Cation exchange capacity
Encyclopedia
In soil science
, cation-exchange capacity (CEC) is the maximum quantity of total cations, of any class, that a soil
is capable of holding, at a given pH value, for exchanging
with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater
from cation contamination. It is expressed as milliequivalent of hydrogen per 100 g (meq+/100g), or centi mol per kg (cmol+/kg). The numeric expression is coincident in both units.
Clay
and humus have electrostatic surface charges that attract the solution ions, and hold them. This holding capacity varies for the different clay types and clay-blends present in soil, and is very dependent of the proportion of clay+humus that is present in a particular soil. A way to increase CEC is to favor the formation of humus.
For agricultural soils, CEC is ideally between 10 and 30 meq/100 g.
and valence
, and estimating the soil depth and its density. Mengel gives the following correspondence for 1 meq/100g:
Calcium, 400 lb/acre
Magnesium, 240 lb/acre
Potassium, 780 lb/acre
Ammonium, 360 lb/acre
saturation, which is the fraction of exchangeable cations that are base cations (Ca
, Mg
, K
and Na
). It can be expressed as a percentage, and called percent base saturation. The higher the amount of exchangeable base cations, the more acid
ity can be neutralised in the short time perspective. Thus, a site with high cation-exchange capacity takes longer time to acidify (as well as to recover from an acidified status) than a site with a low cation-exchange capacity (assuming similar base saturations).
The Base Cation Saturation Ratio
(BCSR) is a method of interpreting soil test results that is widely used in sustainable agriculture, supported by the National Sustainable Agriculture Information Service (ATTRA) and claimed to be successfully in use on over a million acres (4,000 km²) of farmland worldwide.
methods for determining CEC:
There exist slightly conflicting ideas on which mechanisms to include in the term, "cation exchange", in soil chemistry
. From a theoretical point of view, one should distinguish cation exchange from ligand
exchange, and exchange of diffuse layer adsorbed
cations. On the other hand, from a practical point of view, e.g. in forest
and agricultural
management
, what is important is the soils' ability to replace one cation with another rather than the exact mechanism by which this replacement occurs. What is included in the term, "cation exchange", in soil science
thus varies with the scientific context.
These are the values reported by Carroll (1959) for the cation-exchange capacity of minerals in meq./100g at pH
of 7.
Soil science
Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.Sometimes terms which...
, cation-exchange capacity (CEC) is the maximum quantity of total cations, of any class, that a soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
is capable of holding, at a given pH value, for exchanging
Ion exchange
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
from cation contamination. It is expressed as milliequivalent of hydrogen per 100 g (meq+/100g), or centi mol per kg (cmol+/kg). The numeric expression is coincident in both units.
Clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
and humus have electrostatic surface charges that attract the solution ions, and hold them. This holding capacity varies for the different clay types and clay-blends present in soil, and is very dependent of the proportion of clay+humus that is present in a particular soil. A way to increase CEC is to favor the formation of humus.
For agricultural soils, CEC is ideally between 10 and 30 meq/100 g.
Translation into practical units
Translation from meq/100g to an every day unit, like lb/acre, can be made via calculation, considering atomic weightAtomic weight
Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
and valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
, and estimating the soil depth and its density. Mengel gives the following correspondence for 1 meq/100g:
Calcium, 400 lb/acre
Magnesium, 240 lb/acre
Potassium, 780 lb/acre
Ammonium, 360 lb/acre
Base saturation
Closely related to cation-exchange capacity is the baseAcid-base reaction theories
An acid–base reaction is a chemical reaction that occurs between an acid and a base. Several concepts that provide alternative definitions for the reaction mechanisms involved and their application in solving related problems exist...
saturation, which is the fraction of exchangeable cations that are base cations (Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
and Na
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
). It can be expressed as a percentage, and called percent base saturation. The higher the amount of exchangeable base cations, the more acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
ity can be neutralised in the short time perspective. Thus, a site with high cation-exchange capacity takes longer time to acidify (as well as to recover from an acidified status) than a site with a low cation-exchange capacity (assuming similar base saturations).
The Base Cation Saturation Ratio
Base Cation Saturation Ratio
Base-cation saturation ratio is a method of interpreting soil test results that is widely used in sustainable agriculture, supported by the National Sustainable Agriculture Information Service and claimed to be successfully in use on over a million acres of farmland worldwide...
(BCSR) is a method of interpreting soil test results that is widely used in sustainable agriculture, supported by the National Sustainable Agriculture Information Service (ATTRA) and claimed to be successfully in use on over a million acres (4,000 km²) of farmland worldwide.
Laboratory determination
There are two standardised International Soil Reference and Information CentreInternational Soil Reference and Information Centre
World Soil Information, established in 1964 is an independent foundation with a global mandate, funded by the Netherlands Government, and with a strategic association with Wageningen University and Research Centre.Our aims:...
methods for determining CEC:
- extraction with ammonium acetateAmmonium acetateAmmonium acetate is a chemical compound with the formula CH3COONH4 . It is a white solid, which can be derived from the reaction of ammonia and acetic acid...
; and - the silver-thiourea method (one-step centrifugalCentrifugal forceCentrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
extraction).
There exist slightly conflicting ideas on which mechanisms to include in the term, "cation exchange", in soil chemistry
Soil science
Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.Sometimes terms which...
. From a theoretical point of view, one should distinguish cation exchange from ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
exchange, and exchange of diffuse layer adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
cations. On the other hand, from a practical point of view, e.g. in forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
and agricultural
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
management
Management
Management in all business and organizational activities is the act of getting people together to accomplish desired goals and objectives using available resources efficiently and effectively...
, what is important is the soils' ability to replace one cation with another rather than the exact mechanism by which this replacement occurs. What is included in the term, "cation exchange", in soil science
Soil science
Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.Sometimes terms which...
thus varies with the scientific context.
Standard values
| Kaolinite Kaolinite Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra... |
3-15 |
| Halloysite Halloysite Halloysite is a 1:1 aluminosilicate clay mineral with the empirical formula Al2Si2O54. Its main constituents are aluminium , silicon , and hydrogen . Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite, kaolinite, montmorillonite... 2H2O |
5-10 |
| Halloysite Halloysite Halloysite is a 1:1 aluminosilicate clay mineral with the empirical formula Al2Si2O54. Its main constituents are aluminium , silicon , and hydrogen . Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite, kaolinite, montmorillonite... 4H2O |
40-50 |
| Montmorillonite Montmorillonite Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central... -group |
70-100 |
| Illite Illite Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron layers. The interlayer space is mainly occupied by poorly hydrated potassium cations... |
10-40 |
| Vermiculite Vermiculite Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite... |
100-150 |
| Chlorite Chlorite The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:... |
10-40 |
| Glauconite Glauconite Glauconite is an iron potassium phyllosilicate mineral of characteristic green color with very low weathering resistance and very friable.It crystallizes with a monoclinic geometry... |
11-20+ |
| Palygorskite Palygorskite Palygorskite or attapulgite is a magnesium aluminium phyllosilicate with formula 2Si4O10·4 which occurs in a type of clay soil common to the Southeastern United States. It is one of the types of fuller's earth.-Name:... -group |
20-30 |
| Allophane Allophane Allophane is a poorly-crystalline hydrous aluminium silicate clay mineraloid. Its chemical formula is Al2O3·1.3-2·2.5-3. Since it has short-range atomic order, it is a mineraloid, rather than a mineral, and can be identified by its distinctive infrared spectrum and its X-ray diffraction pattern. It... |
~70 |
These are the values reported by Carroll (1959) for the cation-exchange capacity of minerals in meq./100g at pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of 7.