Editing Soil pH (section)

=== Sources of acidity ===
Many processes contribute to soil acidification. These include:<ref name="VanBreemen1984">{{cite journal |last1=Van Breemen |first1=Nico |last2=Driscoll |first2=Charles T. |last3=Mulder |first3=Jan |title=Acidic deposition and internal proton sources in acidification of soils and waters |journal=[[Nature (journal)|Nature]] |date=16 February 1984 |volume=307 |issue=5952 |pages=599–604 |doi=10.1038/307599a0 |bibcode=1984Natur.307..599B |s2cid=4342985 |url=https://www.researchgate.net/publication/40164555 |access-date=19 February 2023}}</ref>
* Rainfall:  Average rainfall has a pH of 5.6 and is moderately acidic due to dissolved atmospheric carbon dioxide ({{chem|link=carbon dioxide|C|O|2}}) that combines with water to form [[carbonic acid]] ({{chem|H|2|C|O|3}}).  When this water flows through the soil it results in the leaching of basic cations as [[Bicarbonate|bicarbonates]]; this increases the percentage of {{chem|Al|3+}} and {{chem|H|+}} relative to other cations.<ref>{{cite web |last=US EPA |first=OAR |date=2016-02-09 |title=What is acid rain?|url=https://www.epa.gov/acidrain/what-acid-rain |access-date=26 February 2023 |website=[[United States Environmental Protection Agency|US EPA]] |language=en}}</ref>
* Root [[Respiration (physiology)|respiration]] and decomposition of organic matter by microorganisms release {{chem|C|O|2}} which increases the carbonic acid ({{chem|H|2|C|O|3}}) concentration and subsequent leaching.
* Plant growth: Plants take up nutrients in the form of ions (e.g. {{chem|N|O|3|-}}, {{chem|N|H|4|+}}, {{chem|Ca|2+}}, {{chem|H|2|P|O|4|-}}), and they often take up more [[cation]]s than [[anion]]s. However, plants must maintain a neutral charge in their roots. In order to compensate for the extra positive charge, they will release {{chem|H|+}} ions from the root. Some plants also exude organic acids into the soil to acidify the zone around their roots to help solubilize metal nutrients that are insoluble at neutral pH, such as iron (Fe).
* Fertilizer use: [[Ammonium]] ({{chem|N|H|4|+}}) fertilizers react in the soil by the process of [[nitrification]] to form nitrate ({{chem|N|O|3|-}}), and in the process release {{chem|H|+}} ions.
* [[Acid rain]]: The burning of fossil fuels releases oxides of sulfur and nitrogen into the atmosphere. These react with water in the atmosphere to form [[sulfuric acid|sulfuric]] and [[nitric acid]] in rain.
* [[Weathering#Oxidation|Oxidative weathering]]: Oxidation of some primary minerals, especially [[Sulfide|sulfides]] and those containing {{chem|Fe|2+}}, generate acidity. This process is often accelerated by human activity: 
** [[Acid mine drainage|Mine spoil]]: Severely acidic conditions can form in soils near some mine spoils due to the oxidation of [[pyrite]].
** [[Acid sulfate soil]]s formed naturally in [[Waterlogging (agriculture)|waterlogged]] coastal and estuarine environments can become highly acidic when drained or excavated.