Leaching is the process whereby water, as it percolates through the soil, dissolves out various mineral salts. Rain water is slightly acidic, because of dissolved carbon dioxide from the atmosphere, and thus important in the leaching of soils. Once removed, the soluble minerals can only be replaced very slowly. As a result, leached soils become coarse and are infertile. Saline soils, can, however, be reclaimed for agriculture by deliberately leaching out salts.

The term is also used to describe the industrial process of extracting a soluble material from a solid by washing the solid with solvents.