There is no strict formula. For soils with less than 1% organic carbon ,clay CEC is generally estimated by using soil CEC divided by percentage of clay, which is finally multiplied by 100.

The cation exchange capacity (CEC) of soil is measured using the Cu-triethylenetetramine method (Meier and Kahr, 1999) with and without Hofmann-Klemen-treatment (Hofmann and Klemen, 1950). The method was applied to the 1 – 2 mm fraction with different reaction times. Standard reaction time for the powders is 2 h shaking in an end-over-end shaker. The 1 – 2 mm samples were additionally investigated with respect to the CEC after 48 h shaking followed by 10 min ultrasonic treatment and finally after additional 72 h shaking followed by additional 10 min ultrasonic treatment. The method is a good tool to get an idea of the quantity of smectite-group minerals in clayish sediments and soil. It is measured in meq/ 100 gr (milligram equivalent per 100 gram).

Meier, L.P., Kahr, G., 1999. Determination of the cation exchange capacity (CEC) of clay minerals using the complexes of Copper (II) ion with Triethylenetetramine and Tretraethylenepentamine. Clays and Clay Minerals 47, 386 - 388.

Hofmann U., Klemen R.,1950. Verlust der Austauschfähigkeit yon Lithiumionen an Bentonit durch Erhitzung. Zeitschrift für anorganische Chemie 262, 95 - 99.

You can deduce the proportion from the values attained by the method

HGD

It is possible to calculate soil CEC using some empirical equation, when organic matter (OM) and clay content of a soil is known. There are many actually. One of them, CEC = (% OM x 200) + (% Clay x 50).

It is even possible to calculate clay CEC without carrying out any experiment in laboratory, when some of the other attributes are known. One of the latest by Kong et al. (2021).

1) Clay CEC = 10.32 − 0.14pH −0.05OM − 0.11Fe2O3 + 0.01Silt − 0.01Clay + 1.17CECsoil

2) Clay CEC = −3.40 + 0.01Sand + 0.02Silt + 1.05CECsoil

You should remember, any empirical equation is valid under some set of condition and usually valid in the similar study area or environment. That is why using any empirical equation to calculate CEC of soil is not advisable.

You are aware that there are many factor which influence CEC of soil.

Even type of clay minerals present in your sample will influence the CEC.

Thus without knowing the type of clay minerals present in soil, calculation of caly CEC will lead to erroneous result and interpretation.

Thanks Dilip Kumar Pal ! The issue is that for soils from the Amazon region (Brazil), organic carbon will be often higher than 1%.

The best and oldest refence is Cmlay and Clay Minerals by Bailey SW (1966)

Many thanks for the reference Temgoua Emile ! I will have a look at it!