Cation exchange capacity (CEC) is the sum of all exchangeable cations (e.g., Na+, Ca2+, K+) that a soil can adsorb. These cations are positively charged ions and are held on the negatively charged surfaces of soil particles (e.g., clay and organic matter) and can be replaced by other positively charged ions in the soil solution. Since they are readily exchanged with other cations, they are also easily available to plants. The CEC of a soil is dependant on the composition (e.g., types and amounts of clay and organic matter) of the soil (permanent charge) and the pH (pH-dependent charge).
The proportion of the CEC occupied by non acid-forming cations (Ca2+, Mg2+, K+ and Na+) is termed the percent base saturation. It is important to note that these cations are not bases and do not necessarily form bases. Cations including Al3+ and Fe3+ are considered acid-forming cations as they can hydrolyze water. The percent base saturation can be calculated as shown in Equation 1.
Consider the method presented in your textbook for determining the CEC for a soil. Similar measurements were conducted for soil samples collected from a Gray Luvisol in BC (forested area; see Table 1 a brief summary of the soil properties). For these measurements, 100 g subsamples of oven-dried soil from each horizon were leached with 400 mL of a NH4+ salt solution (leachate 1). The NH4+ displaces all of the cations adsorbed to soil surfaces. Leachate 1 was analyzed for Ca, Mg, K and Na (Table 2). The excess NH4+ remaining in the soil was removed using alcohol and then the sample was leached again with a 400 mL of a K+ salt solution (leachate 2). Similar to above, the K+ displaces all of the NH4+ from the soil surfaces.
Determine the CEC of the soil sample collected from each soil horizon presented in Table 2.
cmol of exchangeable Ca +Mg+K'+Na cmol of CEC %base saturation
