Cation Exchange Capacity and Base Saturation

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/100g) or as centimoles of charge per kilogram (cmole_c/kg). CEC can range from below 5 meq/100g in sandy soils low in organic matter to over 20 meq/100g 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.

'Base saturation' is the percentage of negatively charged sites on soil colloids that are filled with basic cations. Of the cations that account for the vast majority of those adsorbed on the soil colloids in New England soils—calcium (Ca++), magnesium (Mg++), potassium (K+), hydrogen (H+), and aluminum (Al+++)—Ca++, Mg++, and K+ are considered basic cations and have little to no influence on soil pH, while H+ and Al+++ are considered acidic cations because they tend to lower soil pH. If all the cations in a soil were basic and none were acidic, there would be a 100% base saturation and the soil pH would be close to 7 or neutral. In acid soils there is some amount of acidic cations adsorbed on the soil colloids—called the 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 80–90%.

At one time, many labs provided fertilizer recommendations to achieve very specific ideal Ca++, Mg++, and K+ saturation ratios. This approach was never well supported by data. Research conducted over the last several decades indicates that an ideal basic cation ratio does not exist and fertilizing to achieve a prescribed level of these nutrients is unjustified. Still, base saturation can provide useful information. When base saturation values reported on soil tests are well outside of ranges typically observed in New England soils, it is usually associated with deficient or excessive potassium or very acidic or alkaline soil conditions. Following the fertilizer and lime recommendations provided with your report will generally result in base saturation values within normal ranges.