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...