Many processes contribute to soil acidification. These include:
* Rainfall:  Average rainfall has a pH of 5.6 and is moderately acidic due to dissolved atmospheric carbon dioxide () that combines with water to form carbonic acid ().  When this water flows through the soil it results in the leaching of basic cations as bicarbonates; this increases the percentage of  and  relative to other cations.
* Root respiration and decomposition of organic matter by microorganisms release  which increases the carbonic acid () concentration and subsequent leaching.
* Plant growth: Plants take up nutrients in the form of ions (e.g. , , , ), and they often take up more cations than anions. However, plants must maintain a neutral charge in their roots. In order to compensate for the extra positive charge, they will release  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 () fertilizers react in the soil by the process of nitrification to form nitrate (), and in the process release  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 and nitric acid in rain.
* Oxidative weathering: Oxidation of some primary minerals, especially sulfides and those containing , generate acidity. This process is often accelerated by human activity: 
** Mine spoil: Severely acidic conditions can form in soils near some mine spoils due to the oxidation of pyrite.
** Acid sulfate soils formed naturally in waterlogged coastal and estuarine environments can become highly acidic when drained or excavated.