Soil acidification is the buildup of hydrogen cations, which reduces the soil pH. Chemically, this happens when a proton donor gets added to the soil. The donor can be an acid, such as nitric acid, sulfuric acid, or carbonic acid. It can also be a compound such as aluminium sulfate, which reacts in the soil to release protons. Acidification also occurs when base cations such as calcium, magnesium, potassium and sodium are leached from the soil. Soil acidification naturally occurs as lichens and algae begin to break down rock surfaces. Acids continue with this dissolution as soil develops. With time and weathering, soils become more acidic in natural ecosystems. Soil acidification rates can vary, and increase with certain factors such as acid rain, agriculture, and pollution. Causes Acid rain 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 Al3+ that is toxic to fish, especially newly-hatched fry with immature gill systems through which they pass large volumes of water to obtain O2 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