Soil Absorption Capacity and Cation Exchange Capacity (CEC)

1.1 Mechanical Sorption

This is the ability of soil to retain particles whose size exceeds the diameter of soil pores. Mechanical sorption is essentially a filtration process.

Essence: Solid particles suspended in the soil solution (e.g., silt particles, colloids, organic suspensions) cannot pass through the narrow pores of the soil and are retained within it.

Significance: Mechanical sorption plays an important role in purifying soil water from suspended matter, in soil silting, and in the formation of soil horizons. However, it has practically no effect on the retention of dissolved ions, which is our main topic.

1.2 Physical (Molecular) Sorption

This is the ability of soil to change the concentration of dissolved substances at the phase interface (solid particle — solution) due to surface energy (Eash et al., 2016).

Essence: Molecules of dissolved substances can concentrate on the surface of solid particles due to intermolecular forces (van der Waals forces, hydrogen bonds). This changes the concentration of the solution in the near-surface layer, but its composition may not change. This process is sometimes referred to as molecular adsorption.

Significance: Physical sorption is particularly important for the retention of organic molecules, pesticides, and some nonpolar compounds. However, it is reversible and is not the primary mechanism for retaining mineral ions in soil.

1.3 Chemical Sorption (Chemisorption)

This involves the formation of sparingly soluble or poorly dissociated compounds as a result of chemical reactions between soil components and substances in the soil solution (Eash et al., 2016; Essington, 2015).

Essence: Ions from the solution undergo chemical reactions with mineral or organic components of the soil, forming new chemical compounds. For example, phosphate ions can react with calcium ions to form insoluble calcium phosphate:

Most important feature: Chemical sorption is an irreversible process. The resulting compounds cannot be easily returned to the solution by simply changing the concentration. This is one of the mechanisms of phosphorus "fixation" in soil.

Significance: Chemical sorption plays a key role in the fixation of phosphates, sulfates, carbonates, and some heavy metals in soils. It is also involved in mineral weathering processes.

1.4 Physicochemical (Ion-Exchange) Sorption

This is the most important type of sorption capacity for our course. It determines the soil's ability to retain cations (positively charged ions) on its surface and exchange them.

Essence: On the surface of soil colloids (clay minerals, organic matter), there exist electrical charges. These charges attract ions of the opposite sign from the solution, which are held on the surface by electrostatic forces. The retained ions can be exchanged for other ions from the solution, which is why the process is called ion-exchange sorption (Eash et al., 2016; Scheffer et al., 2018).

Most important feature: The process is reversible. If another cation is added to the solution, it can displace the previously retained cation from the surface.

Significance: This mechanism determines:

• The availability of nutrients to plants (calcium, magnesium, potassium, ammonium);
• The soil's ability to retain toxic cations (heavy metals, radionuclides);
• The overall sorption capacity of the soil.

This type of sorption capacity will be the focus of our attention.

1.5 Biological Sorption

This is the selective accumulation of elements by living organisms (microorganisms, plant roots).

Essence: Plant roots and microorganisms selectively absorb from the soil solution the elements they need for building tissues.

Characteristic: After organisms die and their residues mineralize, the absorbed elements return to the soil solution. Biological sorption is temporary.

Significance: The biological factor plays a huge role in the circulation of substances in nature (protein cycle), however, in the context of our lecture, we consider abiotic sorption mechanisms.

To summarize:

Of all five types of sorption capacity, physicochemical (ion-exchange) sorption is of greatest interest to us because it provides:

• Retention of nutrients (calcium, magnesium, potassium, ammonium);
• Detoxification of toxic substances (heavy metals, radionuclides);
• Soil buffering (the ability to resist pH changes).

In the following sections, we will examine the mechanisms of ion exchange, cation exchange capacity, and the factors influencing these processes.

2.1 What is Ion Exchange?

Ion exchange is a reversible process of stoichiometric (equivalent) substitution of ions located in the electrical double layer on the surface of the solid phase by ions of the same charge from the soil solution (Eash et al., 2016; Essington, 2015). Simply put, it is an exchange of "places" between ions attracted to the surface of a colloidal particle and ions freely floating in the soil moisture.

In the vast majority of soils, negative charges predominate on the surface of colloids (clay minerals, organic matter). Therefore, the main process is cation exchange — the exchange of positively charged ions (cations). However, as we will see later, anion exchange capacity can also be significant in acidic and highly weathered soils.

2.2 Mechanism of Cation Exchange

Imagine a colloidal particle (e.g., a flake of montmorillonite or a fragment of humus). Its surface carries a negative charge, which arises from isomorphous substitution in the crystal lattice (permanent charge) or dissociation of functional groups (variable charge, pH-dependent). This negative charge attracts positively charged ions — cations — from the solution (e.g., Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, H⁺, Al³⁺).

The attracted cations are not chemically bonded to the surface but are held by electrostatic forces. Around each colloidal particle, a so-called electrical double layer forms (Scheffer et al., 2018; White, 2006):

• The inner layer is the directly charged surface of the colloid.
• The outer layer is a "swarm" of counter-ions (ions of opposite charge) that are attracted to the surface but are in constant thermal motion and can exchange with ions from the solution.

The outer layer, in turn, is subdivided into a denser Stern layer (ions directly adjacent to the surface) and a diffuse layer (ions at a greater distance from the surface). In the Stern layer, some cations may lose their hydration shell and form inner-sphere complexes (directly bonded to surface oxygen atoms), while in the diffuse layer, ions retain their hydration shell and are held more weakly — these are outer-sphere complexes (Essington, 2015; Sparks, 2003).

When another cation enters the soil solution (for example, we apply potassium fertilizer), it can approach the colloidal particle, displace one of the previously adsorbed cations from the surface, and take its place. This process is cation exchange.

