Cation Exchange Capacity

The total amount of positively charged atoms or molecules, called ions, held by a soil. Many plant nutrients are available as positively charged ions and so a soil with a high cation exchange capacity, or CEC, will be relatively fertile and nutrient rich. This relative abundance of nutrients within a soil with a high CEC makes them available over an extended period of time reducing the need for the addition of many micro-nutrients as a supplementary fertiliser.

Clay particles and organic matter have primarily negatively charged surfaces, and therefore can attract and retain significant amounts of cations. Clean sand, by contrast, does not hold onto nutrients very well, and therefore this results in leaching in sand rootzones; however, small amounts of nutrients are retained within sand rootzones because they are typically ‘contaminated’ with small amounts of clay and organic matter in practice.

In most soils, the Hydrogen ion (H+) displaces other cations, on a regular and continuous basis, from where they are being held onto by the negatively charged clay particles and/or organic matter. These displaced cations then become available within the soil solution for uptake by a plant.