Soil acidification

Soil acidification is the buildup of hydrogen cations, also called protons, reducing 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 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 these alkali (or base) elements. Plants utilize all these alkali elements as they grow, each important to some aspect of cell tissue development and growth. Where plant material is removed, as when a forest is logged or crops are harvested, the base elements they have taken up are permanently lost from the soil.

Rainfall is acidic

Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. Lightning produces nitric acid in rainwater. Sulfur dioxide from volcanic eruptions can become sulfuric acid within rainwater. These sources of rainfall acidity contribute to soil acidification. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower rainfall pH to 4.5 or even 3.0., which can cause solution weathering to the rocks on which it falls.

Biological weathering

Plant roots acidify soil by releasing protons and organic acids so as to chemically weather ^[1]soil minerals. Decaying remains of dead plants on soil may also form organic acids which contribute to soil acidification.^[2] Acidification from leaf litter on soil is more pronounced under coniferous trees such as pine, spruce and fir, which return fewer base cations to the soil, than under deciduous trees.

Rocks in the soil

Certain parent materials also contribute to soil acidification. Granites and their allied igneous rocks are called "acidic" because they have a lot of free quartz, which produces silicic acid on weathering. Also, they have relatively low amounts of calcium and magnesium. Some sedimentary rocks such as shale and coal are rich in sulfides, which, when hydrated and oxidized, produce sulfuric acid which is much stronger than silicic acid. Many coal spoils are too acidic to support vigorous plant growth, and coal gives off strong precursors to acid rain when it is burned. Marine clays are also sulfide-rich in many cases, and such clays become very acidic if they are drained to an oxidizing state.

Soil amendments

Soil amendments such as fertilizers and manures can cause soil acidification.Sulfur based fertilizers can be highly acidifying, examples include elemental sulfur and iron sulfate while others like potassium sulfate have no significant effect on soil pH. While most nitrogen fertilizers have an acidifying effect, ammonium-based nitrogen fertilizers are more acidifying than other nitrogen sources. Ammonia-based nitrogen fertilizers include ammonium sulfate, diammonium phosphate, monoammonium phosphate, and ammonium nitrate. Organic nitrogen sources, such as urea and compost, are less acidifying. Nitrate sources which have little or no ammonium, such as calcium nitrate, magnesium nitrate, potassium nitrate, and sodium nitrate, are not acidifying.^[3]^[4]^[5]

Pollution

Acidification may also occur from nitrogen emissions into the air, as the nitrogen may end up deposited into the soil.^[6] Animal livestock is responsible for 64 percent of man-made ammonia emissions.^[7]

Effects

Soil acidification results in smaller, less durable roots, sometimes damaging the tips of the roots restricting further growth. Plant height is impaired. Seed germination decreases. Soil acidification can reduce negative plant health, resulting in reduced cover and lower plant density. Soil acidification is directly linked to a decline in endangered species of plants. Soil acidification reduces soil microbial and macrofaunal diversity.

Maple decline is often attributed to soil acidification caused by acid rain.

References

^ Chigira, M.; Oyama, T. (2000-01-01), Kanaori, Yuji; Tanaka, Kazuhiro; Chigira, Masahiro (eds.), "Mechanism and effect of chemical weathering of sedimentary rocks", Developments in Geotechnical Engineering, Engineering Geological Advances in Japan for the New Millennium, vol. 84, Elsevier, pp. 267–278, retrieved 2019-02-10
^ Tom., Nisbet, (2014). Forestry and surface water acidification. Forestry Commission. ISBN 9780855389000. OCLC 879011334.{{cite book}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
^ Schindler, D. W.; Hecky, R. E. "Eutrophication: More Nitrogen Data Needed". Science. 324: 721–722. doi:10.1126/science.324_721b.
^ Penn, C. J.; Bryant, R. B. "Phosphorus Solubility in Response to Acidification of Dairy Manure Amended Soils". Soil Science Society of America Journal. 72: 238–243. doi:10.2136/sssaj2007.0071N. Retrieved 2019-01-13.
^ "Don't let nitrogen acidify your soil". Department of Primary Industries - New South Wales. Retrieved 2019-01-13.
^ USGS. Acid Soils in Slovakia Tell Somber Tale.
^ Henning Steinfeld; Pierre Gerber; Tom Wassenaar; Vincent Castel; Mauricio Rosales; Cees de Haan (2006). "Livestock's Long Shadow: Environmental issues and options". Food and Agriculture Organization of the United Nations. Retrieved 25 October 2012.