Cation Exchange Capacity (CEC): Meaning, Concept and Its Determination | Soil Colloids

After reading this article you will learn about:- 1. Meaning of Cation Exchange Capacity (CEC) 2. Milliequivalent Concept of Cation Exchange Capacity 3. Factors Affecting 4. Percentage 5. Determination 6. Changing Cation Exchange Capacity 7. Root CEC 8. Complementary Ion Effect.

Meaning of Cation Exchange Capacity (CEC):

Cation exchange capacity (CEC) is the amount of exchangeable cations per unit weight of dry soil. It is measured in milliequivalents (me) of cations per 100 gms of soil (recently C mol (P⁺) kg^-1 soil).

So it is the capacity of soil colloidal material in exchanging all its cations with the cations of the soil solution. It is one of the important chemical properties of soils and is usually intimately related to soil fertility and hence plant nutrition.

Milliequivalent Concept of Cation Exchange Capacity (CEC):

For the measurement of CEC, the term equivalent or more especially “Milliequivalent” is used because the number of negative charge sites in a given soil sample does not change, but the weights of the cations that may be adsorbed to those sites at one time do change because they have different weights.

The term “milliequivalent” may be defined as one milligram of hydrogen or the amount of any other ion that will combine with or displace it. The milliequivalent weight of a substance is one thousandth of its atomic weight.

Thus, if a clay has a cation exchange capacity of 1 milliequivalent (1 me/100 g); it is capable of exchanging 1 mg of hydrogen or its equivalent for every 100 g of clay. The weight of one hectare furrow slice (depth 0-15 cm) is 2.2 × 10⁶ kilograms. Therefore, one hectare furrow slice. (0-15 cm) soil is capable of exchanging 22 kilograms of hydrogen.

Calculation:

0.1 kg soil can exchange 1/10⁶ kg of hydrogen

1 kg soil can exchange 1/0.1 × 10⁶kg of hydrogen

2.2 × 10⁶ kg soil can exchange 2.2 × 10⁶/0.1 × 10⁶kg of hydrogen

= 22 kilograms of hydrogen

As for example, calcium, this element has an atomic weight of 40, compared to 1 for hydrogen. Each Ca²⁺ ion has two charges (divalent) and this is equivalent to two H⁺ ions. Therefore, the amount of calcium required to displace 1 mg of hydrogen is 40/2 or 20 mg. So 1 me of Ca²⁺ weighs 20 mg.

If 100 g of a certain clay is capable of exchanging a total of 250 mg of calcium (Ca²⁺), the cation exchange capacity (CEC) is 250/20 or 12.5 me/100 g or 12.5 C mol (P⁺) kg^-1.

Application of Milliequivalent Concepts to the Practical Field:

This concept can be used for the calculation of the amount of lime required in the filed. For instance, 1 me of hydrogen can be replaced on the colloids by 1 me of CaCO₃ (lime stone). Then calculate the amount of lime (CaCO₃) required in kg/hectare furrow slice to replace that 1 me of hydrogen.

We know, 1 me of H⁺/100 g = 1 me of CaCO₃ /100 g.

1 mg of H⁺/100 g can replace 100/2 or

50 mg of CaCO₃/100 g

In other words,

50 mg of CaCO₃/100 g will be required to replace 1 mg of hydrogen/100 g

Since 1 me of hydrogen/100 g can be expressed as 22 kilograms of hydrogen per hectare furrow slice.

So, 1 me of CaCO₃/100 g can also be expressed as 22 × 50 or 1100 kilograms of CaCO₃ per hectare furrow slice.

Factor Affecting Cation Exchange Capacity (CEC):

There are various factors that affect the ion-exchange also affect the cation exchange capacity of the soil namely texture of the soil, organic matter content, amount and kind of clay etc. As for example, soils with large amounts of clay and organic matter will have higher cation as well as anion exchange capacities as sandy soils low in organic matter.

Also, soils with predominately 2: 1 colloids will have higher exchange capacities than soils with predominately 1: 1 mineral colloids. Soil reaction or pH of the soil also affects the cation exchange capacity of the soil.

Generally with the increase in soil pH, the cation exchange capacity increases. The exchangeable aluminium and other polymeric hydroxy-aluminium also affect the cation exchange capacity of the soil by changing the soil reaction or pH.

Percentage of Cation Exchange Capacity (CEC):

The percentage of the total cation exchange capacity (CEC) satisfied with basic cations is termed percent base saturation. It is defined as the extent to which the exchange complex of a soil is saturated with exchangeable cations other than hydrogen and aluminium and it is expressed as a percentage of the total cation exchange capacity.

% BS = S/T× 100

where, BS = base saturation,

S = me of basic cations per 100 g soil

T = total exchange capacity me per 100 g soil.

