Absorption, or more generally "sorption," is the process by which one material (the sorbent) takes up and retains another (the sorbate) to form a homogenous concentration at equilibrium.

The general term is "sorption," which is defined as adhesion of gas molecules, dissolved substances, or liquids to the surface of solids with which they are in contact. In soils, three types of mechanisms, often working together, constitute sorption. They can be grouped into physical sorption, chemiosorption, and penetration into the solid mineral phase. Physical sorption (also known as adsorption) involves the attachment of the sorbent and sorbate through weak atomic and molecular forces. Chemiosorption involves chemical bonds similar to holding atoms in a molecule. Electrostatic forces operate to bond minerals via ion exchange, such as the replacement of sodium, magnesium, potassium, and aluminum cations (+) as exchangeable bases with acid (-) soils. While cation (positive ion) exchange is the dominant...