Soil Absorption Complex

Historical Background: From Observations to Discovery

The history of studying the absorptive capacity of soils began long before the advent of modern soil science. Agronomists in the 19th century already noted that soil has the ability to "retain" dissolved substances, preventing them from being leached out by water. However, these were merely empirical observations.

A turning point came with the work of J. Thomas Way in the 1850s. This English chemist, rightfully called the founding father of soil chemistry, conducted a series of classic experiments. By passing salt solutions, such as ammonium sulfate, through soil columns, he discovered that ammonium ions were retained in the soil, while calcium ions were released into the effluent solution (Sparks, 2003). Thus, the phenomenon of ion exchange was discovered.

Sparks (2003) notes that Way not only discovered this phenomenon but also identified its key properties: the rapidity of the reaction, the crucial role of clay minerals, and the reduction of absorptive capacity upon heating or acid treatment.

However, it was the Russian scientist Konstantin Kaetanovich Gedroits who developed a coherent theory. Working in the early 20th century, he elaborated the doctrine of the Soil Absorbing Complex (SAC), which became the foundation of our modern understanding (Mukha et al., 2003). Gedroits not only systematized knowledge but also proposed the first classification of the types of absorptive capacity of soil, which we will examine later. It is Gedroits who is rightfully considered the founder of the modern theory of the SAC.

Modern Definition: What Exactly is the SAC?

Today, we interpret this concept more broadly than a century ago. The Soil Absorbing Complex (SAC) is the assemblage of highly dispersed (colloidal and pre-colloidal) mineral, organic, and organomineral particles of the soil's solid phase that possess the ability to absorb, retain, and exchange ions and molecules from the soil solution (Mukha et al., 2003; Weil and Brady, 2017).

Let's break down this definition piece by piece.

"Assemblage of highly dispersed particles": This means that the SAC does not include all soil particles, but only the finest ones, smaller than 0.001–0.0001 mm (colloids) and slightly larger ones (the pre-colloidal fraction). It is on these particles that the main chemical activity is concentrated.

• "Mineral, organic, and organomineral": The SAC is heterogeneous in its composition. It includes three main groups of components (Weil and Brady, 2017; White, 2006):
• Mineral colloids: These mainly consist of secondary clay minerals (smectites, vermiculites, illites, kaolinite), as well as oxides and hydroxides of iron, aluminum, manganese, and silicic acid.
• Organic colloids: These are humic substances (humic and fulvic acids), as well as proteins and other high-molecular-weight organic compounds.
• Organomineral complexes: These are complex "unions" of clay particles and humus, linked by chemical bonds. This form often predominates in actual soils, especially in the upper horizons.

"Ability to absorb, retain, and exchange": This is the primary function of the SAC, which makes it the regulator of the soil's chemical life. The SAC acts as a giant ion exchanger or a "reservoir" of nutrients.

It is important to emphasize: the SAC is not a separate phase that can be extracted from the soil. It is a functional complex, determined by particle size and their properties.

The Main Secret: Why Surface, Not Mass?

The key to understanding the unique role of the SAC lies in two fundamental physicochemical phenomena: enormous specific surface area and surface energy.

Imagine a piece of rock with a mass of 1 kg. Its surface area is small, and the chemical activity of this surface is insignificant. Now, crush this rock into colloidal particles. The total mass will remain the same, but the total surface area will increase millions of times! If a cube with a 1 cm edge has a total surface area of 6 cm², when crushed into colloidal particles, this surface area can increase up to 6 hectares (Mukha et al., 2003).

This enormous surface is not just a geometric abstraction. Atoms and ions on the surface of a solid body have unsaturated, incompletely compensated force fields. This creates so-called free surface energy. The system tends to reduce this energy and does so by attracting and retaining other molecules and ions from the surrounding environment (the soil solution). This process is called adsorption (Weil and Brady, 2017).

Thus, the chemically active part of the soil is not its entire mass but its surface, and the contribution of the colloidal fraction here is decisive. It is here, at the phase boundary "solid particle — soil solution," that the main chemical reactions regulating soil fertility take place.

Conclusions from Part One

So, we have formed a basic understanding of the Soil Absorbing Complex.

1. It is a historically established and scientifically substantiated concept, key to understanding soil chemistry. From Way's early observations to Gedroits's fundamental works, our knowledge of the SAC has come a long way.

2. The SAC is not just the "fine fraction." It is a complex, polyfunctional system of highly dispersed particles, consisting of clay minerals, humus, and their complexes.

3. The main reason for its exceptional role is the colossal specific surface area and the associated free surface energy, which transform the finely dispersed fraction into the main chemical reactor of the soil.

In the next part, we will analyze in detail the specific types of absorption carried out by the SAC (mechanical, physical, physicochemical, chemical, and biological) and how this determines its ability to retain ions and molecules, which, in turn, affects all key processes in the soil — from buffering to fertility.

