20.3: Cation Exchange Capacity (CEC) Management

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. Conversely, in sandy loams with little clay, or in some of the soils of the southeastern United States and of the tropics 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. 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 effectiveCEC. 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 and 3% SOM → (5 – 4.5) x 3 = 1.5 me/100g

Example 2: pH = 6 and 3% SOM → (6 – 4.5) x 3 = 4.5 me/100g

Example 3: pH = 7 and 3% SOM → (7 – 4.5) x 3 = 7.5 me/100g Example 4: pH = 7 and 4% SOM → (7 – 4.5) x 4 = 10 me/100g

Soil Acidity

Background

• pH 7 is neutral.
• Soils with pH levels above 7 are alkaline; those of less than 7 are acidic.
• The lower the pH, the more acidic is the soil.
• Soils in humid regions tend to be acidic; those in semiarid and arid regions tend to be around neutral or alkaline.
• Acidification is a natural process.
• Most commercial nitrogen fertilizers are acid forming, but many manures are not.
• Crops have different pH needs, probably related to nutrient availability or susceptibility to aluminum toxicity at low pH.
• Organic acids on humus and aluminum on the CEC account for most of the acid in soils.

Management

• Test soils regularly, every other year if possible, to track soil acidity changes and to make timely adjustments if needed.
• Use limestone to raise the soil pH. (If magnesium is also low, use dolomitic lime, which contains magnesium in addition to calcium.)
• Mix lime thoroughly into the plow layer.
• Spread lime well in advance of planting sensitive crops, if at all possible.
• If the lime requirement is high—some labs say greater than 2 tons, others say greater than 4 tons—consider splitting the application over two years.
• Reducing soil pH (making soil more acid) for acid-loving crops is best done using elemental sulfur (S).