CEC
Ala Khaleel and Amber Anderson
- Identify sources of cation exchange capacity
- Calculate CEC and base saturation given soil test information
- Explain how management may change based upon CEC/AEC
- Predict differences between CEC/AEC could be found given soil characteristics
Nutrients
Nutrients are held (or not) in different ways in the soil:
- Adsorption: is the retention of ions or molecules to a surface. The prefix “ad” describes a reaction “at” the surface of a solid.
- Cations: positively charged ions (for example, calcium, magnesium, potassium, sodium,,,etc)
- Anions: negatively charged ions (for example, chloride)
- Exchangeable cations: cations that are replaced/exchanged by soil solution.
Ion Exchange in soils:
- Ion exchange involves the movement of anions or cations through the soils.
- In ion-exchange reactions, cations or anions that are adsorbed on soil surfaces are exchanged/replaced by another cations or anions in the soil solution.
- Ion exchange in soils occurs on surfaces of:
- o Primarily on clay minerals (layer silicate minerals)
- o Soil organic matter
- Soils in the United States have more negatively charged minerals than positively charged minerals; therefore, cation exchange is much more common.
Cation Exchange Capacity (CEC)
CEC is a measure of the total amount of negative charges on soil surfaces that are available to hold cations, usually plant nutrients. This is based on the organic matter and clay minerals, along with the pH of the soil. Consider it a measure of the soil’s ability to attract and hold nutrient cations or the sum of total exchangeable cations that the soil can absorb. Like a positive side of a magnet attracts the negative, and strength is influenced by factors like size and type of material, not all soils hold equally. CEC is very important to plant productivity as it influences what or what quantity of plant nutrients held and made available in the soil. It is reversible and adjusts to be in equilibrium with the soil solution.
Buffering capacity is the ability to resist those changes-higher CEC values mean the system will be slower to change. We call this a higher buffering capacity.
CEC is very important for management because soils with low CEC cannot hold and retain too many important nutrients (ammonium (NH4+), and base cations (Ca2+, Mg2+, K+, and Na+)) like soils with higher CEC. An anion like nitrate (NO3-) is repelled rather than attracted to soil surfaces in most midwestern US conditions and can leach. In areas with anion exchange capacity, like highly weathered soils where CEC is low, nutrient management strategies change. These areas may rely more heavily on forms of nutrients that can be held and released as plants need them, like organic material. In highly weathered, acidic conditions, anion exchange capacity may dominate. This requires different management strategies as well, as different nutrients are likely limiting plant productivity.
Percent Base Saturation (BS)
Cations in the soil can be classified into base (non-acid forming) cations (Ca+2, Mg+2, K+1, Na+, and NH4+) and acidic cations (Al+3 and H+1). Most bases are plant nutrients, excluding sodium, so higher BS is generally better. However, base saturation is simply a percentage of the total, rather than a total amount available to the plant. Higher values are also considered to have higher ‘buffering capacity’ or ability to resist change.
If you have two soils both with a base saturation of 50%, and want to increase it to near 95%, the soil with the higher CEC will require more material to adjust, even though the percentages are the same.
- I have a soil reported to contain 3 cmolc/kg of Ca, 1 cmolc/kg K, 1 cmolc/kg Mg, and 5 cmolc/kg H.
- Since these numbers add up to 10 (assuming this is all of the cations), then my CEC is about 10 cmolc/kg. Five of these (Ca, K, and Mg) are basic cations, resulting in a 50% (5 bases/10 total) base saturation for this soil.
- CEC results from organic matter and clays within the soil
- CEC is the measure of cations (usually plant nutrients) held within a soil
- Base saturation is a measure of percentage of charges occupied by basic (non-acid forming) cations