The balance of hydrogen ions and basic ions determines soil acidity. A clay particle with abundant hydrogen ions acts as a weak acid, whereas if fully charged with bases (such as calcium, Ca) it has a neutral or alkaline reaction. In practice, soil pH is usually regulated by the presence of calcium cations; soils become more acid as calcium is leached from the soil faster than it is replaced. This is the tendency in temperate areas where rainfall (carbonic acid see p297) exceeds evaporation over the year.
Hydrogen ions take over the soil's cation exchange sites (see p305) and the pH falls. Soils with large reserves of calcium (containing pieces of chalk or limestone) do not become acid because they are kept base-saturated. In contrast, calcium ions are readily leached from free-draining sands in high rainfall areas and these soils tend to go acid rapidly (see podsols p302). In addition to the carbonic acid in rainfall, there are several other sources of acid that affect the soil:
In climates where the evaporation exceeds rainfall over the year, the dissolved salts are brought to the surface. As the water is lost from the soil by evaporation, the dissolved salts accumulate on the surface. These are usually basic (alkaline) in action so the soil pH rises. An extreme example of this is the salt (sodic) desert, e.g. Utah Salt Flats.
Soil pH falls and becomes more acid as 'lime' is lost from the soil.
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