Cation binding and exchange.

(a) Clay and humus particles display negative electrostatic surface charges that bind cations. Bound cations include not only plant mineral nutrients, such as NH4+, but also cations that are not plant nutrients, such as Al3+. (b) Cation exchange occurs when protons displace cations, releasing them to soil water. This process makes cations more available for uptake by plant roots, but it also increases the potential for cations to leach away during rains or floods. Protons in acid soils or arising from acid rain displace so many cations from soil that its fertility decreases, and toxic ions, such as Al3+, more readily damage plants.