Cation-Exchange Capacity (CEC)
Cation-exchange capacity is defined as the degree to which a soil can adsorb and exchange cations.
Cation-a positively charged ion (NH4+, K+, Ca2+, Fe2+, etc...)
Anion-a negatively charged ion (NO3-, PO42-, SO42-, etc...)
Soil particles and organic matter have negative charges on their surfaces. Mineral cations can adsorb to the negative surface charges or the inorganic and organic soil particles. Once adsorbed, these minerals are not easily lost when the soil is leached by water and they also provide a nutrient reserve available to plant roots.
These minerals can then be replaced or exchanged by other cations (i.e., cation exchange)
CEC is highly dependent upon soil texture and organic matter content. In general, the more clay and organic matter in the soil, the higher the CEC. Clay content is important because these small particles have a high ration of surface area to volume. Different types of clays also vary in CEC. Smectites have the highest CEC (80-100 millequivalents 100 g-1), followed by illites (15-40 meq 100 g-1) and kaolinites (3-15 meq 100 g-1).
Examples of CEC values for different soil textures are as follows:
Soil texture
CEC (meq/100g soi)
Sands (light-colored) 3-5
Sands (dark-colored) 10-20
Loams 10-15
Silt loams 15-25
Clay and clay loams 20-50
Organic soils
50-100
In general, the CEC of most soils increases with an increase in soil pH.
Two factors determine the relative proportions of the different cations adsorbed by clays. First, cations are not held equally tight by the soil colloids. When the cations are present in equivalent amounts, the order of strength of adsorption is Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+.
Second, the relative concentrations of the cations in soil solution helps determine the degree of adsorption. Very acid soils will have high concentrations of H+ and Al3+. In neutral to moderately alkaline soils, Ca2+ and Mg2+ dominate. Poorly drained arid soils may adsorb Na in very high quantities.
Base Saturation
The proportion of CEC satisfied by basic cations (Ca, Mg, K, and Na) is termed percentage base saturation (BS%). This property is inversely related to soil acidity. As the BS% increases, the pH increases. High base saturation is preferred but not essential for tree fruit production. The availability of nutrient cations such as Ca, Mg, and K to plants increases with increasing BS%.
Base saturation is usually close to 100% in arid region soils. Base saturation below 100% indicates that part of the CEC is occupied by hydrogen and/or aluminum ions. Base saturation above 100% indicates that soluble salts or lime may be present, or that there is a procedural problem with the analysis.
CEC and Availability of Nutrients
Exchangeable cations, as mentioned above, may become available to plants. Plant roots also possess cation exchange capacity. Hydrogen ions from the root hairs and microorganisms may replace nutrient cations from the exchange complex on soil colloids. The nutrient cations are then released into the soil solution where they can be taken up by the adsorptive surfaces of roots and soil organisms. They may however, be lost from the system by drainage water.
Additionally, high levels of one nutrient may influence uptake of another (antagonistic relationship). For example, K uptake by plants is limited by high levels of Ca in some soils. High levels of K can in turn, limit Mg uptake even if Mg levels in soil are high.
Anion Exchange
In contrast to CEC, AEC is the degree to which a soil can adsorb and exchange anions. AEC increases as soil pH decreases. The pH of most productive soils in the US and Canada is usually too high (exceptions are for volcanic soils) for full development of AEC and thus it generally plays a minor role in supplying plants with anions.
Because the AEC of most agricultural soils is small compared to their CEC, mineral anions such as nitrate (NO3- and Cl-) are repelled by the negative charge on soil colloids. These ions remain mobile in the soil solution and thus are susceptible to leaching.
