Soil acidity can be considered as the capacity of soils to manifest properties of acids or proton donors (Vorob’eva and Avdon’kin 2006). It occurs when acidity-generating processes outweigh acidity-consuming processes (Ulrich 1994). A soil is defined as acid when its pH is lower than 7. Soil acidification has many causes that are natural and unnatural. Although soil acidification is a slow natural process, it can be acce­lerated by plants, animals, and human activities or slowed down or reversed by care­ful management practices (Bolan et al. 1994; Poss et al. 1995).

7.2.1 Some Causes of Soil Acidity

7.2.1.1 Rainfall and Leaching

In climates where rainfall exceeds evapotranspiration, soils with low buffer capac­ity tend to acidify. Excess water infiltrating the soil enhances leaching of basic ions such as calcium (Ca2+) and magnesium (Mg2+) from the exchange complex of soil (clay minerals, humus) and their substitution by protons (H+) and aluminum ions (Al3+) (Mayer 1998). This way, neutral clay may be converted into a hydrogen clay or acid clay, which gradually accumulates and intensifies under increasing amounts of rainfall.