2.3 Stoichiometry and Reversibility

The most important rule of cation exchange is: exchange occurs equivalently in terms of charge. This means that one divalent cation (Ca²⁺) can replace two monovalent cations (e.g., two K⁺ or two Na⁺). Conversely, to displace one Ca²⁺, two monovalent cations are required.

Example reaction:

The double arrow (\rightleftharpoons) indicates that the reaction is reversible. If a large amount of Ca²⁺ is added to the solution, the equilibrium shifts to the left, and ammonium is displaced back into the solution (Eash et al., 2016; Essington, 2015).

2.4 Exchange Selectivity: The Lyotropic Series

Not all cations are held on the colloid surface with equal strength. Some ions are displaced more easily, others with more difficulty. The strength of retention is determined by two main factors:

1. Ion charge (valency). The higher the charge, the stronger the ion is attracted to the negatively charged surface. Therefore, trivalent ions (Al³⁺, Fe³⁺) are held more firmly than divalent ions (Ca²⁺, Mg²⁺), which in turn are held more firmly than monovalent ions (Na⁺, K⁺).

2. Hydrated radius. In the soil solution, ions are always surrounded by a shell of water molecules. The larger the hydration shell, the farther the ion's charge center is from the colloid surface, and the weaker the electrostatic attraction (Coulomb's law). For example, Li⁺ has a small ionic radius but is very strongly hydrated, so its hydrated radius is larger than that of K⁺ or Cs⁺. Consequently, K⁺ is held more strongly than Li⁺. Also important is the ability of some cations (K⁺, NH₄⁺, Cs⁺) to lose their hydration shell and penetrate the interlayer spaces of clay minerals (inner-sphere adsorption), making their retention particularly strong (fixation of potassium and ammonium).

Based on these patterns, the so-called lyotropic series (or selectivity series) for cations has been established (Weil & Brady, 2017; White, 2006):

This series means that an ion located further to the left will more readily displace an ion to its right. For example, calcium will displace potassium and sodium but will not displace aluminum. This is why, in acidic soils where there is much exchangeable Al³⁺, nutrient cations (Ca²⁺, Mg²⁺, K⁺) are often deficient — they are displaced by aluminum.

2.5 Significance of Cation Exchange for Soil Processes

Cation exchange is not merely a physicochemical curiosity. It determines fundamental soil properties:

• Plant nutrition. Most of the cations required by plants (Ca²⁺, Mg²⁺, K⁺, NH₄⁺) are held on exchange sites and are accessible to roots precisely because of cation exchange. Root hairs release H⁺, which displaces the desired cation from the colloid, and it enters the solution, from which it is taken up by the plant. (We do not delve into physiology, but the principle is clear.)

• Protection against leaching. Without exchange capacity, all cations would be rapidly leached from the soil by atmospheric precipitation. Colloids retain them in the root zone, creating an "exchangeable pool" of nutrients.

• Buffering. Soil with a high exchange capacity can resist sharp changes in cation concentration in the solution, i.e., it acts as a buffer medium. For example, the application of potassium fertilizer does not cause a sharp increase in K⁺ concentration in the solution because part of the potassium is sorbed on exchange sites, and the solution remains in equilibrium with the exchange complex.

• Detoxification of pollutants. Heavy metals (lead, cadmium, copper) also participate in cation exchange and can become fixed on colloids, reducing their toxicity and migration into groundwater.

2.6 Cation Exchange and pH

It is important to understand that the exchange capacity of soil is strongly dependent on pH (Scheffer et al., 2018; Weil & Brady, 2017). At low pH (acidic conditions), there is a high concentration of H⁺ in the solution, which competes with other cations for exchange sites. Moreover, at low pH, variable charges (on organic matter and oxides) become less negative or may even acquire a positive charge, reducing the cation exchange capacity. At high pH (alkaline conditions), these groups dissociate, creating more negative charges, and the exchange capacity increases. This effect is particularly noticeable in soils with high organic matter content and iron and aluminum hydroxides.

In the next section, we will quantitatively assess this soil capacity through the concept of cation exchange capacity (CEC).

3.1 Definition and Physical Meaning

Cation exchange capacity (CEC) is the total amount of negative charges on the surface of soil colloids that can be occupied (neutralized) by exchangeable cations under certain conditions (pH, ionic strength, temperature) (Essington, 2015; Weil & Brady, 2017). In other words, CEC is the maximum amount of cations (in charge units) that a soil can retain in exchangeable form.

Important: CEC is not the number of cations but the number of charges. Since exchange proceeds equivalently in terms of charge (see Section 2), we measure the number of negative charges available for exchange, expressed in units of charge per unit mass of soil.

3.2 Units of Measurement

In modern soil science and international practice (including the Soil Taxonomy classification), CEC is expressed in centimoles of charge per kilogram of soil — cmol_c/kg (symbol: cmol_c/kg). This unit is equivalent to the older, but still occasionally encountered, unit — milliequivalents per 100 g of soil (meq/100 g).

Why centi- (1/100 mole)? Because typical CEC values for mineral soils range from 2 to 50 cmol_c/kg, and such numbers are convenient to work with. For comparison: 1 cmol_c/kg means that 1 kg of soil can retain 0.01 mole of monovalent cations (e.g., 0.01 mole of K⁺, corresponding to 0.39 g of potassium).

Other units may occasionally be encountered, such as mol_c/kg — but this is too large a unit for soils (values would be less than 1), or meq/g — too small. Therefore, cmol_c/kg is the standard.

