Leaching usually refers to the movement of dissolved substances with water percolating through soil. Sometimes, leaching may also refer to the movement of soluble chemicals out of biological tissues, as when rainfall causes potassium and other ions to be lost by foliage.
Leaching occurs naturally in all soils, as long as the rate of water input through precipitation is greater than water losses by evapotranspiration. In such cases, water must leave the site by downward movement, ultimately being deposited to deep groundwater, or emerging through springs to flow into surface waters such as streams, rivers, and lakes. As the subterranean water moves in response to gradients of gravitational potential, it carries dissolved substances of many kinds.
Leaching is a highly influential soil-forming process. In places where the climate is relatively cool and wet, and the vegetation is dominated by conifers and heaths, the soil-forming process known as podsolization is important. In large part, podsolization occurs through the dissolving of iron, aluminum, calcium, organic matter, and other chemicals from surface soils, and the downward leaching of these substances to lower soil depths, where they are deposited. Some solubilized materials may also be altogether lost from the soil, ending up in deep groundwater or in surface water. A different soil-forming process known as laterization occurs under the warm and humid climatic conditions of many tropical rainforests, where aluminum and iron remain in place in the surface soil, while silicate is dissolved and leached downward.
The ability of water to solubilize particular substances is influenced to a substantial degree by the chemical nature of the solution. For example, highly acidic solutions have a relatively great ability to dissolve many compounds, especially those of metals. Aluminum (Al), for instance, is an abundant metallic constituent of soils, typically present in concentrations of 7-10%, but occurring as aluminum compounds that are highly insoluble, so they cannot leach with percolating water. However, under highly acidic conditions some of the aluminum is solubilized as positively charged ions (or cations), particularly as Al3+ and AlOH2+. These soluble ions of aluminum are highly toxic to terrestrial plants and animals, and if they are leached to surface waters in large quantities they can also cause biological damage there. Aluminum ions are also solubilized from soils by highly alkaline solutions, in which they occur mostly as the anion Al(OH)–. A large salt concentration in soil, characterized by an abundance of dissolved ions, causes some ions to become more soluble through an osmotic extraction, also pre-disposing them more readily to leaching.
Soils can become acidified by various human activities, including emissions of air pollutants that cause acidic precipitation, certain types of agricultural fertilization, harvesting of biomass, and the mining of coal and sulfide minerals. Acidification by all of these activities causes toxicity of soil and surface waters through the solubilization of aluminum and other metals, while also degrading the fertility and acid-neutralization capacity of soil by causing the leaching of basic cations, especially calcium, magnesium, and potassium.
Another environmental problem associated with leaching concerns terrestrial ecosystems that are losing large quantities of dissolved nitrogen, as highly soluble nitrate. Soils have little capability to bind nitrate, so this anion leaches easily whenever it is present in soil water in a large concentration. This condition often occurs when disturbance, fertilization, or atmospheric depositions of nitrate and/or ammonium result in an availability of nitrate that is greater than the biological demand by plants and microorganisms, so this chemical can leach at relatively high rates. Terrestrial ecosystems of this character are said to be "nitrogen-saturated." Some negative environmental effects are potentially associated with severe nitrogen saturation, including an increased acidification and toxicity of soil and water through leaching of aluminum and basic cations (these positively charged ions move in companion with the negatively charged nitrate), nutrient loading to aquatic systems, potentially contributing to increased productivity there, and possibly pre-disposing trees to suffer decline and die back. If the nitrogen saturation is not excessive, however, the growth of trees and other vegetation may be improved by the relatively fertile conditions.