How to Calculate CEC in MEQ/100 GMS

The Cation Exchange Capacity (CEC) has become an important soil quality for assessing soil fertility. It has also found use in other engineering fields to judge the capacity of soils to uptake heavy metal cations. It is important to understand the general principle of how cation exchange capacity is derived before understanding how to use the CEC number in the field. Then the CEC can be used along with another set of data, base saturation, to compute soil fertility and predict response to available nutrients.

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Cation Exchange Capacity test results

Find the way the cation CEC has been computed in the laboratory report. The CEC can be composed of the capacity of the clay minerals plus the organic fraction of the soil. Therefore, it is common that the CEC is expressed as milliequivalents per 100 grams of soil, abbreviated as MEQ/100 gms. Be sure you know what type of test data you are using before proceeding.

Correct the values of CEC if the test soils are alkaline. The typical test for CEC may overestimate total CEC for soils that are in an alkaline condition, or have a very high percentage of calcium in the soil. If the soils are alkaline, then the total CEC can be adjusted either through adjusting the soil pH or by adjusting the expected CEC.

The laboratory report for CEC should contain a table of Percent Base Saturation values (%BS). These values are the amount of cations that are taking the place of the total capacity of the soil to accept cations.

The CEC value and the percent base saturation levels are used to compute the remaining CEC for the soil, and to compute the amount to exchangeable cations for each nutrient in the soil.

The value of the CEC in mEQ/100 gms is the amount of H+ (hydrogen ions) that the laboratory found the soil can hold. Since H+ has an atomic weight of 1, the capacity for heavier cations will increase depending on the atomic weight of the soil.

For instance, the atomic weight of Potassium (K+) is 39. If a soil has a CEC of 1, it takes 39 times by weight of K+ ions to fill all the H+ sites. Calcium (Ca++) has an atomic weight of 40, but since is has a charge of +2, it takes two sites for every ion. Therefore, Calcium would take 20 times by weight of H+ to fill all the H+ exchange sites.

Compute the amount of CEC that remains in the soil by multiplying the CEC by 100 - %BS and dividing by 100. If the %BS is 92%, and the CEC is 50 MEQ/100 gms, then the available CEC is 50(100-92)/100 = 5 MEQ/100 gms. This is the amount of cation sites that are freely available in the soil.

The next most easily exchangeable cation sites are the H+ nutrient sites. These sites, along with Al+3 are also directly responsible for soil acidity. Replacing these sites with Ca+2 ions will raise soil pH and usually result in a looser soil. Since aluminum ions do not exchange readily, especially at lower pH, is more likely that adding soil amendments, especially calcium, will exchange the H+ ions before the Al+3 ions.

Compute the exchangeable CEC for the H+ ions the same way as for the free sites. If a soil report shows the nutrient saturation for H+ is 5%, and the total CEC is 50 MEQ/100 gms, then the CEC for the H+ sites is: 50(5)/100 = 2.5 MEQ/100 gms.

Compute the total weight of the soil to which the soil amendment is being added. The upper 6 inches of soil is considered the plow zone, and is usually used for determining the amount of nutrients to be added. The proportion of CEC on a MEQ /100 gm basis is 0.001/100, or 0.00001:1. The weight of the plow zone per acre is 2,000,000 lbs. Therefore, each MEQ is equal to 2000000/0.00001 = 20 lbs. The amount of Mg+2, with an atomic weight of 24, to be added per acre is:12 MEQ/100 gms * 20 lbs/acre/MEQ/100gms = 240 lbs per acre for each MEQ weight to be added.

So to raise by 3 MEQ, add 720 lbs magnesium per acre. Check the available magnesium from the soil amendment label to find how much to add of the amendment.