3.3 How is CEC Measured? (Principle of the Method)

Without going into the details of laboratory protocols (of which there are many), we can outline the general principle of CEC determination (Eash et al., 2016; White, 2006; Weil & Brady, 2017):

1. Saturation of the exchange complex with an "indicator" cation. The soil sample is treated with a concentrated salt solution containing a cation that is not present in the soil in large amounts (usually NH₄⁺ or Ba²⁺). Due to the mass action effect (high concentration), this cation displaces all the "native" exchangeable cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, Al³⁺, H⁺) and itself occupies all exchange sites. The soil is now saturated with the indicator cation.

2. Removal of excess indicator cation. The soil is washed with distilled water or an organic solvent (e.g., ethanol) to remove the indicator cation that is not in exchangeable form but simply in the pore solution.

3. Displacement of the indicator cation. The soil is then treated with a solution of another salt containing a cation with a higher exchange capacity (e.g., K⁺ or Ca²⁺), which displaces the indicator cation from the exchange sites.

4. Determination of the amount of displaced indicator cation. The concentration of the indicator cation in the resulting solution is measured (e.g., NH₄⁺ colorimetrically or Ba²⁺ by atomic absorption spectrometry). The amount of displaced indicator cation (in moles of charge) is the CEC of the given soil sample.

3.4 Effective and Potential CEC

It is necessary here to make an important distinction that is often overlooked.

• Potential CEC (CEC_pot) — is the CEC measured at a standardized pH, usually at pH 7 or 8.2 (using ammonium acetate or barium acetate with buffer). At this pH, virtually all pH-dependent charges (on organic matter, oxides, edge surfaces) are dissociated (negative). Therefore, potential CEC reflects the maximum possible exchange capacity of the soil, which can be realized under neutral or slightly alkaline conditions. This value is used in soil classification (e.g., to differentiate Alfisols and Ultisols — Weil & Brady, 2017).

• Effective CEC (ECEC) — is the CEC measured at the actual pH of the soil, without buffering, typically using a BaCl₂ or NH₄Cl solution in a neutral medium, but without pH adjustment. In this case, only the portion of negative charges that is actually dissociated at the existing soil pH is measured. For acidic soils, ECEC is always lower than potential CEC because some of the pH-dependent charges are protonated and do not carry a negative charge. ECEC better reflects the actual exchange capacity of the soil under field conditions (Scheffer et al., 2018; White, 2006).

For example, for an acidic forest soil with pH 4.5, the potential CEC (at pH 7) might be 15 cmol_c/kg, while the ECEC (at actual pH) is only 5 cmol_c/kg. The difference is due to charges that are "turned on" only when the pH increases.

3.5 Typical CEC Values for Different Soils and Soil Components

Soil CEC is determined by its composition: the amount and type of colloids. The following table provides approximate values (based on Weil & Brady, 2017; Essington, 2015; Eash et al., 2016).

Table 1. CEC of various soil components (at pH 7, cmol_c/kg)

Table 2. Typical CEC ranges for soils of different taxonomic groups (according to Weil & Brady, 2017)

From these tables, it is evident that humus has the highest CEC per unit mass (200–500 cmol_c/kg), far exceeding even the most active clay minerals. Therefore, even a small amount of organic matter (e.g., 2–3%) can make a substantial contribution to the total soil CEC, especially in sandy soils where the clay fraction is small.

Among clay minerals, vermiculite and smectite have very high CEC (up to 150 cmol_c/kg), due to numerous isomorphous substitutions and the accessibility of interlayer spaces for cations. Kaolinite has low CEC because its permanent charge is virtually absent, and variable charge appears only on edge surfaces.

To summarize:

• CEC is a quantitative measure of the soil's ability to retain exchangeable cations.
• It is expressed in cmol_c/kg.
• A distinction is made between potential CEC (at pH 7–8) and effective CEC (at the actual soil pH).
• CEC varies greatly depending on the content and type of clay minerals and organic matter.
• High CEC is indicative of good buffering capacity and high potential fertility.

In the next section, we will analyze what specifically CEC depends on and which factors determine its magnitude in real soils.

4.1 Three Main Sources of Negative Charge

To understand what CEC depends on, we must first recall where negative charges on the colloid surface originate. There are three sources (Essington, 2015; Scheffer et al., 2018; Weil & Brady, 2017):

1. Permanent (structural) charge — arises from isomorphous substitution in the crystal lattice of clay minerals. This charge is independent of pH and ionic strength. It is inherent in 2:1 minerals (smectite, vermiculite, illite, chlorite) and to a lesser extent in 1:1 minerals (kaolinite).

2. Variable (pH-dependent) charge — arises on surface hydroxyl groups (≡S–OH) and organic functional groups (–COOH, –OH). This charge appears upon proton dissociation (loss of H⁺) in alkaline conditions or disappears (protonates) in acidic conditions. It is characteristic of organic matter, iron and aluminum oxides, and the edge surfaces of clay minerals.

3. Charge associated with adsorption of polyvalent cations (e.g., aluminum hydroxides) — in some soils, this mechanism may contribute additionally, but it is usually of secondary importance.

Thus, CEC is the sum of the permanent and variable charges available for cation exchange under given conditions. Consequently, any factors affecting the magnitude or availability of these charges will determine CEC.

4.2 Mineralogical Composition — The Basis of Permanent Charge

Clay minerals are the main carriers of permanent negative charges in the soil. Their contribution to CEC depends on:

1) The amount of clay fraction. The higher the content of physical clay (particles <0.002 mm), the higher the potential CEC (other things being equal). However, this relationship is not linear, as different clay minerals have different specific CEC values.

