Cation-exchange capacity
Cation-exchange capacity (CEC) is a measure of how many
Definition and principles
Cation-exchange capacity is defined as the amount of positive charge that can be exchanged per mass of soil, usually measured in cmolc/kg. Some texts use the older, equivalent units me/100g or meq/100g. CEC is measured in moles of electric charge, so a cation-exchange capacity of 10 cmolc/kg could hold 10 cmol of Na+ cations (with 1 unit of charge per cation) per kilogram of soil, but only 5 cmol Ca2+ (2 units of charge per cation).[1]
Cation-exchange capacity arises from various negative charges on soil particle surfaces, especially those of
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.[4] Exchangeable cations thus form part of the diffuse layer 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
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.[1] Barium (Ba2+) and ammonium (NH4+) are frequently used as exchanger cations, although many other methods are available.[4][5]
CEC measurements depend on pH, and therefore are often made with a buffer solution 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.[1][5]
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.
Soil Taxonomy order | CEC (cmolc/kg) |
---|---|
Ultisols | 3.5 |
Alfisols | 9 |
Spodosols | 9.3 |
Entisols | 11.6 |
Mollisols | 18.7 |
Vertisols | 35.6 |
Histosols | 128 |
Base saturation
Base saturation expresses the percentage of potential CEC occupied by the cations Ca2+, Mg2+, K+ or Na+.[1][4] These are traditionally termed "base cations" because they are non-acidic, although they are not bases in the usual chemical sense.[1] Base saturation provides an index of soil weathering[4] and reflects the availability of exchangeable cationic nutrients to plants.[1]
Anion-exchange capacity
Positive charges of soil minerals can retain anions by the same principle as cation exchange. The surfaces of kaolinite,
References
- ^ a b c d e f g h i j k l Brady, Nyle C.; Weil, Ray R. (2008). The nature and properties of soils (14th ed.). Upper Saddle River, USA: Pearson.
- ^ a b Birkeland, Peter W. (1999). Soils and geomorphology (3rd ed.). Oxford: Oxford University Press.
- ^ Zech, Wolfgang; Schad, Peter; Hintermeier-Erhard, Gerd (2014). Böden der Welt (in German) (2nd ed.). Berlin: Springer Spektrum.
- ^ a b c d Schaetzl, Randall J.; Thompson, Michael L. (2015). Soils: Genesis and geomorphology (2nd ed.). Cambridge: Cambridge University Press.
- ^ a b Pansu, Marc; Gautheyrou, Jacques (2006). Handbook of Soil Analysis. Berlin: Springer-Verlag. pp. 709–754.
- .
- ^ "Cations and Cation Exchange Capacity". Retrieved June 23, 2017.
- .
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