1. Charge as the Basis of Chemical Activity

So, we have established that the chemical reactor of the soil is the surface of its colloidal particles. However, the surface itself is merely a "stage" or a "platform." For reactions to begin on this platform, it must be charged. Imagine a piece of paper and a charged comb. It is the charge that makes the light pieces of paper attract to the comb. Similarly, the electrical charge on the surface of colloidal particles causes them to attract and retain ions and molecules from the soil solution.

The magnitude of this charge, its origin, and its ability to change determine the unique chemical activity of the soil. Without a charge, colloidal particles would be just inert filler. Thanks to the charge, they become sorption centers, retaining a huge number of nutrients, water, and even entire molecules of organic substances (Weil and Brady, 2017).

2. Two Sources of Charge: Permanent and Variable

Colloidal particles in the soil carry an electrical charge that arises from two fundamentally different sources. The properties of a particular soil depend on which source predominates.

3. The Double Electric Layer (DEL): How Charge Retains Ions

The presence of a charge on the surface of a colloidal particle creates a powerful electric field around it. This field attracts ions of the opposite sign (counterions) from the soil solution. As a result, a special structure forms near the surface — the double electric layer (DEL) (Eash et al., 2016; Weil and Brady, 2017).

It can be represented simply as follows:

1. Core (granule): This is the colloidal particle itself, carrying a negative (in most cases) charge.

2. Adsorption (or Stern) layer: This is a layer of counterions (cations) that are attracted to the surface very closely and firmly. They partially lose their hydration shell and are retained on the surface by electrostatic forces (White, 2006).

3. Diffuse layer: This is a layer of counterions located slightly further from the surface. Their bond with the particle is weaker; they are surrounded by a water shell and are in constant but more free motion than the ions in the Stern layer (Eash et al., 2016).

It is the ions in the diffuse layer that are the exchangeable cations, which can readily exchange with other cations from the soil solution. This constant exchange occurring on the colloid surface is the main mechanism by which the soil can retain nutrients, preventing them from being leached, while still making them available to plant roots (Weil and Brady, 2017). This system is electrolytically balanced, and the sum of all charges (positive and negative) within the DEL is always zero.

4. Conclusions from Part Two

Now we can answer the question "why is soil chemically active?".

1. Because its colloidal fraction carries an electrical charge. This charge can be permanent (resulting from isomorphous substitution in clays) or variable (resulting from the dissociation of surface OH groups, pH-dependent).

2. It is the presence of the charge that creates a double electric layer around the particles, serving as a kind of "trap" for ions from the soil solution.

3. Thus, the chemical activity of the soil is not a property of its mass but a property of its charged surface. This fundamental principle underlies all further processes we will study: ion exchange, buffering, acidity, and the aggregation of soil particles.

In the next part of the lecture, we will move on to the main topic — the examination of "Reactive Soil Surfaces." We will analyze which specific ions and molecules can interact with the SAC and how this interaction determines its ability to be the central regulator of all soil processes.

1. From Macro to Micro- and Nano-Levels

So, we know that colloidal particles are tiny "chemical reactors." But what exactly makes their surface capable of participating in reactions? The answer lies in its chemical structure. The surface of a colloidal particle is not smooth and uniform. It consists of numerous functional groups — atoms or groups of atoms capable of chemical interactions (Sposito, 1984; Sparks, 2003). These functional groups determine the reactivity of the surface, its ability to adsorb specific ions and molecules.

Modern science uses the concept of "Reactive Soil Surfaces" to emphasize that the chemical activity of soil is not an abstract property but the result of the work of specific, well-studied molecular "instruments" on the surface of colloidal particles (Huang et al., 2012). The key idea here is that different surfaces have different reactivity. The surface of a humus particle will behave differently than the surface of a montmorillonite crystal or an iron oxide.

2. Types of Reactive Surfaces in Soil

Three main types of surfaces can be distinguished in soil, differing in their nature and, consequently, in the type and strength of interaction with substances from the soil solution.

3. Inner- and Outer-Sphere Complexes: Two Types of Binding

The presence of different types of reactive surfaces leads to ions and molecules being retained in two fundamentally different ways. This has significant implications for their availability to plants and their mobility in the soil (Weil and Brady, 2017).

1. Outer-sphere complex: The ion (typically a cation) is retained on the surface purely by electrostatic attraction. The ion retains its hydration shell (water molecules) between it and the surface. Such a complex is weak and reversible. Ions held in outer-sphere complexes are readily exchangeable and easily available to plants. This is typical for binding on the siloxane surfaces of 2:1 clays (Eash et al., 2016).

