1. Exchange capacity:
   
   1. Permanent charges
      
   2. Variable charges
      
      1. Al-oxides
         
      2. Humus
         
2. Adsorption of cations and anions
   
   1. Inner-sphere complex and outer-sphere complex
      
3. Cation exchange capacity (CEC)
   
   1. Concepts
      
   2. Methods
      
   3. Comparison of CEC and ECEC
      
      1. Organic soils
         
      2. Variable-charge Ferralsols
         
      3. Permanently charged Vertisols
         
   4. Ambiguity: which method to use
      

Fig.1 Isomorphic substitution

( Source: Brady and Weil 2002, p. 323.)

• The electric charge on clay particles results from isomorphic substitution in the crystal lattices of clay minerals. One atom or element fills a position usually filled by another of similar size without alteration or modification of the lattice structure.
  
• It takes place at the formation of minerals from lava flux (not afterwards during weathering) due to deficiency or surplus of individual elements in the lava flux.
  
• The molecules fit by magnitude (size), but posses a different charge.
  
• However, as the number of O- or OH-ions in the tetrahedron or octahedron is fixed, negative surplus charges are generated. Hence neutralizing (counter-balancing) cations are inserted. They determine the cation-exchange capacity ( CEC) and the nutrient availability of soils.
  
• Note: The charge deficit is permanent in the sense that it is a structural feature of the clay particle and persists irrespective of changing conditions such as soil pH and soil solution composition.
  

Fig.2 Clays

( Source: Brady and Weil 2002, p. 328.)

Most of the permanent charge in soils resides:

• on surfaces of layer-silicates mainly the 2:1 layer silicates (illite, vermiculite, smectite)
  
• the 2:2 clays (chlorite)
  
• the 1:1 clays (kaolinite) -> contain only a small amount of permanent charge on their planar surfaces
  
• Note: surface varies from 10 m²/g for Kaolinite to 800 m²/g for montmorillonite.
  

Fig.3 Major Properties of Selected Soil Colloids

( Source: Brady and Weil 2002.)

• Variable charges or pH-dependent charges depend on the pH of the solution. They result mainly through protonation (release of protons) or deprotonation of H⁺ from hydroxyl-groups (OH) at the edges or surface of inorganic and organic colloids:
  
  1. the edges of 1:1 layer silicate clays (kaolinite)
     
  2. on very poorly ordered aluminosilicates (allophane and imogolite)
     
  3. on crystalline hydroxides of Al (gibbsite) and Fe (goethite, hematite)
     
  4. OH combined with C of humus (SOM)
     
• Thus, depending on the pH of the solution, the same location produces both negative and positive net charges.
  

1.2.1. Al-oxides

• Two examples of negative and positive charges of Al-colloids are illustrated below.
  

  Fig.4 Negative charges of Al-oxides

  ( Source: Brady and Weil 2002.)

• When pH raises, increase of hydroxylions (OH-) in the soil solution. H⁺ ions dissociate and negative surface charges result, leading to CEC (see fig.4).
  

Fig.5 Positive charges of Al-oxide

( Source: Brady and Weil 2002.)

• When pH declines, H⁺ ions in the soil solution accumulate and protonation of H⁺ at OH^- ions take place. This results in positive surface charges and thus the  AEC.
  
• Note: Under normal to strongly acid conditions: Fe/Al-Oxides and clay minerals have positive surface charges.
  

1.2.2. Humus

Fig.6 Humus

( Source: Brady and Weil 2002.)

• Humus has a small permanent negative charge and a large pH-dependent charge.
  
• Humus contains complex compositions with many active groups.
  
• The functional groups of humic materials contain carboxyl – and phenolic groups which become dissociated at high pH.
  
• A simplified schema depicts 3 main types with OH-groups which are responsible for the high sorption capacity.
  
• Similar to the sesquioxides and clay minerals negative and positive charges develop based on loss of H⁺ of the different groups.
  