2) The type of clay minerals. This factor is decisive (Eash et al., 2016; Weil & Brady, 2017):

• Smectites (montmorillonite) — have very high CEC (80–120 cmol_c/kg) due to numerous isomorphous substitutions in octahedral and tetrahedral sheets and the accessibility of interlayer space for hydrated cations.
• Vermiculites — have even higher CEC (100–150 cmol_c/kg), but their interlayer space is not accessible to all cations: K⁺ and NH₄⁺ are fixed there very firmly (almost irreversibly).
• Illite (hydromica) — has CEC of 10–40 cmol_c/kg, because most of its structural charge is neutralized by non-exchangeable K⁺ in interlayer spaces.
• Kaolinite — CEC only 3–15 cmol_c/kg, as it has virtually no permanent charge, and the main charge comes from edge hydroxyl groups (pH-dependent charge).
• Chlorite — CEC of 10–40 cmol_c/kg, but its exchange capacity is reduced due to the presence of hydroxide interlayers blocking some of the charges.

Thus, soils enriched with smectite and vermiculite (e.g., Vertisols) have high CEC, while soils composed mainly of kaolinite and oxides (e.g., Oxisols, Ultisols) have low CEC.

4.3 Organic Matter — The Main Source of Variable Charge

Soil organic matter (humus, humic and fulvic acids) carries a large number of pH-dependent negative charges, mainly due to dissociation of carboxyl groups (–COOH → –COO^- + H⁺) and, to a lesser extent, phenolic hydroxyls (–OH → –O⁻ + H⁺).

• Specific CEC of humus is very high — 200–500 cmol_c/kg (Weil & Brady, 2017; White, 2006). This means that even 1% organic matter can contribute 2–5 cmol_c/kg of CEC.
• The contribution of humus to total CEC is particularly noticeable in sandy soils, where the clay fraction is small. In such soils, CEC often correlates with the organic carbon content (Weil & Brady, 2017).
• pH dependence — since humus charge is variable, its contribution to CEC increases greatly with rising pH (from acidic to neutral to alkaline). At pH 4–5, carboxyl groups are partially protonated and do not carry a charge; therefore, the ECEC of such soils can be significantly lower than the potential CEC measured at pH 7.

In soils with high organic matter content (e.g., Histosols, as well as the humus horizons of Mollisols), the organic component can provide up to 50–80% of the total CEC.

4.4 Soil pH — Regulator of Variable Charge

pH is the most important factor modifying CEC, especially for soils rich in organic matter and iron/aluminum oxides (Scheffer et al., 2018; Essington, 2015).

• At low pH (acidic soils), most pH-dependent groups are protonated (≡S–OH₂⁺, –COOH undissociated). The negative charge decreases, and CEC (especially ECEC) declines. Moreover, in acidic conditions, aluminum solubility increases, and exchange sites may be occupied by Al³⁺ and hydroxyaluminum ions, reducing the available capacity for other cations.
• At high pH (alkaline and neutral soils), deprotonation occurs: –COOH → –COO^- + H⁺, ≡S–OH → ≡S–O⁻ + H⁺. The negative charge increases, and CEC rises. This is exactly why potential CEC is measured at pH 7 or 8.2 — to "activate" all possible charges.

This effect is clearly seen in graphs of CEC versus pH: for kaolinite and humus, it increases with pH, while for smectite (where permanent charge dominates), it changes little (Weil & Brady, 2017).

4.5 Texture and Particle-Size Composition

Although texture itself is not a "source" of charge, it determines the total surface area of colloids available for exchange. The more clay and fine silt particles in the soil, the higher the specific surface area and, consequently, the potentially higher CEC. This is especially important for soils dominated by highly active 2:1 minerals.

However, texture is only an indirect factor; the decisive factor is the mineralogical composition of the clay fraction and the content of organic matter.

4.6 Ionic Strength and Composition of the Soil Solution

Although CEC is usually defined as a property of the solid phase, its measured value can depend on the concentration and composition of the solution (Essington, 2015; White, 2006).

• At high ionic strength (e.g., in saline soils), the diffuse layer contracts, and some cations, especially monovalent ones, may transfer from the diffuse layer to the Stern layer, formally increasing the "exchangeable" capacity. However, this is more of a measurement artifact than a change in the true number of charges.
• At low ionic strength (e.g., after rain), the diffuse layer expands, and some cations become less accessible for direct exchange, potentially reducing the measured ECEC.

Therefore, in laboratory practice, CEC is determined at a standardized ionic strength (usually 0.1–1 M) to minimize the influence of this factor.

4.7 Interaction of Factors in Real Soils

In real soils, all the aforementioned factors act together. For example:

• High CEC is characteristic of soils with high smectite or vermiculite content and/or high humus content, under neutral or slightly alkaline conditions.
• Low CEC is typical for sandy soils with low organic matter content, as well as for highly weathered acidic soils where kaolinite and oxides dominate (permanent charge is small, and variable charge is suppressed by low pH).

This is why Oxisols and Ultisols (tropical weathered soils) have low CEC, while Vertisols and Mollisols have high CEC.

To summarize:

CEC is determined by three main groups of factors:

1. The amount and type of clay minerals (permanent charge).

2. The content of organic matter (variable charge).

3. Soil pH (regulates the expression of variable charge).

Additional influence is exerted by texture (through specific surface area) and ionic strength of the solution. Understanding these factors allows predicting soil behavior upon application of fertilizers, amendments, and pollutants.

In the next section, we will move on to consider anion sorption — a process that, although less well-known than cation exchange, plays a huge role in the behavior of phosphates, sulfates, arsenates, and other anions in soils.

5.1 Why Can Anions Be Sorbed at All?

It would seem that if the colloid surface is negatively charged, it should repel anions. Indeed, for most temperate soils, where 2:1 clay minerals with high permanent negative charge predominate, anions (especially Cl^- and NO₃^-) are practically not sorbed and are easily leached. However, there are two mechanisms that enable anion sorption (Essington, 2015; Sparks, 2003; Scheffer et al., 2018):

1. Sorption on positively charged sites. On the surface of some colloids (Fe and Al oxides, edge surfaces of kaolinite, allophane), there are positively charged groups, especially in acidic conditions. These positive charges attract anions from the solution.

