Cation exchange capacity (CEC) is a measure of the soil’s ability to retain and supply nutrients, specifically the positively charged nutrients called cations. These include calcium (Ca++), magnesium (Mg++), potassium (K+), ammonium (NH4+), and many of the micronutrients. Cations are attracted to negatively charged surfaces of clay and organic particles called colloids. CEC is reported as milli-equivalents per 100 grams of soil (meq/100 g) or as centimoles of charge per kilogram (cmole+/kg). CEC can range from below 5 meq/100 g in sandy soils low in organic matter to over 20 meq/100 g in finer textured soils and those high in organic matter. Low CEC soils are more susceptible to cation nutrient loss through leaching, and may not be able to hold enough nutrient cations for a whole season of crop production.
The cations Ca++, Mg++, K+, hydrogen (H+) and aluminum (Al+++) account for the vast majority of cations adsorbed on the soil colloids in New England soils. It is important to note that H+ and Al+++ are not plant nutrients. Both H+ and Al+++ are considered acidic cations because they tend to lower soil pH while Ca++, Mg++, and K+ are considered basic cations and have little to no influence on soil pH. If all the cations are basic and none are acidic, there would be a 100% base saturation and the soil pH would be close to 7 or neutral. In acid soils there are acidic cations adsorbed on the soil colloids (called exchangeable acidity) and the percent base saturation is less than 100. A soil with a pH between 6.5 and 6.8 will typically have a base saturation of 80 to 90%.