Exchangeable cations and base saturation

Calcium (Ca²⁺) — "The Guardian of Fertility"

This is, without exaggeration, the most important and noble resident of the SAC in most fertile soils. It is often called the "guardian of soil fertility" (Sokolovsky, cited in Mukha et al., 2003). Calcium enters the soil through the weathering of minerals such as feldspars and in the form of carbonates (limestone).

Its main role is not only to nourish plants but also to be a structure-former. Calcium cations, having two positive charges and a relatively small hydrated radius, are attracted to negatively charged colloids more strongly than monovalent cations. They act to pull colloidal particles together, causing their coagulation (clumping). This leads to the formation of water-stable aggregates (the very granular or crumbly structure so valued by agronomists) and the stabilization of humus (Sparks, 2003; White, 2006).

Soils in whose SAC calcium dominates (e.g., Chernozems) have a neutral or slightly alkaline reaction, good water-physical properties, and high natural fertility.

Magnesium (Mg²⁺) — "The Faithful Companion"

Magnesium is the second most important divalent cation. It is in many ways similar to calcium: it is also an essential nutrient (it is a component of chlorophyll) and promotes the coagulation of colloids, although to a lesser extent (Scheffer & Schachtschabel, 2018). Its content in the SAC often correlates with calcium content.

However, it is important to know that a high magnesium content, especially in combination with sodium, can be a sign of trouble. On magnesian solonetz soils, dispersion (breakdown) of the soil structure, deterioration of physical properties, and increased alkalinity are observed (Mukha et al., 2003). Thus, even though it is in the "base" group, magnesium in excess can behave like a "pest."

Potassium (K⁺) — "The Valuable Professional"

Potassium is a monovalent cation. It is vital for plants (plays a key role in osmotic pressure, enzyme activation), and its presence in the SAC is a guarantee of good nutrition. However, its retention on colloids is much weaker than that of divalent calcium and magnesium (Sparks, 2003). Therefore, it is more mobile and subject to leaching.

A feature of potassium is its ability for non-exchangeable absorption. It fits perfectly in size into the "windows" of the crystal lattice of some 2:1 clay minerals (especially illite and vermiculite) and can become fixed there in interlayer spaces, becoming unavailable to plants (Eash et al., 2016; Weil & Brady, 2017). This temporary "immobilization" is an important process regulating potassium availability.

Sodium (Na⁺) — "The Structure Destroyer"

Sodium is the most insidious "resident" of the SAC. In small amounts, it is present in most soils, but with a high proportion of sodium among the exchangeable cations (as in solonetz soils), its influence becomes catastrophic.

What is the problem? First, like potassium, it is a monovalent cation that is weakly retained. But most importantly: it becomes highly hydrated (attracts a large number of water molecules), which increases its "effective radius." Such a large and weakly bound ion does not pull colloids together but, on the contrary, swells them from within (dispersion), creating thick water shells around the colloidal particles. This leads to:

• Structure breakdown: aggregates are destroyed.
• Soil swelling: it becomes viscous, sticky, and poorly permeable to water and air.
• Crust formation: after drying, the surface turns into a dense crust.
• pH increase to alkaline values.

As a result, soil with a high content of exchangeable sodium becomes "dead" for most plants (Weil & Brady, 2017; White, 2006).

Hydrogen (H⁺) and Aluminum (Al³⁺) — "Sources of Acidity"

These two cations are the main "pests" in acidic soils. It is their presence in the SAC that makes the soil acidic, and the hydrogen ions determine the actual acidity of the soil solution (Scheffer & Schachtschabel, 2018). It is important to understand that in acidic mineral soils, the bulk of the "acidic charge" is not on hydrogen itself, but on exchangeable aluminum.

How does this work? The aluminum cation (Al³⁺) in solution is always surrounded by water molecules, forming the aqua complex Al(H₂O)₆³⁺. Under the action of an alkaline salt solution (e.g., when determining hydrolytic acidity), this complex hydrolyzes, releasing hydrogen ions (Eash et al., 2016; Mukha et al., 2003):

Thus, each aluminum ion in the SAC can become a source of three hydrogen ions, acidifying the soil solution. In strongly acidic podzolic soils and ferralitic soils of the tropics, it is aluminum that dominates the SAC, which determines their extremely unfavorable properties: toxicity to plants, low fertility, and structure destruction (Weil & Brady, 2017; Huang, 2012).

