Cation exchange capacity (CEC) is the ability of a given soil mass to hold positively charged ions, or cations. This is important because CEC sites allow the soil to hold and release cations, many of which are essential nutrients, into the soil solution for plant uptake. Knowing the basics of CEC can help you understand why some soils retain nutrients better than others as well as help you develop a sound fertility plan for your home lawn.
Each nutrient ion has a net electric charge based on its loss or gain of electrons. Cations are ions with a positive charge and anions are ions with a negative charge. As with a magnet, ions can either attract or repel one another. Ions with unlike charges (i.e., positive and negative) will attract one another, and ions with like charges (i.e., positive and positive) will repel each other. In general, soil particle surfaces mainly have negative charges. Attraction of positive ions by the soil particles allows retention of positively charged ions, such as potassium (K+). Ions can move into the soil solution for plant uptake or continue to be held by the soil. Ions with greater positive charges can also exchange weaker positive ions on soil particles. Cation exchange capacity is a measure of the total amount of exchangeable cations that can be held by a given soil mass. The CEC of a soil can be measured by soil analysis laboratories if requested.
Soil pH: Soil pH can affect CEC of some soil types and organic matter. This is known as pH-dependent CEC. Higher soil CEC can reduce leaching losses of cations and help buffer the soil. Buffering capacity of a soil helps the soil to resist chemical changes, such as pH, that affect nutrient availability and microbial activity. For more information on how to adjust soil pH see publication 3624-BBB.
Soil organic matter: In general, higher CEC is associated with higher soil organic matter. However, differences in CEC can exist based on the state of decomposition of the organic matter. The addition of well-decomposed organic matter, such as humus, can increase soil CEC. Remember, microorganisms are involved in nutrient transformations in soil as well as decomposition of organic matter, such as leaf clippings and thatch. Therefore, maintaining suitable environmental soil conditions is important to microbial activity.
Soil texture: In general, CEC is influenced by soil texture. Soil texture is determined by its respective percentages of sand, silt and clay particles. Soils with high percentages of clay content typically have higher CEC. Sandy textured soils generally have lower CEC and may require addition of soil organic matter. For more information on how to determine soil texture, see publication 3624-CCC.
|Nitrogen (N), Nitrate||NO3-|
|Nitrogen (N), Ammonium||NH4+|
|Phosphorus (P)||H2PO4- , HPO42-|
|Iron (Fe)||Fe2+, Fe3+|
|Copper (Cu)||Cu2+, Cu+|