Cation-exchange capacity (CEC) is a measure of how many
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s can be retained on soil particle surfaces.
Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules (cations), but allow these to exchange with other positively charged particles in the surrounding soil water.
This is one of the ways that solid materials in soil alter the chemistry of the soil. CEC affects many aspects of soil chemistry, and is used as a measure of
soil fertility
Soil fertility refers to the ability of soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality. , as it indicates the capacity of the soil to retain several nutrients (e.g. K
+, NH
4+, Ca
2+) in plant-available form. It also indicates the capacity to retain pollutant cations (e.g. Pb
2+).
Definition and principles
![CEC concept](https://upload.wikimedia.org/wikipedia/commons/7/7c/CEC_concept.svg)
Cation-exchange capacity is defined as the amount of positive charge that can be exchanged per mass of soil, usually measured in cmol
c/kg. Some texts use the older, equivalent units me/100g or meq/100g. CEC is measured in
mole
Mole (or Molé) may refer to:
Animals
* Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America
* Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
s of electric charge, so a cation-exchange capacity of 10 cmol
c/kg could hold 10 cmol of Na
+ cations (with 1 unit of charge per cation) per kilogram of soil, but only 5 cmol Ca
2+ (2 units of charge per cation).
[
Cation-exchange capacity arises from various negative charges on soil particle surfaces, especially those of ]clay minerals
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay mineral ...
and soil organic matter Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesize. SOM provides numerous b ...
. Phyllosilicate
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually consid ...
clays consist of layered sheets of aluminium and silicon oxides
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
. The replacement of aluminium or silicon atoms by other elements with lower charge (e.g. Al3+ replaced by Mg2+) can give the clay structure a net negative charge.[ This charge does not involve deprotonation and is therefore pH-independent, and called permanent charge.][ In addition, the edges of these sheets expose many acidic ]hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
groups that are deprotonated to leave negative charges at the pH levels in many soils. Organic matter also makes a very significant contribution to cation exchange, due to its large number of charged functional groups
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
. CEC is typically higher near the soil surface, where organic matter content is highest, and declines with depth. The CEC of organic matter is highly pH-dependent.[
Cations are adsorbed to soil surfaces by the electrostatic interaction between their positive charge and the negative charge of the surface, but they retain a shell of water molecules and do not form direct chemical bonds with the surface.] Exchangeable cations thus form part of the diffuse layer
A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The D ...
above the charged surface. The binding is relatively weak, and a cation can easily be displaced from the surface by other cations from the surrounding solution.
Soil pH
Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the neg ...
The amount of negative charge from deprotonation of clay hydroxy groups or organic matter depends on the pH of the surrounding solution. Increasing the pH (i.e. decreasing the concentration of H+ cations) increases this variable charge, and therefore also increases the cation-exchange capacity.
Measurement
Cation-exchange capacity is measured by displacing all the bound cations with a concentrated solution of another cation, and then measuring either the displaced cations or the amount of added cation that is retained.[ Barium (Ba2+) and ammonium (NH4+) are frequently used as exchanger cations, although many other methods are available.]
CEC measurements depend on pH, and therefore are often made with a buffer solution
A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is ...
at a particular pH value. If this pH differs from the natural pH of the soil, the measurement will not reflect the true CEC under normal conditions. Such CEC measurements are called "potential CEC". Alternatively, measurement at the native soil pH is termed "effective CEC", which more closely reflects the real value, but can make direct comparison between soils more difficult.[
]
Typical values
The cation-exchange capacity of a soil is determined by its constituent materials, which can vary greatly in their individual CEC values. CEC is therefore dependent on parent materials from which the soil developed, and the conditions under which it developed. These factors are also important for determining soil pH, which has a major influence on CEC.
Base saturation
Base saturation expresses the percentage of potential CEC occupied by the cations Ca2+, Mg2+, K+ or Na+.[ These are traditionally termed "base cations" because they are non-acidic, although they are not bases in the usual chemical sense.][ Base saturation provides an index of soil weathering][ and reflects the availability of exchangeable cationic nutrients to plants.][
]
Anion-exchange capacity
Positive charges of soil minerals can retain anions by the same principle as cation exchange. The surfaces of kaolinite, allophane
Allophane is an amorphous to poorly crystalline hydrous aluminium silicate clay mineraloid. Its chemical formula is Al2O3·(SiO2)1.3-2·(2.5-3)H2O. Since it has short-range atomic order, it is a mineraloid, rather than a mineral, and can be ident ...
and iron and aluminium oxides often carry positive charges.[ In most soils the cation-exchange capacity is much greater than the anion-exchange capacity, but the opposite can occur in highly weathered soils,][ such as ]ferralsols
Oxisols are a soil order in USDA soil taxonomy, best known for their occurrence in tropical rain forest within 25 degrees north and south of the Equator. In the World Reference Base for Soil Resources (WRB), they belong mainly to the ferralsol ...
(oxisols
Oxisols are a soil order in USDA soil taxonomy, best known for their occurrence in tropical rain forest within 25 degrees north and south of the Equator. In the World Reference Base for Soil Resources (WRB), they belong mainly to the ferralsols ...
).
References
General References
Ramos, F.T.; Dores E.F.G.C.; Weber O.L.S.; Beber D.C.; Campelo Jr J.H.; Maia J.C.S. (2018) "Soil organic matter doubles the cation exchange capacity of tropical soil under no-till farming in Brazil". ''J Sci Food Agric.'
10.1002/jsfa.8881
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