Fig.7 Dissociation of functional groups of humic substances at high pH

( Source: Juma, 2002.)

• Example: dissociation of carboxyl-groups at high pH (see fig.7).
  
• Note: normally, the negative surface is higher than positive one.
  

Fig.8 Humus components

( Source: Brady and Weil 2002.)

Fig.9 Positive and negative charges of various soil colloids

( Source: Brady and Weil 2002.)

Fig.10 CEC

( Source: Brady and Weil 2002.)

• Figure 10 shows how both anions and cations are adsorbed at the same colloid.
  
• Normally soil colloids have negative surface charges and are enclosed by layer of cations.
  
• Concentration is highest at the stern layer at the surface of the clay minerals and lowest in the bulk solution.
  
• Adsorption occurs via 2 possibilities: Inner-sphere-complex and Outer-sphere-complex.
  
• Inner-sphere-complex: For example H₂PO₄^-: binds directly at negatively charged surface and cannot easily be replaced.
  
• Outer-sphere-complex: e.g. Ca²⁺: water molecules build bridges between adsorbed cations and negatively charged surface.
  
• Results in weak binding through electrostatic attraction and these ions can easily be exchanged or replaced.
  
• Exchangeable cations are cations of the outer-sphere complexes. They can be replaced by other cations.
  
• This happens always charge-for-charge basis, e.g. H⁺ and Na⁺

  Fig.11 Exchange of Na

  ( Source: Brady and Weil 2002.)

  but 2 H⁺ and one Ca²⁺.

  Fig.12 Exchange of Ca

  ( Source: Brady and Weil 2002.)

  
  
• This complex is named cation/anion exchange complex.
  

Fig.13 Ionic radius of exchangeable cations

( Source: Juma 2002, p. 194.)

• The adsorption is based on number of charges and the hydrated radius.
  
• The sequence of preferential adsorption is Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH⁴⁺ > Na⁺.
  
• Rule: the higher the charge and the smaller the hydrated radius the stronger the attractive force between the cation and the surface (are less easily removed).
  

Fig.14 Anion exchange capacity (AEC)

( Source: Juma, 2002.)

• Same principle as for ( CEC) .
  
• Positive charges of Fe/Al-Oxides and SOM attract negative ions like NO₃^- and sulfate (SO₄^3-).
  
• Only variable-charge soils ever develop significant  AEC.
  
• In tropics: When kaolinite and Fe/Al dominate the  AEC can be higher than the ( CEC) .
  
• Important: impedes leaching of NO₃^- from the subsoil.
  

Fig.15 Determination of cation exchange capacity (CEC)

( Source: Brady and Weil 2002.)

• ( CEC) is an important soil chemical property. It is used to: Classify soils and assess their fertility.
  
• The measurements of ( CEC) are rather empirical, and several different analytical methods are used that yield different results. This depends on:
  1. The pH value at which the ( CEC) determination is made. The ( CEC) increases with increasing pH (pH dependent variable charge).
  2. The species of ions used in the exchanging or displacing solutions, e.g. ion species differ in ease of displacement, and certain species (K especially) may be trapped or fixed by some clay mineral species.
  
• Standard method: In general, a concentrated solution of a particular exchanger cation (NH₄⁺, Ba²⁺) is used to leach the soil sample.
  
• ( CEC) can be determined by:
  1. Measuring the number of exchanger cations adsorbed (see Fig.15 c +d).
  2. The amounts of each of the elements originally held on the exchange complex (Ca, Mg, Al, K, Na -> Fig.15 a+b).
  

3 types of CEC determinations are commonly employed

1. NH₄-saturation displacement method (CEC₇) conducted at pH 7.

2. Sum of cations method (CEC_8.2) performed at pH 8.2 in which the acid-generating H + Al (exchange acidity) are added to the basic cations. This method is buffered against fluctuation and yields CEC values higher than those from the NH₄ saturation method due to the fact that CEC values increase with higher pH in  OM, 1:1 clay minerals, and amorphous or short-range order minerals.

