organic matter also buffer pH, and decrease the load of H+ that subsequently leaches through underlying mineral horizons.