2. Ligand exchange (specific adsorption). This is a mechanism where an anion (ligand) replaces a hydroxyl group (OH⁻) or water molecule (H₂O) in the coordination sphere of a surface metal (Fe³⁺, Al³⁺). A strong covalent bond is formed, and the anion is held even on a negatively charged surface. This process is largely analogous to the formation of inner-sphere complexes for cations, but with the opposite charge.

5.2 Sources of Positive Charge

Positive charge on the surface of soil colloids arises from the protonation of hydroxyl groups in acidic conditions (pH below the point of zero charge, PZC) (Essington, 2015; White, 2006):

The lower the pH, the more such positively charged groups exist. This effect is especially pronounced in:

• Iron and aluminum oxides and hydroxides (goethite, hematite, gibbsite, ferrihydrite) — their PZC lies in the pH range of 6–9, so in acidic soils they carry significant positive charge.
• Edge surfaces of clay minerals — especially in kaolinite, where the ratio of edge surface to basal surface is large.
• Allophane and imogolite (volcanic soils, Andisols) — their amorphous structure creates many reactive hydroxyl groups, and they can have high anion exchange capacity (AEC) at both low and high pH.
• Organic matter — although it mainly carries negative charge, at very low pH (<3), some amino groups can become protonated, creating positive centers, but the contribution of this mechanism in most soils is negligible.

5.3 Mechanisms of Anion Sorption

Anion sorption can occur by two fundamentally different pathways (Essington, 2015; Sparks, 2003; Scheffer et al., 2018):

5.4 Anions — From Indifferent to Strongly Sorbing

By the degree of interaction with soil colloids, anions can be arranged in a series (Essington, 2015; Sparks, 2003; Scheffer et al., 2018):

Phosphates are a classic example of anions with very high sorption capacity. They form strong inner-sphere complexes with the surface of iron and aluminum hydroxides, often bidentate (with two bonds) or even tridentate. This is precisely why phosphorus in acidic soils is rapidly "fixed" and becomes poorly available to plants.

Arsenates (AsO₄³⁻) behave similarly to phosphates, which explains their accumulation in soils rich in iron oxides and their low mobility.

Sulfates occupy an intermediate position. In neutral soils, they are weakly sorbed, but in acidic conditions they can form inner-sphere complexes on aluminum hydroxides, leading to their retention in podzolic and acidic soils.

Borates (B(OH)₄⁻) and molybdates (MoO₄²⁻) also actively participate in ligand exchange, especially on aluminum oxides.

5.5 Factors Affecting Anion Sorption

As with cation exchange, the magnitude of anion sorption depends on several key conditions (Essington, 2015; Scheffer et al., 2018):

1. pH. This is the main factor. The lower the pH, the more protonated hydroxyl groups (≡S–OH₂⁺), the higher the positive charge, and the more active both outer-sphere sorption and ligand exchange become. However, the optimal pH differs for different anions: phosphates are maximally sorbed at pH 4–5, while borates at pH 7–9 (near their pKa).

2. Anion concentration. Sorption increases with increasing concentration, but at high concentrations, saturation of sorption sites may occur.

3. Competition between anions. Different anions compete for the same sorption sites. For example, phosphate, having high binding energy, can displace sulfate or selenate. This is relevant for phytotoxicity of heavy metals and radioactive elements.

4. Surface composition. The nature of the oxide (iron vs. aluminum vs. manganese) affects affinity for anions. For example, goethite (FeOOH) and gibbsite (Al(OH)₃) may differ in their sorption capacity for phosphates and arsenates.

5. Ionic strength. Outer-sphere sorption depends on ionic strength (decreasing with increasing salt concentration), while ligand exchange is virtually independent.

6. Time. As with cations, anion sorption can have a fast (outer-sphere) and a slow (inner-sphere, diffusion-controlled) stage. Over time, some anions may transform into difficult-to-desorb forms (the "aging" effect of sorbed phosphate).

5.6 Significance of Anion Sorption for Soil Science

Anion sorption is not just an academic curiosity. It has enormous practical significance (Sparks, 2003; Weil & Brady, 2017; Eash et al., 2016):

• Phosphorus behavior. Up to 90% of phosphorus fertilizers applied to acidic soils can be "bound" by iron and aluminum hydroxides into inaccessible forms. Understanding the mechanisms of ligand exchange underlies the development of methods to improve phosphate fertilizer efficiency (localized application, co-application with organic acids, etc.).
• Toxic elements. Arsenic, selenium, chromium (VI) are also actively sorbed by soil oxides. Their environmental behavior (toxicity, migration) directly depends on anion sorption. For example, As(V) sorption is analogous to phosphate and is firmly retained on iron hydroxides in acidic soils, while in alkaline soils it is more readily leached.
• Agroecology. Sorption of nitrates on anion-exchange sites in some soils (e.g., in Andisols and Oxisols) can slow their leaching into groundwater, which is relevant for preventing eutrophication.
• Wastewater treatment. The ability of minerals (especially ferrihydrite) to sorb phosphates, arsenates, and chromates is used in water and sediment treatment technologies.
• Sulfur cycle. Sulfate sorption in acidic soils (podzols, brown forest soils) can significantly affect sulfur availability to plants and its export to water bodies.

