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Leaching is the leaching of substances from a solid by a solvent. Often it is the leaching of relatively readily water-soluble substances such as salt , gypsum or lime from a solid with water.

Leaching can often be observed in building materials such as concrete . It also plays a role in geology in the chemical weathering and subrosion of rock or in soil science in the transport of nutrients from the upper soil layer to deeper. In the field of environmental geology , the "leaching" (referred to here as leaching or mobilization ) of pollutants from contaminated sites and waste is an important issue. The process ( maceration ) plays a role in the extraction of ingredients in medicine and in food technology, for example in the production of liqueurs .

Leaching as a geological process, often referred to as leaching or subrosion , mostly affects rocks such as rock salt , gypsum or limestone . For example, if groundwater has access to underground salt, gypsum or limestone layers and removes these substances in dissolved form, cavities can arise as a result of this leaching into which the rock layers above can sink or collapse. Subrosion or leaching depressions then develop on the surface . In the case of limestone, one speaks generally of karstification, whereby this term is understood to mean both the superficial and underground solution of limestone. The breaking of the outer layers into the cavities creates a special form of breccia , the leaching breccia or collapse breccia .

Due to chemical weathering in the relatively aggressive climate of the tropics, aqueous solutions also remove silicate and metal compounds from rocks and precipitate them again elsewhere . The leached rock is called saprolite .

In the late Middle Ages and early modern times, many water-permeable soils - arable and forest soil - were severely depleted. In the 19th century the causes were recognized in many places, crop rotations and fertilization were improved and ecological measures were taken (for example reforestation of hilltops). As a result of increased coal mining and urbanization, less firewood was taken from the forests (see Niederwald ).

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