All cations are included in the cation exchange capacity but in case of base saturation, all cation excluding hydrogen (H⁺) and aluminium, are considered. Because these two groups of cations contribute H⁺ ions in the soil solution. Adsorbed H⁺ contributes directly to the H⁺ ion concentration in the soil solution.

Aluminium ion (Al³⁺) also contributes H⁺ ions to the soil solution through hydrolysis as follows:

Therefore, an aluminium (Al³⁺) ion is not considered as basic cations and other cations are called exchangeable bases. The degree of base saturation is an important property of soil which usually reflects the extent of leaching and weathering of the soil. It increases at the expense of the exchange acidity (acidity develops due to adsorbed H⁺ and Al³⁺ on the colloidal surfaces).

Example:

How base saturation is calculated?

Assuming that the following ion quantities were found in the ammonium acetate extract from the leaching of 20 g of soil.

So the milliequivalents of calcium (Ca²⁺ atomic weight 40, equivalent weight 40/2 = 20) ion present in 20 g soil is:

= 0.02 × 10³/20

= 1 me.

In a similar way, the milliequivalent of other ions present in 20 g of soil is being calculated and the total amount of those basic cations is 2.5 me per 20 g of soil or 12.5 me per 100 g of soil [(2.5 × 100)/20 = 12.5 me]. Suppose the total cation exchange capacity of the soil was 15 me per 100 g. So the percentage of base saturation will be calculated as follows:

% BS = (100 × S/T)

= (100 × 12.5/15)

= 83.3

As a general rule, the degree of base saturation of normal uncultivated soils is higher for arid than for humid region soils. But this is not always true, especially in humid regions; the degree of base saturation of soils formed from limestone’s or basic igneous rocks is greater than that of soils formed from sandstones or acid igneous rocks. Base saturation is closely related to pH of soil and also to the level of soil fertility.

The ease with which cations are absorbed by plants is related to the degree of base saturation. For any given soil the availability of plant nutrients like calcium, magnesium and potassium increases with the degree of base saturation.

It is evident that soils with large amounts of organic colloids or 1: 1 type clay colloids can supply the nutrient cations to plants at a much lower degree of base saturation than soils high in 2: 1 type clay colloids.

Problem:

Different soils contain predominant clay mineral, clay humus and the value of measured CEC as follows:

Calculate the estimated value of CEC of those soils.

Solution:

Using average value from Table, assume each 1 per cent of humus (1 g of humus per 100 g soil) contributes 2 C mol (P⁺) kg^-1 of CEC [(200 me/100 g or 200 C mol (P⁺) kg^-1 × 0.01 = 2 C mol(P⁺) kg^-1].

In a similar manner, 1 per cent of montmorillonite contributes [100 C mol (P⁺) kg^-1× 0.01] = 1.0 C mol (P⁺) kg^-1 of CEC; 1 percent of kaolinite contributes (8 C mol (P⁺)kg^-1× 0.01) = 0.08 C mol (P⁺) kg^-1; and 1 per cent sesquioxides contributes about (2 C mol (P⁺) kg^-1× 0.01) = 0.02 C mol (P⁺) kg^-1 of CEC (assuming that CEC value of sesquioxides is 2.0 C mol (P⁺) (kg^-1).

Site 1 soil entisol:

With 0.5 per cent humus, it should have 2 C mol (P⁺) kg^-1 of CEC for humus. Assuming the clay is montmorillonite, the 2.6 per cent clay contributes 2.6 × 1.0 C mol (P⁺) kg^-1 = 2.6 C mol (P⁺) kg^-1. Adding the CEC contributed by clay and humus, this type of soil has an estimated value of CEC = (2.6 + 1.0) = 3.6 C mol (P⁺) kg^-1 of soil.

Site 2 soil (vertisol):

This soil has montmorillonite clay, so its 36 per cent clay contributes 36 × 1.0 C mol (P⁺) kg^-1 soil = 36 C (P⁺) kg^-1 soil of CEC and its 1.7 per cent humus contributes 1.7 × 2.0 C mol (P⁺) kg^-1 = 3.4 C mol (P⁺) kg^-1 of CEC. Therefore, the soil has an estimated value of CEC = 36 + 3.4 = 39.4 C (P⁺) kg^-1 soil.

Site 3 soil (Spodosol, organic layer):

This organic layer of spodosol with 85 per cent humus has 2 C mol (P⁺) kg^-1× 85 = 170 C mol (P⁺) kg^-1 of CEC contributed by humus. The 4.1 per cent kaolinite clay provides 0.08 C mol (P⁺) kg^-1× 4.1 = 0.3 C mol (P⁺) kg^-1. So the total estimated CEC of the soil will be summation of CEC contributed by humus and clay mineral i.e. 170 + 0.3 C mol (P⁺) kg^-1 of soil.

