Cation exchange capacity indicates the ability of a soil to "hold" onto positively charged ions (cations) including plant nutrients such as potassium, calcium, magnesium and ammonium. The CEC is largely determined by clay content and organic matter. Clay has the greatest ability to hold cations, as it has a very large surface area compared to sand or silt (see Soil Texture in the Lab factsheet). Organic matter also has a high cation exchange capacity (up to 30 times greater than clay).
The soil quality indicator values presented here are “Effective CEC” values, which is calculated by summing the exchangeable base cations (Na+ , K+ , Mg2+ and Ca2+), as well as exchangeable aluminium (Al3+) and hydrogen (H+).