Where does exchangeable aluminum come from?

Hydrogen ions (H⁺), entering the soil with acid rain or formed as a result of organic matter decomposition, do not remain passive. They react with soil minerals—aluminosilicates, which form the basis of clay particles. H⁺ attacks the crystal lattice, displacing metal ions from it, primarily aluminum cations (Al³⁺) (White, 2006). This process is called acid hydrolysis (Eash et al., 2016). The released Al³⁺ ions, in turn, occupy the vacated sites on the exchange complexes, displacing other cations into the solution, such as Ca²⁺ or Mg²⁺, which can then be leached.

Thus, exchangeable aluminum is a marker of a deep degree of weathering and depletion of the soil absorbing complex. Its appearance indicates that the soil has gone through a stage of acid hydrolysis, and its buffering capacity has significantly changed (Weil & Brady, 2017; Huang et al., 2012).

Why is Al³⁺ a "hidden" acid?

Here lies the most important nuance. The aluminum ion itself in the SAC does not make the soil solution acidic. It, like calcium, simply sits on the exchange site. However, unlike calcium, it is a potential, or hidden, source of acidity. The whole point is hydrolysis.

In an aqueous solution, the Al³⁺ cation is always surrounded by six water molecules, forming the aqua complex [Al(H₂O)₆]³⁺. This complex is capable of dissociating (releasing hydrogen ions) according to the following mechanism (Scheffer & Schachtschabel, 2018; Mukha et al., 2003):

1. [Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(OH)(H₂O)₅]²⁺ + H₃O⁺

This reaction proceeds already at pH ~ 5.0. In the second stage, the process can continue further:

2. [Al(OH)(H₂O)₅]²⁺ + H₂O ⇌ [Al(OH)₂(H₂O)₄]⁺ + H₃O⁺

As we can see, at each stage of hydrolysis, a hydronium ion (H₃O⁺) is formed, which acidifies the soil solution. Aluminum ions are thus sources of protons. This mechanism explains why, in strongly acidic mineral soils (pH < 4.5), the bulk of the exchangeable acidity is associated not with hydrogen, but with aluminum (Eash et al., 2016). Aluminum continuously "supplies" new portions of H⁺ to the solution.

The role of aluminum in shaping the properties of acidic soils

Understanding this role of aluminum is the key to answering the main question of our lecture. The presence of exchangeable aluminum in the SAC radically changes the "character" of the soil compared to a soil where the SAC is saturated with calcium, even with the same CEC.

What does this mean in practice?

1. High buffering capacity against acidification: Soils containing exchangeable aluminum are powerful buffers. To raise the pH of such a soil (e.g., by liming), it is necessary to neutralize not only the active acidity of the solution but also the entire "reserve" of potential acidity associated with exchangeable aluminum. Until the Al³⁺ ions are converted to the insoluble form Al(OH)₃, the soil will tend to return to its original low pH. Therefore, changing the pH by 1 unit in a clayey acidic soil requires tens and hundreds of times more lime than in a sandy soil (Weil & Brady, 2017).

2. High content of "acidic" cations → low base saturation: From a classification point of view, this is a fundamental difference. It is the high percentage of exchangeable Al³⁺ (and H⁺) that makes soils unsaturated with bases, unlike Chernozems, where bases (Ca²⁺, Mg²⁺) occupy almost all exchange sites (Mukha et al., 2003). This division underlies the separation into such important soil orders as, for example, Alfisols (base-saturated) and Ultisols (base-unsaturated) (Huang et al., 2012; Weil & Brady, 2017). This feature is manifested in diagnostic horizons.

Thus, exchangeable aluminum is not just an element. It is an "acidity generator" that determines the chemical background, buffering properties, and, ultimately, the classification position of the soil. A soil with a high CEC but with Al³⁺ predominating in the SAC is a fundamentally different object than a soil with the same CEC but saturated with calcium.

What is base saturation?

Base Saturation (BS) is the proportion (in percent) of the total cation exchange capacity (CEC) that is occupied by bases (cations Ca²⁺, Mg²⁺, K⁺, Na⁺). This is one of the key "diagnostic" indicators of soil (Weil & Brady, 2017; Eash et al., 2016).

