Under conditions in which rainfall exceeds evapotranspiration (leaching) during most of the year, the basic soil cations (Ca, Mg, K) are gradually depleted and replaced with cations helds in colloidal soil reserves, leading to soil acidity. Clay soils often contain Fe and hydroxy Al, which affect the retention and availability of fertilizer cations and anions in acidic soils.
Soil acidification may also occur by addition of hydrogen, due to decomposition of organic matter, acid-forming fertilizers, and exchange of basic cations for H+ by the roots.

Soil acidity is reduced by volatilization and denitrification of nitrogen. Under flooded conditions, the soil pH value increases. In addition, the following nitrate fertilizers -- calcium nitrate, magnesium nitrate, potassium nitrate and sodium nitrate -- also increase the soil pH value.
Some alkaline soils have Calcium in the form of limestone that is not chemically available to plants. In this case sulfuric acid or Sulfur may be added to reclaim the soil.