CEC, an abbreviation for Cation Exchange Capacity, refers to the amount of negative charges available on the surface of soil particles.
It gives an indication of the potential of the soil to hold plant nutrients, by estimating the capacity of the soil to retain cations, which are positively-charged substances.
Therefore, the CEC of the soil directly affects the amount and frequency of fertilizer application.
Clay soil particles and organic matter carry a negative charge on their surfaces. Cations are attracted to the negatively-charged particles by electrostatic forces. The net charge of the soil is, therefore, zero.
Soils with high CEC typically have a high clay and organic matter content. These soils are considered to be more fertile, as they can hold more plant nutrients. Sandy soils typically have a lower CEC and require more frequent fertilizer applications.
Typical Cation Exchange Capacities of soil components and soil types
Loamy Sand to Sandy Loam
The predominant cations in agricultural soils are: K+, Ca2+, Mg2+, Na+, Al3+ and H+. These are also being referred to as "Exchangeable Cations", because they can be replaced by other cations present in the soil solution.
Other positively-charged plant nutrients, that are present at lower quantities in the soil, are NH4+, Fe2+, Mn2+ and Cu2+.
Only a small portion of the plant nutrient cations are in the soil solution. The exchangeable cations , which are bound to the soil surfaces, are in equilibrium with soil solution. The CEC, therefore, provides a reservoir of nutrients to replenish those removed by plant uptake or leached out of the root zone.
The units in which CEC is measured are meq/100g. This unit takes into account the charge of the ion. For example, Calcium carries a charge of (+2). Therefore, 1mmol of calcium is equivalent to 2meq of calcium. On the other hand, potassium (K) carries a charge of (+1) and 1mmol K+ = 1meq K+ .
From the example above, it can be noted that Ca ions will be adsorbed to as twice as many exchange sites as the same number of K+ ions.
The electrical charge of some of the soil components that contribute to the CEC is affected by the pH of the soil.
These components have an OH- group on their edges. OH- group can release or absorb protons. At high pH protons are released from these groups, their charge becomes negative and, as a result, CEC of the soil increases.
OH- groups are present on the surfaces of Kaolinite clay, hydroxides (primarily Al and Fe hydroxides) and organic matter.
In fact, there are two types of CEC measured in laboratories: CEC at neutral conditions and CEC at the actual soil pH. The values resulting from these two different measurements may vary greatly. The CEC measured at the actual soil pH is referred to as "Effective CEC" and abbreviated as CECe.
Inaccuracies in the measurement occur when soil pH is greater than 7.5 or when lime was recently applied. The resulting CEC is overestimated under such conditions.