To summarize:

• Anion sorption is the retention of negatively charged ions on positively charged sites of soil colloids (primarily on Fe and Al hydroxides) by means of electrostatic forces or, more importantly, ligand exchange.
• Distinction is made between outer-sphere (electrostatic, reversible) and inner-sphere (ligand exchange, poorly reversible) sorption.
• Anions differ greatly in their sorption capacity: Cl^- and NO₃^- are almost not sorbed, while phosphates, arsenates, and molybdates are sorbed firmly and actively.
• pH is the main regulator of anion sorption: in acidic environments it is enhanced, in alkaline environments it weakens.
• Anion sorption is of immense importance for soil fertility (phosphorus nutrition) and for ecology (pollutant behavior, water purification).

In the next section, we will link cation and anion exchange into a unified picture — considering the amphoteric nature of the soil surface and introducing the concept of AEC (anion exchange capacity) in comparison with CEC.

6.1 Amphotericity: Two Sides of the Same Surface

Amphotericity means that surface functional groups can behave both as acids (donating H⁺) and as bases (accepting H⁺). This is characteristic of:

• Hydroxyl groups on oxides (\equivFe–OH, \equivAl–OH),
• Edge groups of clay minerals (aluminols Al–OH and silanols Si–OH),
• Organic functional groups (carboxyls, phenolic hydroxyls, amines).

In acidic conditions (high H⁺ activity), these groups become protonated:

In alkaline conditions, conversely, deprotonation occurs:

Thus, the same group can be a source of both cation and anion exchange, depending on pH (Essington, 2015; Sparks, 2003).

6.2 Point of Zero Charge (PZC) and Point of Zero Net Charge (PZNC)

For any amphoteric surface, there exists a pH value at which the total positive and negative charges are equal, i.e., the net surface charge is zero. This value is called the point of zero charge (PZC).

• At pH < PZC, the surface has a net positive charge — anion sorption dominates.
• At pH > PZC, the surface has a net negative charge — cation exchange dominates.
• At pH = PZC, the net sorption capacity for cations and anions is theoretically zero (although in reality, due to specific adsorption, this may not hold).

Important: PZC is a property of an individual surface (e.g., pure goethite or gibbsite). In a real soil composed of a mixture of different minerals and organic matter, one refers to the point of zero net charge (PZNC) — the pH value at which the total CEC balances the total AEC (i.e., CEC = AEC) (Sposito, 1984; Essington, 2015).

6.3 Dependence of CEC and AEC on pH

The classic graph of CEC and AEC versus pH for a soil with variable charge (e.g., an Oxisol or Andisol) looks as follows (Weil & Brady, 2017; Scheffer et al., 2018):

1) At low pH (acidic conditions):

• Most hydroxyl groups are protonated → positive charge increases → AEC is high.
• Carboxyl and phenolic groups of organic matter are weakly dissociated → CEC is low.
• In acidic soils rich in Fe/Al oxides, AEC may even exceed CEC (especially in podzolic horizons and some tropical soils).

2) At high pH (alkaline conditions):

• Hydroxyl groups are deprotonated → negative charge increases → CEC is high.
• Positive charge disappears → AEC approaches zero.
• Organic matter is fully dissociated, further increasing CEC.

Thus, CEC and AEC are not constant characteristics but functions of pH. This is precisely why CEC measurements always specify the pH (e.g., CEC at pH 7 or ECEC at soil pH). AEC measurement is performed at soil pH (usually under acidic conditions) or at a standardized low pH.

Graphical illustration (see, e.g., Essington, 2015, Fig. 8.9; Weil & Brady, 2017, Fig. 8.23): the CEC curve rises with pH, the AEC curve falls, and they intersect at the PZNC.

6.4 Typical AEC Values and the CEC/AEC Ratio in Different Soils

In most temperate soils (Alfisols, Mollisols, Vertisols), where 2:1 clay minerals with high permanent negative charge dominate, AEC is negligibly small (usually < 1–2 cmol_c/kg), while CEC ranges from 20–50 cmol_c/kg and above. These soils are classic cation exchangers.

However, in acidic, highly weathered soils (Oxisols, Ultisols, many Andisols), as well as in podzolic horizons (Spodosols), where kaolinite, Fe/Al oxides, and amorphous phases dominate, AEC can reach 5–15 cmol_c/kg, while CEC at low pH is very low (2–5 cmol_c/kg). In such soils, CEC and AEC become comparable, and AEC may even exceed CEC (Essington, 2015; Scheffer et al., 2018).

Example: in the acidic B horizon of an Oxisol (pH 4.5), CEC might be 3–5 cmol_c/kg, while AEC could be 3–8 cmol_c/kg. This means such a soil simultaneously retains cations (on residual negative charges) and anions (on positive centers of oxides). This phenomenon has fundamental significance for the behavior of phosphates, arsenates, sulfates, and many trace elements.

6.5 Permanent and Variable Charge: A Modern View

In modern soil chemistry, it is customary to distinguish between permanent charge (structural, isomorphous substitution) and variable charge (pH-dependent). In this context:

• Permanent charge (mainly negative) is inherent in 2:1 minerals and is independent of pH. It forms the basis of CEC in neutral and alkaline soils.
• Variable charge (can be both positive and negative) is inherent in organic matter, oxides, and edge surfaces. It is precisely this charge that determines amphotericity and the pH dependence of CEC/AEC.

Total soil CEC = permanent negative charge + variable negative charge (at a given pH).

AEC = variable positive charge (at a given pH).

In acidic soils, permanent charge is small (due to kaolinite and oxide dominance), while variable positive charge is significant, so AEC is high. In neutral and alkaline soils, permanent negative charge dominates, and variable positive charge disappears, so CEC is high and AEC is low.

6.6 Surface Models and Modern Approaches

To describe ion interactions with amphoteric surfaces, surface complexation models (SCM) have been developed (Essington, 2015; Sparks, 2003). Among them:

• Constant Capacitance Model — assumes all charges are concentrated in one plane and accounts for inner-sphere complex formation.
• Triple Layer Model — distinguishes between inner (Stern) and outer (diffuse) layers, allowing separation of outer-sphere and inner-sphere sorption.
• CD-MUSIC Model — accounts for crystallographic heterogeneity of the surface and distinguishes types of surface hydroxyl groups (mono-, di-, tridentate).