The formula for its calculation is simple and elegant:

Where the Sum of exchangeable bases (S) is the sum of all exchangeable cations that are bases, measured in cmol(+/kg) or meq/100 g of soil (Eash et al., 2016; White, 2006).

How to interpret this indicator?

The base saturation percentage is a direct indicator of how "occupied" the sites on the colloids are with useful (from an agronomic point of view) cations.

• High base saturation (BS > 80%): Most sites in the SAC are occupied by Ca²⁺, Mg²⁺, K⁺. This is a sign of fertile, well-structured soils with a neutral or slightly alkaline reaction. A classic example is Chernozems (Mollisols) and many carbonate soils (Eash et al., 2016; Huang et al., 2012). Such soils are rich in nutrients, have good structure, and high buffering against acidification.
• Low base saturation (BS < 50%): A significant part of the exchange sites is occupied by acidic cations—H⁺ and, more importantly, Al³⁺. This is a sign of acidic, leached soils. Examples are Podzolic soils (Spodosols), Ultisols, and Ferralitic soils (Oxisols) (Weil & Brady, 2017; Huang et al., 2012). Such soils, as a rule, have unfavorable physical properties (destroyed structure), low biological activity, and are deficient in essential nutrients (Ca, Mg).

Key nuance: "effective" and "potential" saturation

In soil science, there are two approaches to determining CEC (and, consequently, to calculating base saturation), and this is very important for a correct understanding of the issue (Weil & Brady, 2017; Scheffer & Schachtschabel, 2018).

1. Potential CEC (CEC at pH 7 or 8.2): This method determines CEC at an artificially created high pH. It accounts for not only permanent charges but also all pH-dependent charges that are "turned on" only in an alkaline medium (e.g., on humus and oxides). For acidic soils, this number will always be overestimated.

2. Effective CEC (ECEC): This method determines CEC at the real, natural pH of the soil. It shows the actual ability of the soil to retain cations in its natural state. This value most accurately reflects the availability of nutrients for plants.

An important conclusion follows from this. If we calculate the "potential" base saturation (using CEC at pH 7), then in an acidic soil, it will be unreasonably low due to the overestimated denominator. At the same time, "effective" saturation (using ECEC) will show that in an acidic soil with low pH, almost all available exchange sites are indeed occupied by bases, just there are very few of them (Weil & Brady, 2017). This helps to take a fresh look at the problem: in a strongly acidic soil, we have not only a high proportion of acidic cations but also an overall degradation of the SAC itself, a loss of its "capacity."

Base saturation as a key to the "character" of the soil

Now we are approaching the answer to the main question of our lecture. Imagine two soils with the same CEC (e.g., 20 cmol(+/kg)).

• In the first soil, these 20 units are 90% occupied by calcium (BS = 90%). This means that the soil is calcium-saturated, has a neutral pH, good structure, and high fertility.
• In the second soil, these same 20 units are 90% occupied by hydrogen and aluminum (BS = 10%). This is an acidic, "leached" soil, with poor structure, low fertility, and aluminum toxic to plants.

So, we see: identical CEC does not mean identical properties. The key role is played by the qualitative composition of the exchangeable cations. It is the ratio of bases to acidic cations (base saturation) that determines whether the soil will be a fertile Chernozem or an infertile Podzol.

Thus, base saturation is not just a number. It is an integral indicator that connects the chemical composition of the SAC with the physical properties and fertility of the soil (White, 2006). In the next part, we will analyze this thesis using specific examples and move on to considering how this indicator is related to soil genesis and classification.

4.1. Calcium and Aluminum soils — two poles

To understand the scale of the differences, let's compare two soils with the same CEC = 20 cmol(+/kg), but with fundamentally different SAC composition.

Type One: Calcium soil (e.g., Chernozem).

SAC composition: 90% occupied by calcium (Ca²⁺), 5% by magnesium (Mg²⁺), 5% by potassium and sodium (K⁺, Na⁺). Acidic cations (H⁺, Al³⁺) are practically absent.

Base Saturation (BS): ~95% (high).

Reaction (pH): Neutral or slightly alkaline (pH 6.5–7.5).

Behavior: Agronomically valuable properties.

