I think without the texture or clay content and broad mineralgy of the soil,it may not be possible to calculate the CEC from organic matter.In very coarse sands or highly calcareous/lime(with very less clay) soils the contribution of mineral matter to CEC can  sometimes be  ignored,giving most of the weightage to organic matter.

CEC is easily measured, but you need to define the pH

Thank you for your replies. I can obtain the soil texture (e.g. loamy sand), organic matter content, and an approx. bulk density. Would this be sufficient to estimate the CEC? and if so, which formula or steps should I use?

To determine/predict  the CEC of a soil from its clay and organic carbon contents,one has to search in the literature the previously established relationships between CEC (dependent variable) and that of clay and organic carbon(independent variables).Also if one knows the CEC per unit of organic matter and clay mineral composition  of the soil,one can approximately compute the CEC of the soil.CECs of dominant clay minerals are availanle in literature.The first approach is easy ,if prediction equations are available from your country.As an example I am providing reference of a paper I published long back.

Prediction of cation exchange capacity from clay and organic carbon of smectite and kaolinite-rich soils. Subba Rao,A.,Srinivasa Rao,Ch. and Pal,S.K. Journal of the Indian Society of Soil Science,44(2):214-219,1996.

Similar papers are availble from other countries.

Kamprath,E.J.&Welch,C.D.(1962) Proc(now J.).Soil Science.Soc.Am.,26,263.

Sahrawat,K.J.(1983) Commun.Soil Sci.Pl.Anal.,14,803.

Yuan,J.L.Nathan Gammon,Jr. and Leighty,R.G.(1967) Soil Sci.104,123.CEC

 soils have CEC , primarily because of clay particles and organic matterin the soil , tends to be negatively charged. Organic matter can have a 4 to 50 times higher CEC per given weight than clay. The source of negative charge in organic matter is different from that of clay minerals; the dissociation (separation into smaller units) of organic acids causes a net negative charge in soil organic matter, and again this negative charge is balanced by cations in the soil. Because organic acid dissociation depends on the soil pH, the CEC associated with soil organic matter is called pH-dependent CEC. This means that the actual CEC of the soil will depend on the pH of the soil. Given the same amount and type of organic matter, a neutral soil (pH ~7) will have a higher CEC than a soil with e.g. pH 5, or in other words, the CEC of a soil with pH-dependent charge will increase with an increase in pH. So some approximation is possible ..for sure..

CEC is the total capacity of a soil to hold exchangeable cations. Thats why soils with a higher clay fraction tend to have higher CEC. Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the top soil. Organic materials in the soil increase the CEC through an increase in available negative charges.Organic matter build up in soil usually positively impacts soil fertility. Therefore, a relationship is possible depends on the soil and type of organic matter present on it. However, organic matter CEC is heavily impacted by soil acidity, as acidity causes many organic compounds to release ions to the soil solution.

With this information, you can at best get a very rough estimate of CEC at a given pH. You will have to measure the CEC separately, but if you assume the pH is 7, you can use the "rule of thumb" value of 320 cmol/kg of SOM for the SOM only. If you know the approximate mineralogy of the soil clay fraction, you can then estimate the contribution from clay. 

Short answer: no, you do not have enough information to precisely predict the CEC.

If it helps, note that some soil classification systems contain general estimates of CEC. For example, US soil taxonomy uses a 4-level classificaiton for mineral soils to reflect the CEC of the clay fraction:

Page 827-828 of Soil Taxonomy (2nd ed, 1999):

A. Soils that are not Histosols, Histels, or Oxisols, that are not in “kandi” or “kanhap” great groups or subgroups of Alfisols and Ultisols, that are in either a mixed or siliceous mineralogy class, that are not in a fragmental, sandy, or sandyskeletal particle-size class or any substitute for a particle-size class, and that have a ratio of cation-exchange capacity (by NH4OAc at pH 7) to clay (percent by weight) of:

1. 0.60 or more. - Superactive

2. 0.40 to 0.60. - Active

3. 0.24 to 0.40. - Semiactive

4. Less than 0.24. - Subactive

Here is the link

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051232.pdf

Direct estimation of CEC is always the perfect method; all other methods provide only an approximate estimation. Direct estimation is not a very tedious procedure.

