Soil acidification

Soil acidification is the buildup of hydrogen cations, also called protons, reducing the soil pH. This happens when a proton donor gets added to the soil. The donor can be an acid, such as nitric acid and sulfuric acid (these acids are common components of acid rain). It can also be a compound such as aluminium sulfate, which reacts in the soil to release protons. Many nitrogen compounds, which are added as fertilizer, also acidify soil over the long term because they produce nitrous and nitric acid when oxidized in the process of nitrification.

Acidification also occurs when base cations such as calcium, magnesium, potassium and sodium are leached from the soil. This leaching increases with increasing precipitation. Acid rain accelerates the leaching of bases. Plants take bases from the soil as they grow, donating a proton in exchange for each base cation. Where plant material is removed, as when a forest is logged or crops are harvested, the bases they have taken up are permanently lost from the soil.

Many plants produce organic acids. Where plant litter accumulates on or is incorporated to the soil, these acids (including acetic acid, humic acid (see http://www.suprahumic.unina.it/) , oxalic acid, and tannic acid) are liberated. This is especially acute in soils under coniferous trees such as pine, spruce and fir, which return fewer base cations to the soil than do most deciduous trees.

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