• Structure: The high content of Ca²⁺ causes coagulation of colloids, forming water-stable aggregates—the very granular or crumbly structure that provides optimal water-air regime (Sparks, 2003; White, 2006).
• Buffering: The soil has a high ability to resist acidification. Calcium cations serve as a reserve that can neutralize incoming acids.
• Nutrient regime: A constant source of Ca and Mg for plants. K⁺ and Na⁺ are readily available.
• Biological activity: A favorable environment for the development of beneficial microflora.

Type Two: Aluminum soil (e.g., Podzol or Ferralitic soil).

SAC composition: 80% occupied by exchangeable aluminum (Al³⁺), 15% by hydrogen (H⁺), and only 5% by bases (Ca²⁺, Mg²⁺, K⁺, Na⁺).

Base Saturation (BS): ~5% (very low).

Reaction (pH): Strongly acidic (pH < 4.5).

Behavior: Agronomically unfavorable properties.

• Structure: The predominance of Al³⁺ leads to dispersion of colloids, structure destruction, crust formation, and deterioration of water permeability (Weil & Brady, 2017).
• Buffering: The soil has high buffering specifically against alkalinization. To neutralize this "reserve" of acidity, a huge amount of lime is required. It is the hydrolysis of exchangeable Al³⁺ that is the main supplier of H⁺ to the solution, maintaining a low pH even when fertilizers are applied (Scheffer & Schachtschabel, 2018).
• Nutrient regime: Base cations are in deficit and easily leached. Plant nutrition is extremely limited.
• Toxicity: High content of Al³⁺ in the solution is toxic to the root systems of most cultivated plants (Eash et al., 2016).

4.2. The role of "neighbors" (complementary ions)

The differences between soils are not limited only to the dominant cation. The environment, i.e., the complementary ions, also plays a huge role. As we remember from the section on cation exchange, the strength of retention of a particular cation depends on who surrounds it (Sparks, 2003; White, 2006).

• In calcium soil: The strongly retained Ca²⁺ surrounds the weakly retained K⁺. This makes potassium more available to plants, as root exudates (H⁺) are more likely to "knock out" the weakly bound K⁺ rather than Ca²⁺.
• In aluminum soil: The very strongly retained Al³⁺ and H⁺ surround the weakly retained K⁺. This, on the contrary, makes potassium less available, as root exudates will "bind" with strong competitors, leaving K⁺ in place.

Thus, the availability of the same nutrient (e.g., potassium) can differ dramatically in two soils with the same CEC simply because the set of its "neighbors" in the SAC is different.

4.3. The role of colloidal "architecture"

It is important to remember that the SAC is not just a homogeneous mass but a complex system consisting of different types of colloids (clay minerals, humus, oxides). Each of them interacts differently with cations. For example:

• 2:1 clays (smectites, vermiculites) have the ability for non-exchangeable absorption of K⁺, "locking" it in interlayer spaces (Eash et al., 2016; Weil & Brady, 2017). This affects potassium availability, and this property does not depend directly on CEC.
• Humus creates a huge number of pH-dependent exchange sites. In an acidic environment, these sites are occupied by H⁺ and Al³⁺, which sharply reduces the effective CEC and makes the soil practically infertile.

Final conclusion

We see that CEC is just an "entry ticket." True understanding of soil properties begins with the analysis of the qualitative composition of the SAC and, first of all, with the indicator Base Saturation. It is this indicator, and not the absolute value of CEC, that determines:

• Structural state (aggregated or dispersed soil).
• Chemical buffering (resistance to acidification or alkalinization).
• Nutrient regime (availability of elements to plants).
• Toxicological situation (presence of toxic forms of aluminum).
• Classification position (to which type and order the soil belongs).

Thus, the key to understanding soil behavior lies not in the capacity of its "storehouse," but in what "goods" are stored in it and how they are packaged.

5.1. Two main paths of SAC evolution

Depending on the ratio of input and output of substances in the soil, two fundamentally different types of SAC are formed.

Path One: Accumulation of bases (path of fertility).

This path is characteristic of soils forming under conditions of non-leaching water regime—in steppes, forest-steppes, and semi-deserts. Here, the amount of precipitation is less than or equal to evapotranspiration. Dissolved substances are not leached from the upper horizons but, on the contrary, can be drawn to the surface with capillary moisture.