These models allow quantitative prediction of cation and anion sorption as a function of pH, ionic strength, concentration of competing ions, and other factors. They are actively used for modeling pollutant behavior, developing remediation technologies, and optimizing fertilization.

6.7 Practical Significance of the CEC/AEC Ratio

Understanding that CEC and AEC are two sides of the same coin has numerous applications:

• Fertility and fertilization. In acidic soils with high AEC, phosphate fertilizers are rapidly "fixed" (ligand exchange) and become unavailable to plants. Knowledge of AEC helps adjust P-fertilizer doses and application methods.
• Pollutant behavior. Arsenic, chromium (VI), selenium, fluorides — all participate in anion exchange. In acidic soils, they can be firmly retained; in alkaline soils, they may leach. This determines the risk of groundwater contamination.
• Buffering capacity. Soils with a high proportion of variable charge (i.e., with noticeable AEC and pH-dependent CEC) possess strong buffering capacity against changes in pH and solution ionic composition.
• Soil diagnostics. In soil classification (Soil Taxonomy), the ratio of CEC to clay fraction (cation exchange activity) is used to separate soils into groups with high and low clay activity (kaolinitic vs. smectitic).

To summarize:

• The surface of soil colloids is amphoteric: it can carry both negative and positive charges depending on pH.
• CEC and AEC are quantitative measures of negative and positive charges, respectively. Their sum is the total ion sorption capacity.
• The CEC/AEC ratio changes with pH: at low pH, AEC dominates; at high pH, CEC dominates.
• The point of zero charge (PZC) and the point of zero net charge (PZNC) are key parameters characterizing amphoteric properties of soils.
• Modern surface complexation models allow quantitative description of cation and anion competition for sorption sites.
• Knowledge of CEC and AEC is essential for fertility management, environmental protection, and soil diagnostics.

In the final section, we will discuss the significance of exchange processes for soil ecosystems and their practical application, without delving into plant physiology, but presenting the overall picture.

7.1 Functions of the Exchange Complex: A Systemic View

The totality of all colloidal particles capable of ion exchange is called the soil sorption complex (SSC). It is the SSC that acts as the "accumulator" and "distributor" defining the chemical regime of the soil. Its functions can be grouped into four main categories (Eash et al., 2016; Weil & Brady, 2017; Scheffer et al., 2018):

1. Accumulative-distributive function. The SSC retains ions in the root zone, preventing their leaching, but at the same time ensures their availability to biota (through reversible exchange). This is a kind of "chemical bank" of the soil.

2. Buffering function. The SSC can resist sharp changes in the composition and concentration of the soil solution — whether it be the influx of acids or bases (pH buffering), fertilizer application (ion-exchange buffering), or pollutants (sorption buffering).

3. Regulatory function. Exchange processes control the migration of elements within the soil profile, influence the formation of soil horizons, and determine the direction of soil formation (e.g., carbonate accumulation, base leaching, podzolization).

4. Protective (ecological) function. The SSC fixes toxic elements and organic pollutants, reducing their bioavailability and mobility, thus ensuring water purification and biota protection.

Now let us consider each of these functions in more detail.

7.2 Fertility and Plant Nutrition (Without Delving into Physiology)

Although we avoid a detailed discussion of plant physiology, it is important to understand the fundamental link between exchange processes and plant nutrition.

• Exchangeable nutrient pool. Most cationic nutrients (Ca²⁺, Mg²⁺, K⁺, NH₄⁺) and some micronutrients (Zn²⁺, Cu²⁺, Mn²⁺) are present in the soil in exchangeable form. If there were no exchange capacity, they would be rapidly leached by precipitation. The SSC creates a "reserve pool" from which plants can draw elements as needed.
• Mechanism of root uptake. Plant roots release H⁺ and organic acids into the rhizosphere, which displace the desired cations from the colloid. Thus, the plant "activates" the exchange process locally, obtaining nutrition exactly where it grows. This is a natural self-regulation mechanism.
• Role of CEC. The higher the CEC, the greater the capacity of the "chemical bank." Soils with high CEC (Mollisols, Vertisols) can retain more nutrients and maintain fertility longer without fertilization. Soils with low CEC (sands, Oxisols) require more frequent and careful fertilization, as their retention capacity is limited.

Thus, CEC is one of the key integral indicators of potential soil fertility. That is why it is included in all agrochemical soil passports and used in calculating fertilizer rates (though the calculation itself is a task for agrochemistry).

7.3 Soil Buffering Capacity

Buffering is the ability of soil to resist changes in pH and solution composition under external influences. The exchange complex is the main mechanism of this buffering (Essington, 2015; White, 2006).

• pH buffering. When acid enters the soil (e.g., from acid rain or nitrification), H⁺ is first neutralized by exchangeable bases (Ca²⁺, Mg²⁺, K⁺, Na⁺), which are displaced into the solution, and then by aluminum and hydroxyaluminum forms. As long as there is a reserve of exchangeable bases and aluminum, pH changes slowly. Only when the buffer systems are depleted does pH begin to drop sharply.
• Salt buffering. When fertilizers are applied or salinization occurs, a sharp increase in cation concentration in the solution is mitigated by their sorption on the SSC, and upon leaching — by desorption. This is especially important for preventing osmotic shock to plants and preserving soil structure.
• Buffering against pollutants. When heavy metals, radionuclides, or organic cations enter the soil, they also participate in exchange reactions, reducing their toxicity.

