Cation exchange capacity

Cation exchange capacity (CEC) is a measure of how well soil can hold onto positively charged ions (cations). CEC affects nutrient availability, soil structure, water availability and soil pH.

Clay and soil organic matter have negative electric charges that attract and hold positively charged ions (cations). Cations include important nutrients such as potassium (K), magnesium (Mg), ammonium, calcium and zinc. These cations become attached to the soil through a process called adsorption.

The adsorbed cations are easily exchanged with other cations and are available to plants.

High CEC soils hold onto cations more effectively. Soils with a low CEC are more likely to acidify quickly and to develop deficiencies in potassium (K), magnesium (Mg) and other cations.

The CEC of a soil depends on the percentage of clay particles, the type of clay, soil pH and the amount of organic matter.

Different primary particles define a soil's texture and its ability to hold cations.

The CEC of sand and silt in most soils is negligible. As clay is negatively charged, CEC increases with the amount of clay.

Clay particles also have a greater surface area than sand particles and so clay soils can naturally hold more organic matter than sandy soils. Clay helps to stabilise organic matter in soil.

Well-broken down soil organic matter (often known as humus) has a high CEC and adds to the soils existing cation exchange capacity.

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