There are quite a number of studies which offer equations to estimate CEC from soil texture, pH and SOM (e.g., Geoderma, 109, 41–73; Soil Science Society of America Journal, 59, 865–871). However, if you want a fast, fairly accurate method to estimate CEC, recent work (including one of my publications) only require a measure of water content of an air dry soilsample at any humidity level between 10 and 90%. See the publications below:

1. Rapid estimation of cation exchange capacity from soil water content. European Journal of Soil Science, 2017 doi: 10.1111/ejss.12418 

2. Torrent, J., Del Campillo, M.C. & Barron, V. 2015. Predicting cation

exchange capacity from hygroscopic moisture in agricultural soils of

Western Europe. Spanish Journal of Agricultural Research, 13, 1–3. doi:

10.5424/sjar/2015134-8212.

If you have the information of soil organic matter, texture, clay content and pH then probably you may derive a formula to guess the CEC of the soil. But, I should prefer the direct and accurate estimation which is relatively easy procedure and less time consuming.

As mentioned by me earlier if one knows the organic matter status and clay content or clay mineral composition of soil one can compute the soil CEC.The following presentation explains well the concept of CEC and its determination in detail.Page 9 in the presentation gives an example of estimation of CEC.

Soil colloids and cation exchange capacity what should you know.Chapter 8 , p.316-362

web.utk.edu/~drtd0c/Soil Colloids.pdf

CEC can be measure first you have to establish soil pH

I used a methodology in which you determine the CEC from duplicated samples. In one of them you destroy the OM through dry combustion (furnace) or wet disgestion (dichromate) and than determine the CEC in both samples and calculated the CEC from OM by the difference.

Citation from "Building soils for better crops" Magdoff & Van Es (2009)

Estimating organic matter's contribution to a soil's CEC

The CEC of a soil is usually expressed in terms of the numer of milliquivalents (meq) of negative charges per 100 grams of soil.

(The actual number of charges represented by one meq is about 6 followed by 20 zeros) - avogadro / 1000 : (6 X 10^23) / 1000 = 6 X 10^20

A useful rule of thumb for estimating the CEC due to organic matter is as follows:

For every pH unit above pH 4.5, there is 1 meq of CEC in 100 grams of soil for every percent of organic of organic matter.

(Don't forget that there will be CEC due to clays)

Example 1: pH = 5.0 and SOM = 3% ---> (5.0-4.5) X 3 = 1.5 meq/100g

Example 2: pH = 6.0 and SOM = 3% ---> (6.0-4.5) X 3 = 4.5 meq/100g

Example 3: pH = 7.0 and SOM = 3% ---> (7.0-4.5) X 3 = 7.5 meq/100g

Example 4: pH = 7.0 and SOM = 4% ---> (7.0-4.5) X 4 = 10 meq/100g

If you have the clay content, you can do a "better" estimate:

Calculation of CEC with %clay and %OM.

With this formula assume average CEC for % OM=200 meq/100g

and the average CEC for % clay=50 meq/100g .

Therefore:

CEC = (%OM x 200) + (%Clay x 50)

From soil data:

Soil with 2% (0.02) OM

and 10% (0.1) Clay

CEC = (200 x 0.02 + (50 x 0.1) = (4 + 5) = 9 meq/100 g

Be aware that clay types have an important influence.

I think it will be a rough estimation of CEC only considering the organic matter of the soil since CEC of a given soil greatly influenced not only by organic matter content but also by clay type and amount. Therefore, considering clay mineralogy of the soil is very crucial to determine the CEC of the soil.