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Berlin Environmental Atlas

01.06 Soil-Scientific Characteristic Values (Edition 2002)

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01.06.9 Mean Effective Cation Exchange Capacity


Description

The effective cation exchange capacity (KAKeff) represents the quantity of cations bound to soil colloids, taking into consideration the strongly pH-value-dependent charge of the organic substances. The interchangeable cations are bound to clay minerals and humus colloids. In neutral to weakly acidic soils, calcium (Ca), magnesium (mg), potassium (K) and sodium (Na) dominate the sorption complex; in acidic soils, e.g. pine and heath locations, aluminum (Al), hydrogen (H) and iron (Fe) predominate. The binding capacity of organic substances is considerably higher than that of the clay minerals. The strength of the bond with the organic substance is pH-dependent, while the bond with clay minerals is independent of the pH value. Thus, the binding capacity of the humus drops with the pH value. Clay and humus-rich soils with neutral soil reaction can therefore bind considerably more nutrients and pollutants, and prevent a washout of these substances into the groundwater, than can sandy, humus-poor locations. Effective cation exchange capacity is therefore useful for describing the nutrient and pollutant binding potentials of soils.

Methodology

In calculating the KAKeff, a simple method was used which was on the one hand to show the characteristic value of the respective soil association, while on the other also taking into account the strong deviations from the typical value within the respective soil association.
The KAKeff of the soil associations is derived from the main soil type of the topsoils and subsoils, as in Tab. 1. The topsoil is assumed to have a depth of 0 - 1 dm; the subsoil 3 - 15 dm. In order to ascertain unusual features of a soil association, the subsidiary soil type with the greatest difference in clay content to the main soil type is also taken into account. This permits deviations from the main soil type to be considered; however, this method lends very great weight to the subsidiary soil type, so that the exchange capacities of the soil associations are in some cases estimated too high or too low. The exchange capacity of the humus, corrected by a pH-dependent factor, is added to the averaged cation exchange capacity of the main and subsidiary soil types. Since both the humus contents and the thickness of the humus layer may differ, depending on soil genesis and use, and since these are also incorporated into the calculation of the KAK, different use-specific values are ascertained for each soil association.

Table 1: Average KAK Values of the Soil Types
Type of soil KAKeff
[cmol/kg]
fS 2
G 2
gS 2
Ls2 13
Ls3 12
Ls4 12
Lt2 17
Lt3 22
Lts 19
Lu 15
mS 2
Sl2 4
 
Type of soil KAKeff
[cmol/kg]
Sl3 6
Sl4 9
Slu 9
St2 6
St3 11
Su2 2
Su3 4
Su4 4
Tl 29
Ts2 28
Ts3 20
   
 
Type of soil KAKeff
[cmol/kg]
Ts4 15
Tt 39
Tu2 29
Tu3 21
Tu4 18
Uls 9
Us 5
Ut2 9
Ut3 11
Ut4 14
Uu 6
   
Table 1: Average KAK Values of the Soil Types
(Soil-Scientific Mapping Directive, 1994)

Table 2: pH factors for the Determination of the Effective KAK of the Humus Fraction
pH value
(CaCl2)
pH factor
<3.5 0.15
3.5 - <4.5 0.25
4.5 - <5.5 0.4
5.5 - <6.5 0.6
6.5 - <7.5 0.8
≥7.5 1.0
Table 2: pH factors for the Determination of the Effective KAK of the Humus Fraction (Soil-Scientific Mapping Directive, 1994)

Table 3: Relationship between Humus Content and Potential KAK
Humus content
[Masse-%]
KAKpot
[cmolc/kg]
0 - <1 0
1 - <2 3
2 - <4 7
4 - <8 15
8 - <15 25
15 - <30 50
30 - 100 110
Table 3: Relationship between Humus Content and Potential KAK
(Soil-Scientific Mapping Directive 1994), supplemented by Z3 peat

The values ascertained have been assigned to the stages 1 -5, from very slight - very great, according to the Soil-Scientific Mapping Directive (1994).

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