The CEC in soils is due to well-humified (“very dead”) organic matter and clay minerals. The total CEC in a soil is the sum of the CEC due to organic matter and due to clays. In fine-textured soils with medium to high-CEC clays, much of the CEC may be due to clays. On the other hand, in sandy loams with little clay, or in some of the soils of the southeastern U.S. that contain clays with low CEC, organic matter may account for an overwhelming fraction of the total CEC.
There are two practical ways to increase the ability of soils to hold nutrient cations such as potassium, calcium, magnesium, and ammonium:
- Add organic matter by using the methods discussed in earlier chapters.
- If the soil is too acidic, use lime (see “pH Management”) to raise its pH to the high end of the range needed for the crops you grow.
One of the benefits of liming acid soils is increasing soil CEC. Here’s why: As the pH increases, so does the CEC of organic matter as well as some clay minerals. As hydrogen (H+) on humus is neutralized by liming, the site where it was attached now has a negative charge and can hold Ca++, Mg++, K+, etc.
Many soil testing labs will run CEC if asked. However, there are a number of possible ways to do the test. Some labs determine what the CEC would be if the soil’s pH was 7 or higher. They do this by adding the acidity that would be neutralized if the soil was limed to the current soil CEC. This is the CEC the soil would have at the higher pH but is not the soil’s current CEC. For this reason, some labs total the major cations actually held on the CEC (Ca++, K+, Mg++) and call it effective CEC. It is more useful to know the effective CEC—the actual current CEC of the soil—than CEC determined at a higher pH.
ESTIMATING ORGANIC MATTER’S CONTRIBUTION TO A SOIL’S CEC
The CEC of a soil is usually expressed in terms of the number of milliequivalents (me) of negative charge per 100 grams of soil. (The actual number of charges represented by one me is about 6 followed by 20 zeros.) 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 me of CEC in 100 grams of soil for every percent of organic matter. (Don’t forget that there will also be CEC due to clays.) SOM = soil organic matter.
Example 1: pH = 5.0 and 3% SOM → (5.0 – 4.5) x 3 = 1.5 me/100g
Example 2: pH = 6.0 and 3% SOM → (6.0 – 4.5) x 3 = 4.5 me/100g
Example 3: pH = 7.0 and 3% SOM → (7.0 – 4.5) x 3 = 7.5 me/100g
Example 4: pH = 7.0 and 4% SOM → (7.0 – 4.5) x 4 = 10.0 me/100g