Salts of strong acids and weak bases form acidic solutions. Unpolluted precipitation has a pH of 5.6 because of the presence of carbonic acid. Production of organic acids upon the decomposition of plant residues. Oxidation of reduced substances. Acid mine drainage.
Similar reactions can occur upon draining wetlands.
Recycling of basic cations by rooted plants 3.2.3. Similar reactions control the solubility of Mn in the soil solution. Al and Mn can be toxic to plants and animals. This is a severe problem in the acidic soils of the tropics. The concentration of a cation on the exchange complex determines the concentration of that cation in the soil solution.