Search
Contact Us

t: (440) 561 0510

Linkedin Facebook-f X-twitter

Cation exchange capacity

Abbreviation
CEC
Also known as
CEC, base saturation

Cation exchange capacity (CEC) is a soil's ability to hold positively charged nutrients, such as calcium, magnesium, potassium and ammonium, on negatively charged sites on clay and organic matter. It is reported on soil tests in milliequivalents per 100 grams or centimoles of charge per kilogram.

What is cation exchange capacity?

Clay particles and soil organic matter carry negative electrical charges on their surfaces. These charges attract and hold cations, positively charged ions, including the plant nutrients calcium (Ca2+), magnesium (Mg2+), potassium (K+) and ammonium (NH4+), along with hydrogen and aluminum ions in acidic soils. Held cations can be exchanged with others in the soil solution and taken up by roots.

CEC measures the total quantity of these exchangeable cations a soil can hold. The two common units are numerically equal: 1 milliequivalent per 100 grams (meq/100 g) equals 1 centimole of charge per kilogram (cmol(+)/kg).

What determines CEC

  • Clay content and type. More clay generally means higher CEC, and some clay minerals hold far more charge than others.
  • Organic matter, which has a very high CEC for its weight.
  • Soil pH. Part of the charge on organic matter depends on pH, rising as pH increases.

Sandy, low-organic-matter soils typically have low CEC, while clay and high-organic-matter soils have much higher values.

Why it matters

  • Nutrient holding. Low-CEC soils hold fewer nutrients and lose cations such as potassium more easily to leaching, so they may benefit from smaller, more frequent applications.
  • Lime requirement. High-CEC soils resist changes in pH and need more lime to raise it.
  • Base saturation, the share of CEC occupied by calcium, magnesium, potassium and sodium, is often reported alongside CEC.

CEC changes slowly, mainly through changes in organic matter, which is one reason organic matter is central to soil health.

Frequently asked questions

No. Nitrate is negatively charged and is not held by cation exchange, which is why it leaches readily. See nitrogen loss.

Slowly, by increasing organic matter. Clay content cannot practically be changed.

  • Soil organic matter SOM

    Soil organic matter (SOM) is the fraction of soil made of living organisms, fresh and decomposing plant and animal residues, and stable, well-decomposed material called humus. Although it usually makes up only a few percent of soil by weight, it strongly affects fertility, structure and water holding.

  • Soil health

    Soil health is the continued capacity of a soil to function as a living ecosystem that sustains plants, animals and people. A healthy soil holds and supplies water and nutrients, supports roots and soil life, cycles organic matter and resists erosion.

Sources

  1. Cation exchange capacity and base saturation, Land-grant university extension (e.g. University of Georgia, Penn State)
  2. Soil quality indicators, USDA Natural Resources Conservation Service