• Climate: Arid and semi-arid.
• Vegetation: Herbaceous (steppes, prairies). The root systems of grasses penetrate the soil thickness, actively extracting nutrients from the lower horizons and returning them to the upper layers in the form of organic litter. This phenomenon is called the "biological pump" or basophilic cycle (Weil & Brady, 2017; White, 2006).
• Processes: Under such conditions, the weathering of silicate minerals occurs, but the products of weathering (Ca²⁺, Mg²⁺, K⁺, Na⁺, H₄SiO₄) are not removed. They accumulate in the soil profile. Calcium and magnesium carbonates (CaCO₃, MgCO₃) are not leached and may even accumulate in the form of carbonate horizons.
• Result: The SAC of such soils is base-saturated, primarily with calcium. A classic example is Chernozems (Mollisols) (Huang et al., 2012). High base saturation is their diagnostic feature.

Path Two: Leaching and acidification (path of SAC degradation).

This path is characteristic of soils forming under conditions of percolative water regime—in humid areas where precipitation significantly exceeds evapotranspiration.

• Climate: Humid (wet forests of the temperate zone, wet tropics).
• Vegetation: Forest. Litter of conifers and many deciduous species is rich in organic acids. During its decomposition, aggressive humic acids (fulvic acids) and carbon dioxide are formed (Eash et al., 2016; Scheffer & Schachtschabel, 2018).
• Processes: Downward flows of water actively leach all soluble compounds from the upper horizons. The podzolization process begins, in which acidified solutions destroy silicate minerals, remove bases and iron and aluminum oxides, and the remaining silica enriches the upper horizon, giving it a whitish color (Eash et al., 2016).
• Result: Bases (Ca²⁺, Mg²⁺, K⁺, Na⁺) are leached from the SAC. Their place is taken by acidic cations—first H⁺, then, as further destruction of aluminosilicates occurs, exchangeable aluminum (Al³⁺). The soil becomes base-unsaturated. Typical examples: Podzols (Spodosols) in the taiga, Ferralitic soils (Oxisols) in the tropics, many Ultisols in the subtropics (Huang et al., 2012; Weil & Brady, 2017). Low base saturation is their distinguishing feature.

5.2. The role of time and vegetation in SAC formation

Genesis is a long process. Young soils often inherit the SAC composition from the parent material. But over time, under the influence of climate and biota, the composition of exchangeable cations changes.

For example, even on carbonate rock (rich in calcium) in a humid tropical climate, after millions of years, a soil will form in whose SAC aluminum will dominate. All the calcium will be leached, and the silicate minerals will be destroyed to iron and aluminum oxides. This is how Ferralitic soils (Oxisols) are formed—the most weathered soils on the planet, with a CEC often not exceeding 2–3 cmol(+/kg), almost entirely composed of Al³⁺ (Huang et al., 2012). This is the final stage of SAC degradation in a humid tropical climate.

On the other hand, in steppe conditions, in soil with high calcium saturation, a strong water-stable structure is formed. This, in turn, creates a favorable water-air regime, which promotes humus accumulation and even greater fixation of calcium in the SAC. This positive cycle reinforces the fertility of Chernozems.

5.3. Genesis as an explanation of the spatial distribution of soils

Thus, we see that the composition of the SAC is the "passport" of the soil, inextricably linked to the conditions of its formation. Understanding genesis allows us to explain why:

• In the Chernozem zone of the steppes, we see powerful, dark, crumbly soils with a neutral reaction (calcium-saturated).
• In the taiga zone—light gray, poor, acidic Podzols with a well-developed eluvial horizon (base-unsaturated, Al³⁺ dominates).
• In tropical forests—deep, bright red, but very poor Ferralitic soils with an extremely low ability to retain cations (Al³⁺ and Fe oxides dominate).

That is why, by determining base saturation, we make a conclusion not only about the current state of fertility but also about the path of development the soil has taken. This is the connection between the chemistry and the history of the soil.

6.1. Ferralsols (Oxisols) — "Impoverished giants of the tropics"

These are soils that have undergone the longest and most intense weathering path. They form in the humid tropics on ancient planation surfaces (Huang et al., 2012).