3. In the ECEC method the pH of the soil is not controlled and the basic cations displaced by NH₄ are added to the KCl-extractable Al.

• 2 common methods are presented here: CEC buffered at pH 7 and unbuffered method or effective cation exchange capacity (ECEC)
  

1. Buffered CEC:

• Mostly with NH₄ at pH 7 (sometimes with Ba at pH 8.2).
  
• If the native soil pH is less than the pH of the buffered solution, the method measures not only the cation exchange sites active at the pH of the particular soil but also any pH-dependent exchange sites that would become negatively charged at pH 7.0.
  
• This method measures potential or maximum CEC.
  
• This method gives reliable estimates for temperate soils with predominantly permanent-charge clays.
  
• However, its reliability in many tropical soils is being increasingly questioned as the artificially high pH of the exchange solutions results in an overestimate of the variable-charge CEC.
  

2. Unbuffered method or effective CEC:

• The exchange takes place at the actual pH of the soil.
  
• Thus, measures how many cations the particular soil can hold at the natural soil pH.
  
• The net negative charge of the exchange complex is neutralized by exchangeable bases (Na, K, Ca, Mg) plus exchangeable acidity (Al³⁺ , H⁺). The ECEC, i.e. the sum of bases and exchangeable acidity is thought to represent the soil’s cation exchange capacity at field conditions.
  
• Important: Method used is be known. Both methods yield significantly different values of CEC especially if the soil pH is much below the buffer pH used.
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Fig.16 CEC of various soil materials

( Source: Buol et al., 2003.)

The following 3 examples explain the impact of the pH value at which the CEC determination is made for

1. organic soils (histic epipedon),
   
2. variable-charged ferralsols and
   
3. permanently charged Vertisols.
   

a) Histic epipedon (according to Soil Taxonomy, USA) has > 16 % OM

• CEC of OM is very pH-dependent:
  When the sample pH is at 3.7, the CEC increases greatly from ECEC value to CEC₇.
  When the sample pH is at 6.4, there is no significant difference between the ECEC value and CEC₇ value.
  

b) Oxisol (Soil Taxonomy, USA) corresponds to Ferralsol (WRB classification)

• Predominant clay mineral in this soil is kaolinit. It is greatly affected by the pH of the method (variable-charge soils).
  At pH 5 + 5.7 = ECEC < CEC₇ < CEC_8.2
  

c) Vertisol

• Predominant clay in this soil is Montmorillonit. It is only slightly affected by the pH of the method (permanent-charge soils).
  ECEC = CEC₇ = CEC_8.2
  

Fig.17 Charge Characteristics of Colloids

( Source: Brady and Weil 2002.)

• The example in Fig.17 explains how to calculate the ( CEC) of a soil when the amount of the various colloids is known: 5 % SOM, 20 % Montmorillonite (Smectite) and pH = 7.
  
• Exchange capacity: = [SOM (%) x total charge] + [clay (%) x total charge] = [(5/100) kg x 200 cmol_c/kg] + [(20/100) kg x 100 cmol_c/kg] = 10 + 20 = 30 cmol_c/kg
  

1. Which calculation is correct or the right one? -Depends on the purpose:
   
2. Potential CEC:
   
   • Used for soil classification. Soil classification tries to utilize criteria that are difficult to alter by common soil management practices such as lime application.
     
   • Same conditions for all soils.
     
   • Shows degree of acidification (e.g. CEC of 75 %: 25 % is occupied by H and Al)
     
3. ECEC:
   
   • More relevant for soil fertility surveys and nutrient availability assays.
     
   • Indicates, what fraction of exchangeable cations at the natural pH is occupied by basic cations.
     
   • Soil fertility specialists prefer ECEC measurements because they reflect conditions at the soil pH and can be manipulated by adding lime to the soil.