What Is Cation Exchange Capacity & Base Saturation Percent?

Source Credit: https://extension.uga.edu/publications/detail.html?number=C1040&title=Cation+Exchange+Capacity+and+Base+Saturation#What

CATION EXCHANGE CAPACITY (CEC)

Soil clay minerals and organic matter tend to be negatively charged, thus attracting positively charged ions (cations) on their surfaces by electrostatic forces. As a result, the cations remain within the soil root zone and are not easily lost through leaching. The adsorbed cations may easily exchange with other cations in the soil solution, hence the term “cation exchange.” The adsorbed cations replenish the ions in the soil solution when concentrations decrease due to uptake by plant roots.

Cation exchange capacity (CEC) is a measure of the total negative charges within the soil that adsorb plant nutrient cations such as calcium (Ca²⁺), magnesium (Mg²⁺) and potassium (K⁺). As such, the CEC is a property of a soil that describes its capacity to supply nutrient cations to the soil solution for plant uptake. Figure 1 illustrates cations retained on soil clay minerals that can exchange with those in the soil solution. Plant roots can remove nutrients from the soil solution, which results in nutrients moving away from the clay particles. Addition of fertilizer to soil causes an initial increase in nutrient concentration in the soil solution, which results in nutrients moving toward clay particles.

The nutrient cations plants use in the largest amounts are potassium (K⁺), calcium (Ca²⁺) and magnesium (Mg²⁺). Other cations adsorbed on exchange sites are ammonium (NH⁴⁺), sodium (Na⁺), hydrogen (H⁺), aluminum (Al³⁺), iron (Fe²⁺ or Fe³⁺), manganese (Mn²⁺), copper (Cu²⁺) and zinc (Zn²⁺). Micronutrient cations such as zinc, copper, iron and manganese are typically present at very low concentrations in soils. Ammonium concentrations are also typically very low because microorganisms convert ammonium to nitrate in a process called nitrification.

How CEC Changes With Soil pH

The CEC of soil organic matter and some clay minerals varies with pH. Generally, the CEC is lowest at soil pHs of 3.5 to 4.0 and increases as the pH is increased by liming an acid soil, as shown in Figure 2. Because CEC may vary considerably with soil pH, it is a common practice to measure a soil’s CEC at a pH of 7.0. Also note that some positive charges may occur on specific soil mineral surfaces at low pH. These positive charges retain anions (negatively charged ions) such as chloride (Cl^–) and sulfate (SO4^2-).