Calculation of cation exchange capacity and percent base saturation

Cation exchange capacity (CEC) is a term used to describe the retention capacity of a particular soil for positively charged elements (cations). It can also be described as the ability of one soil to exchange cations for another. In simple terms, the higher the clay content, the higher the CEC since the clay particles have the greatest surface area per unit volume of soil and therefore can contain the greatest amount of cations.

The CEC is part of the soil test that is calculated from the levels of potassium, magnesium, calcium, sodium and hydrogen that were extracted in the soil analysis. These cations are the ones with the highest concentration in most soils. The calculations will be explained below:

Each of these elements has its individual atomic weight as found on the periodic table.

By obtaining the equivalent weights, we now have each element in equal terms or in other words, 20 ppm of calcium can displace 12 ppm of magnesium in the soil complex.

However, CEC is reported in meq/100 gm. Equivalent weights are reported as equivalents per gram. Therefore, the equivalent weights must be multiplied by ten to be converted to meq/100 gm.

Now that we have these values, we can calculate the CEC from the ppm of each of these elements in the soil test. The following will be used as an example:

A soil test shows we have 379 ppm potassium, 133 ppm magnesium, 1600 ppm calcium, 22 ppm sodium, and a pH of 8.0. (Since the pH is above 7.0, no hydrogen has been reported, but if the pH were 6.0, I would use the meq/100 gm of hydrogen given in the Agrotec soil analysis in the calculation.)

As you can see, the most important factor in the calculation is calcium. Soil exchange sites contain mainly calcium; However, in higher pH soils, large amounts of free calcium may also be present in the soil solution, which may also be extracted in the test, reflecting a greater retention capacity than texture analysis would really indicate. Agrotec's testing procedures attempt to reduce the effects of this excess calcium, but the CEC may still reflect an inflated number compared to what the texture would indicate.

The percent base saturation is calculated from these values by dividing the milli-equivalents of each by the CEC as shown below:

The percent saturation of the base could be defined as the relative availability of each of these cations.