PO 10. Describe how cation exchange capacity (CEC) influences nutrient mobility and uptake.

• High CEC means more nutrients are held on the soil, decreasing their mobility and uptake
• Low CEC means that more nutrients are in the soil solution, making them available to plants but also increasing the likelihood of leaching

CEC is defined by measurement of the amount of positively-charged ions (cations) which can be bound by a given weight of soil.  Cations bound on the soil surface can exchange places with cations in the soil solution, making them available to the plants and subjecting them to leaching.

Examples of cations include: K⁺, Ca²⁺, Mg²⁺, NH₄⁺, Cu²⁺, Fe²⁺ or Fe³⁺, Mn²⁺, Al³⁺, Zn²⁺

A larger CEC implies a greater capacity to retain K⁺, Ca²⁺, Mg²⁺, and NH₄⁺.  Soils with large CEC are typically high in clay minerals and soil organic matter (OM), which have a lot of negative charges.  CEC increases with pH, due to variable charge on the organic matter; the CEC measured at the pH of the soil is called the effective CEC.  The CEC is calculated from exchangeable cations, and is only seldom measured in a soil testing lab.

Low CEC means that fewer nutrients can be held by the soil, implying a need for more frequent nutrient additions. As CEC increases, more nutrients are attached to soil particles, and fewer remain in the soil solution.  Since the nutrients in soil solution are available to plants, this means that while there are plenty of nutrients in the soil, the plants may not be able to take advantage of them.  At the same time, they are less likely to leach.  Addition of cations to the soil, through acidification, liming, or fertilization, will release cations into the soil solution as the new cations swap places on the CEC.