Cation exchange capacity
The cation exchange capacity ( abbreviation : KAK , T value ) is a measure of the exchangeable cations and thus the number of negative binding sites of cation exchangers in the soil . It represents an important characteristic of the soil and depends not only on the chemical composition, but also on the mechanical properties of the soil, since the effective surface area is related to the size of the colloids .
The sum of the exchangeable cations - in addition to H +, mainly simple metal ions such as Na + and K + as monovalent, Mg 2+ and Ca 2+ as divalent and Al 3+ and Fe 3+ as trivalent - which are attached to the negative binding sites of inorganic and organic Exchangers are bound in the soil is determined experimentally by exchanging them with a neutral salt solution (often barium, ammonium or calcium chloride).
Negative charges in the inorganic soil substance arise through isomorphic replacement of the clay minerals and through the pH- dependent dissociation of hydroxyl groups of aluminum and iron oxides such as clay mineral earths. Negative charges also arise during the pH-dependent dissociation of carboxy groups in the organic substance. The soil capacity of exchangeable cations is essentially dependent on clay minerals, oxides and humic substances , which are smaller than 2 mm, the soil colloids .
The cation exchange capacity is given in cmol / kg or mmol c / kg. The specification in m val / 100 g is no longer used .
The individual symbols stand for the following quantities :
- k - total number of cation species sorbed
- n i - amount of mole of cation i
- z i - valency of the cation i (example: Al 3+ has the valency 3)
- m - reference ground
The potential cation exchange capacity KAK pot indicates the maximum number of possible free cation binding sites and, according to DIN ISO 13536, relates to the cation exchange capacity at a slightly basic pH value (pH = 8.1) of the soil. International classifications (FAO - UNESCO, US Soil Taxonomy) determine the potential KAK at pH = 7 (neutral).
The effective cation exchange capacity KAK eff indicates the number of actually free cation binding sites at the current pH value of the soil.
meaning
Exchangeable cations can be shifted within the soil, relocated to neighboring ecosystems (e.g. bodies of water) or taken up by plant roots. They therefore directly affect the nutrient supply of plants and the material balance of landscapes. In addition, the exchangeable cations influence important soil properties such as the structure, the water and air balance, the soil reaction and, last but not least, the biological activity. The cation exchange capacity of a soil can therefore be viewed as a soil evaluation measure (e.g. for agricultural soil estimation maps).
See also
literature
- Ad-hoc Working Group Soil: Soil Science Mapping Instructions , Ed .: Federal Institute for Geosciences and Raw Materials in cooperation with the State Geological Services, 5th edition, 438 p .; 41 figs., 103 tabs., 31 lists, Hannover 2005. ISBN 3-510-95920-5
Web links
- nachgeschlaut.de: Cation exchange capacity
- GeoLexicon: cation exchange capacity