Exchange capacity
The exchange capacity (AK) for ions - also sorption capacity - is, in addition to the pH value and the redox properties, an important chemical soil property in relation to the soil pH , the soil structure , soil development , soil fertility and plant nutrition .
Definitions
Ions are reversibly deposited (adsorbed) on the surfaces of the organic and mineral soil particles , mainly in the size class <2 µm. Therefore, they can be exchanged (desorbed). The exchange capacity of a soil describes the sum of the exchangeable ions in mol or mmol c / kg substance (the latter is not SI-compliant).
The exchange capacity depends on the pH value of the soil in relation to the pH value of the exchange solution used and the defined time constant - ions that cannot be exchanged for a short time are not taken into account. Usually the maximum ( potential ) AK with the opposite pH value of the exchange solution and soil pH is measured - and not the current ( effective ) AK with equivalent pH values.
Since the ion exchange in the soil takes place with both positively charged cations and negatively charged anions , a distinction is made between the following properties:
- Cation exchange capacity (KAK) mainly ions: Ca 2+ , Mg 2+ , K + , Na + , Al 3+ , H +
- Anion exchange capacity (AAC) in the acidic range: HPO 4 2− , SiO 3 2− , SO 4 2− , NO 3 - , Cl -
Effect of cations and anions
The exchangers in the soil colloids are mainly negatively charged. Therefore, the cation exchange capacity in the basic to moderately acidic pH range of the soil is the predominantly effective variable. The "used" cations are replaced by liming and fertilization. However, if there is long-term soil depletion with accompanying soil acidification , the cation exchangers are chemically changed - the KAK decreases.
The anion exchange is of greater importance in the acidic range; in soils that are chemically strongly weathered (e.g. podzols ) or in purely organic soils (e.g. bog soils ), the excess anions are responsible for a higher availability of Fe, Al, Mn.
See also
literature
- Arnold Finck : Plant nutrition in brief . 3rd revised edition. Hirt, Kiel 1976, ISBN 3-554-80197-6 , ( Hirt's key word books ), pp. 65–74.