• SAC composition: 2:1 clay minerals are almost completely lost. The SAC is represented mainly by kaolinite (1:1 clay) and iron and aluminum oxides (goethite, hematite, gibbsite). These minerals have a very low CEC (often < 5 cmol(+/kg)) and carry almost entirely pH-dependent charge. The dominant exchangeable cation is Al³⁺, as all bases have long been leached.
• Base Saturation: Extremely low, often < 35%.
• Diagnostic feature: For Ferralsols (Oxisols), the key is the presence of an oxic horizon. Its main criteria are:
• Low CEC (< 16 cmol(+/kg) clay) and low effective CEC (ECEC < 12 cmol(+/kg) clay) (Huang et al., 2012; Weil & Brady, 2017).
• Very low content of primary (easily weatherable) minerals (< 5%).
• ese two criteria are a direct consequence of long-term weathering, leading to SAC degradation and its saturation with aluminum. It is these parameters, not just low CEC, that make Oxisols diagnosable.

6.2. Chernozems (Mollisols) — "Kings of fertility"

These are soils formed under herbaceous vegetation in conditions of insufficient moisture (steppes and prairies). Their genesis is the path of base accumulation (Eash et al., 2016; Weil & Brady, 2017).

• SAC composition: The SAC is saturated with Ca²⁺ (up to 80–90% of CEC) and Mg²⁺. Most of the CEC is provided by humus and 2:1 clays (smectites, illites), which carry a significant permanent charge. CEC can be high (30–50 cmol(+/kg)).
• Base Saturation: Very high, often > 80% and even > 95%.
• Diagnostic feature: For Mollisols (which include Chernozems), the main feature is the presence of a mollic epipedon—a powerful, dark, humified surface horizon. One of its mandatory conditions is base saturation > 50% (Soil Survey Staff, cited in Weil & Brady, 2017). High base saturation is a mandatory condition for assigning a soil to this order. If such a powerful humus horizon had a low base saturation, it would be diagnosed as an umbric epipedon, and the soil would be assigned to other orders (e.g., Inceptisols or Ultisols).

6.3. Luvisols (Alfisols) — "Soils with an argic horizon"

These are widespread soils of the temperate zone, forming under broad-leaved forests with a leaching regime, but on carbonate rocks or with a high initial base content.

• SAC composition: The SAC in the upper part of the profile may be unsaturated, but in the argic (clay-enriched) horizon (Bt), Ca²⁺ and Mg²⁺ dominate, and the proportion of H⁺ and Al³⁺ is small. Clay minerals are mainly 2:1 (illite, smectite, vermiculite), carrying a high permanent charge.
• Base Saturation: In the lower part of the profile (in the Bt horizon), base saturation exceeds 35%. For soils developed on carbonate rocks, it can be much higher (50–80%).
• Diagnostic feature: Alfisols (and their European counterpart Luvisols) are distinguished based on the presence of an argic horizon—a horizon with an increased clay content, brought from the upper layers. And the key criterion for assigning a soil to Alfisols, rather than Ultisols, is base saturation in this horizon > 35% (Huang et al., 2012; Weil & Brady, 2017). This is an indicator that the leaching process has not gone too far, and the soil has retained its base reserve.

6.4. Podzols (Spodosols) — "Products of acid hydrolysis"

These are soils formed under coniferous or heath forests in cold and humid climates. Here, the podzolization process dominates (Eash et al., 2016; Scheffer & Schachtschabel, 2018).

• SAC composition: The SAC is strongly destroyed by acid hydrolysis. The upper eluvial horizon (E) is almost completely devoid of clay and oxides. The main charge carrier is humus, but its CEC is strongly suppressed due to low pH. In the illuvial horizon (Bh, Bs), Al³⁺ and Fe³⁺ accumulate (in the form of complexes with organic matter). CEC is very low.
• Base Saturation: Extremely low, often < 10% and even < 5%.
• Diagnostic feature: Spodosols are distinguished based on the presence of a spodic horizon—an illuvial horizon enriched with amorphous Al-humus and Fe-humus complexes. Its diagnostic criteria include high content of Al and Fe extractable by oxalate, as well as low pH values (Weil & Brady, 2017). Low base saturation and high content of exchangeable aluminum are a direct consequence of the processes forming this horizon.

Summary table

For clarity, let's summarize the information in a table: