The clay and organic components of the soil have a negative charge. As a result of these charges, positively charged ions (cations) such as hydrogen H⁺, potassium K⁺, ammonium NH⁺₄, calcium Ca²⁺, magnesium Mg²⁺, aluminum Al³⁺, etc. may be held at the surface of the clay or organic particles and exchanged with other ions in the solution or with ions at the plant root's surface.

The ability of a soil to hold cations is termed its cation exchange capacity (CEC). Since many cations are plant nutrients the cation exchange capacity is a measure of the soil's ability to hold such nutrients.

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