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Leaching is the process of extracting a substance from a solid by dissolving it in a liquid. In the chemical processing industry, leaching is known as extraction. Leaching has a variety of commercial applications, including separation of metal from ore using acid, and sugar from beets using hot water.

In agriculture, leaching refers to the loss of water-soluble plant nutrients from the soil, due to rain and irrigation. Soil structure, crop planning, type and application rates of fertilizers, and other factors are taken into account to avoid excessive nutrient loss.

Leaching is an environmental concern when it contributes to groundwater contamination. As water, from rain, flooding or other sources, seeps into the ground, it can dissolve chemicals and carry them into the underground water supply. Of particular concern are hazardous waste dumps and landfills, and, in agriculture, excess fertilizer and improperly stored animal manure.