Soils with high CEC (especially due to organic matter and smectite) have significantly greater buffering capacity than soils with low CEC. This is why sandy soils, poor in colloids, require more frequent liming and fertilization — they simply cannot "soften" external impacts.

7.4 Regulation of Migration and Accumulation of Elements in the Profile

Exchange processes determine the vertical distribution of chemical elements in the soil profile and, consequently, the direction of soil formation (Eash et al., 2016; Weil & Brady, 2017).

• Leaching. In humid regions with a leaching regime, some cations are displaced by H⁺ and Al³⁺ and are leached into lower horizons or out of the profile. The enrichment of upper horizons in bases results from biogenic accumulation (plant litter) and, to a lesser extent, weak sorption.
• Accumulation in podzolic and illuvial horizons. In podzolic soils (Spodosols), Fe and Al are removed from upper horizons as organo-mineral complexes, which then precipitate in the illuvial horizon (Bh, Bs). In carbonate soils, calcium and magnesium accumulate as carbonates in the lower part of the profile.
• Solonetz and solonchak. In arid conditions, sodium displaced from the SSC can accumulate in lower horizons (solonetzic horizon), leading to specific morphological features (columnar structure) and strong alkalinity.
• Anion accumulation. In acidic highly weathered soils, phosphates, arsenates, and sulfates can be fixed on Fe and Al hydroxides, forming horizons with high bound phosphorus content (so-called "phosphorus" horizons in some tropical soils).

Thus, the exchange complex actively participates in the geochemical cycling of elements and determines the evolution of soils under different climatic conditions.

7.5 Detoxification and Immobilization of Pollutants

In the modern world, where anthropogenic soil pollution is becoming increasingly significant, the role of exchange processes in immobilizing toxicants cannot be overstated (Essington, 2015; Sparks, 2003; Scheffer et al., 2018).

• Heavy metals. Cations of heavy metals (Pb²⁺, Cd²⁺, Cu²⁺, Zn²⁺, Hg²⁺) are strongly sorbed on negatively charged colloids, especially on organic matter (chelation) and smectitic minerals (inner-sphere adsorption). The higher the CEC, the more soil can retain these metals, reducing their migration into groundwater and uptake by plants. However, if CEC is low (sands, Oxisols), metals may be more mobile.

• Radionuclides. ¹³⁷Cs, ⁹⁰Sr, and other radioactive isotopes actively participate in cation exchange. For example, cesium (Cs⁺) is strongly fixed in the interlayer spaces of vermiculite and illite, making it virtually immobile in some soils. This is a key factor in assessing radiation hazards after nuclear accidents (Weil & Brady, 2017).

• Organic cations and polar molecules. Many pesticides, herbicides, and antibiotics, under certain conditions (low pH, high ionic strength), acquire a positive charge and are sorbed on cation exchange sites. This reduces their bioavailability and slows degradation but also reduces the risk of leaching into water bodies.

• Anionic toxicants. Arsenic (as arsenate), chromates, selenates, and fluorides are actively sorbed on Fe and Al hydroxides (anion exchange and ligand exchange) in acidic soils, limiting their migration. In neutral and alkaline soils, this sorption weakens, and they become more mobile.

Thus, the exchange capacity of soil acts as a kind of natural filter, reducing the toxicity of pollutants and protecting groundwater and food chains from contamination.

7.6 Participation in Structure Formation and Physical Properties

Although this extends beyond pure chemistry, exchange processes directly affect the physical properties of soil, especially aggregation and water stability (Weil & Brady, 2017; White, 2006).

• Divalent cations (Ca²⁺, Mg²⁺) promote flocculation (coagulation) of colloidal particles, forming microaggregates resistant to water erosion. This improves structure, water permeability, and aeration.
• Monovalent cations (Na⁺), on the other hand, cause dispersion (breakdown of aggregates), leading to compaction, crust formation, and reduced infiltration (sodification phenomenon).
• Organic matter, enriching the SSC, acts as a cementing agent, binding mineral particles into aggregates.

Therefore, regulating the composition of exchangeable cations (e.g., gypsum application to displace sodium) is an important management practice for soil physical properties, which we leave for discussion in courses on agriculture and land reclamation.

7.7 Role in Element Cycling and Global Cycles

At a higher level, exchange processes in soils influence the global biogeochemical cycles of carbon, nitrogen, sulfur, phosphorus, and other elements.

• Nitrogen. Ammonium (NH₄⁺) is a cation actively sorbed on the SSC. In soils with high CEC, it is retained in the root zone and slowly nitrified, reducing nitrogen losses as NO₃^- (which is not sorbed). Thus, CEC affects nitrogen fertilizer efficiency and N₂O emissions.
• Carbon. Organic matter is not only a source of CEC but also a carbon sink. Soils with high humus content (and high CEC) serve as significant carbon stores, important for climate regulation.
• Phosphorus and sulfur. Anion sorption controls the availability of phosphorus and sulfur. In acidic soils, phosphorus is fixed on Fe/Al oxides, reducing its input into ecosystems and limiting productivity. On the other hand, sulfates may be retained in podzolic horizons, affecting acidity and the sulfur cycle.

Thus, soil exchange capacity is not only an agronomic characteristic but also an element of global ecological systems.

7.8 Concluding Remarks

Soil sorption capacity, realized through cation and anion exchange, is a fundamental property that permeates all aspects of soil science:

• It determines potential fertility and fertilizer efficiency.
• It provides buffering and resilience of soils to external impacts.
• It regulates element migration and the formation of soil horizons.
• It serves as a critical natural filter, detoxifying pollutants.
• It influences soil structure and physical properties.
• It participates in global elemental cycles.

Understanding the nature of exchange processes and the ability to quantify CEC and AEC is not merely theoretical knowledge but a practical tool for rational use of soil resources, environmental protection, and sustainable development of the agrosphere.