Site 4 soil (Oxisol):

This soil of 73 per cent clay probably has mixed kaolinite and sesquioxide (metal oxides) clays (assume half of each). It would have (73/2) × 0.08 (kaolinite) plus (73/2) × 0.02 (sesquioxides) = 3.7 C mol (P⁺) kg^-1 of CEC and CEC contributed by humus 4.7 × 2.0 C mol (P⁺) kg^-1 = 9.4 C mol (P⁺) kg^-1. So the total estimated CEC of this soil will be (3.7 + 9.4) = 13.1 C mol (P⁺) kg^-1.

When comparing estimated values to measured values, there will be considerable error in some soils. This may be due to a poor choice of “average” value the extent of humus decomposition, an incorrect estimate of the kind of clay, or the presence of an unusual colloid in that soil. It seems that extremes in percentage of clay or humus produce less accurate estimates.

However, using an estimation for more common soils that have 10^-5 per cent clay and 1-6 per cent humus can provide a fair approximation of the soils CEC. Consider that clays in humid climates are mostly kaolinite and hydrous mica, and soils in climates with less than 600 mm (24 inch) annual precipitation or rainfall are montmorillonite plus hydrous mica or vermiculite.

Determination of Cation Exchange Capacity (CEC) and Effective CEC of Soils:

Suppose that 0.054 g of NH₄⁺ were found is 20 g of soil extracted as above (0.054 g is 3 meq i.e. 0.054/0.018 = 3 as 0.018 g is the milliequivalent weight of 1 me of NH₄⁺). Because 3 me were present in 20 g of soil, the CEC of the soil is 3 × 15 me/100 g soil or 15 C mol (P⁺) kg^-1 soil.

Effective CEC:

The cation exchange capacity can better be estimated by extraction with an un-buffered salt which would give a measure of the CEC at the soils normal pH.

Use of neutral N ammonium acetate (NH₄OAc) will result in a high CEC value of the soil is acid simply because of the adsorption of NH₄⁺ ions to the so-called pH dependent exchange sites where intensity of negative charge increases with increasing pH caused by the ionization of OH groups at the edges of the clay lattice and on the hydrous Al and Fe oxides and from the carboxyl (—COOH) and phenolic (—C₆H₄OH) groups present in soil organic matter.

The sum of the milliequivalents of calcium, magnesium potassium and sodium, plus aluminium and hydrogen is the effective CEC.

Changing Cation Exchange Capacity:

The cation exchange capacity of a soil changes with a change in pH (acidity or basicity). Although most of the negatively charged exchange sites are from the isomorphous substitutions and are a permanent charge, sesquioxides (metal oxides) and kaolinite clay have only a few lattice sites with isomorphous substitution.

The various hydroxyls of clays, humus and organic acids do ionize H⁺ into the soil solution, thereby producing negatively charged cation exchange sites on these soil particles (Fig. 9.13).

In acid solutions (high H⁺ concentration); fewer H⁺ ionize off the —O^–. In basic solutions, fewer H⁺ are in solution to adsorb to the —O^– and more ionize off the –O^–, leaving more cation exchange sites. In stronger acid solutions fewer hydroxyls (—OH) have the H⁺ ionized, thereby leaving fewer pH-dependent CEC sites ionized. A relationship between soil pH and negative charge exchange sites is given in Fig. 9.14.

Root CEC:

Soil colloidal materials are not the only component of the soil-plant system to exhibit cation exchange properties. It is evident that plant roots may also possess these cation exchange properties. The cation exchange capacity of roots of different plant species ranges from less than 10 to almost 100 me/100 g.

The exchange properties of roots appear to be attributable mainly to carboxyl groups (—COOH) present in pectic substances. Such sites are considered about 70-90% of the exchange properties of roots. Root CEC varies with the plant species.

Legumes and other dicotyledons generally have values at least double the CEC of monocotyledons, including grasses. Legumes and other plant with high CEC values tend to absorb divalent cations such as calcium (Ca²⁺) preferentially over monovalent cations, whereas the reverse occurs with grasses where more absorption of monovalent cations like potassium (K⁺) takes place.

Complementary Ion Effect:

Complementary ion effect is defined as the influence of one adsorbed ion on the release of another from the surface of a colloid.

For example:

The complementary ion effect can be well explained where ammonium (NH4) in the soil solution is exchanging with calcium (Ca²⁺) on soil colloids. This exchange will take place more readily when the complementary cation on the exchange complex is aluminium rather than sodium. Ammonium will replace much more sodium that it willsaluminium.

By ammonium becoming involved in exchange for sodium, there is less of it available for replacement of calcium. The more strongly held trivalent aluminium tends to satisfy a greater part of the cation exchange capacity and permits the exchange of ammonium for calcium to proceed more completely.