2. Inner-sphere complex: The ion loses its hydration shell and forms a direct, stronger chemical bond with the surface functional group. Such a complex forms as a result of a chemical reaction (e.g., ligand exchange on hydroxyl surfaces). It is much stronger, often irreversible or weakly reversible. Ions in inner-sphere complexes are less available to plants and move slowly in the soil. This is typical for binding phosphates, heavy metals, and some organic molecules (Sparks, 2003; White, 2006).

4. Conclusions from Part Three

We have moved to a new level of understanding. We have seen that:

1. The concept of "Reactive Soil Surfaces" is the key to understanding the molecular mechanisms of the SAC's operation.

2. There are three main types of such surfaces in the soil:

• Siloxane (mainly on 2:1 clays) with a permanent charge, providing weak and reversible cation binding.
• Hydroxyl (on oxides, kaolinite, crystal edges) with a variable, pH-dependent charge, capable of binding both cations and anions, sometimes very strongly (ligand exchange).
• Organic (humus), with exceptionally high diversity of functional groups, providing powerful cation exchange and complexation capacity.
• The nature of substance binding on these surfaces can vary: from weak and reversible (outer-sphere complex) to strong and often irreversible (inner-sphere complex).

Understanding the nature of reactive surfaces allows us to answer fundamental questions: why nitrogen (as nitrates) is readily leached while phosphorus is fixed in the soil; why some soils retain potassium well while others do not; why aluminum becomes toxic in acidic soils; and even why some pollutants can be firmly bound while others migrate. This is the foundation upon which all complex chemical processes in the soil are built.

In the next, concluding part of our introductory lecture, we will move on to the practical significance of this knowledge and discuss which specific processes the SAC regulates and how this relates to soil fertility.

1. Introduction: The SAC — The Soil's Central Processor

So, armed with knowledge about the nature of colloidal surfaces, we can see that the SAC is not a passive accumulator of substances. It is an active regulator, the "central processor" of the soil, managing a cascade of key processes. By interacting with the soil solution, the solid phase, the gas phase, and plant roots, the SAC performs a function without which the soil would simply be an inert mixture of minerals and water.

We can distinguish five main groups of processes that are under the control of the SAC:

1. Ion retention and exchange (sorption and ion exchange).

2. Providing soil buffering capacity.

3. Formation and regulation of acidity and alkalinity.

4. Influence on soil aggregation and structure.

5. Regulation of element migration in the soil profile and landscape.

Let's examine each of them in more detail.

2. Ion Retention and Exchange (Ion Exchange)

This is undoubtedly the main and most well-known function of the SAC. We have already discussed the double electric layer and that ions in the diffuse layer are in a state of constant but weak retention. This is the basis for a reversible process — ion exchange (Eash et al., 2016; Weil and Brady, 2017).

How does it work?

There is a constant "exchange of places" between ions adsorbed on the colloid surface (e.g., Ca²⁺) and ions in the soil solution (e.g., K⁺). If a large amount of potassium ions enters the solution, they can displace calcium ions from the surface, which then enter the solution, and vice versa (Sparks, 2003). This exchange is strictly equivalent: two monovalent K⁺ ions replace one divalent Ca²⁺ ion (Weil and Brady, 2017).

• Cation Exchange (CEC): Since most soil colloids are negatively charged, the soil possesses cation exchange capacity (Cation Exchange Capacity, CEC). The CEC is a quantitative measure of the soil's ability to retain exchangeable cations, expressed in cmol(equiv)/kg (Weil and Brady, 2017). The CEC determines the "capacity" of the soil's nutrient reservoir for cations (Ca²⁺, Mg²⁺, K⁺, NH₄⁺). The higher the CEC, the more these cations the soil can retain, protecting them from leaching.
• Anion Exchange (AEC): In soils where colloids with a variable positive charge predominate (acidic, rich in Fe and Al oxides, kaolinitic), the ability for anion exchange (Anion Exchange Capacity, AEC) is also manifested (White, 2006). Such soils can retain anions, such as sulfates, chlorides, nitrates, and also phosphates (the latter through the mechanism of ligand exchange, see below).

Agronomic significance: Thanks to ion exchange, the soil can accumulate reserves of nutrients in a form available to plants and gradually release them as needed. This is a natural ion-exchange buffer for plant nutrition (Eash et al., 2016; Weil and Brady, 2017).

3. Soil Buffering

Buffering is the soil's ability to resist changes in its reaction (pH) when acids or bases are added (Mukha et al., 2003). The SAC plays a key role here, acting as a huge reservoir of ions that can neutralize incoming acids or bases.

How does it work?

During acidification (addition of acid): If an acid enters the soil, excess H⁺ ions appear in the soil solution. These ions can displace calcium, magnesium, and other base cations from the colloid surface. The bases released into the solution (e.g., Ca²⁺) bind with the acid anions, neutralizing it. The SAC itself becomes saturated with hydrogen ions. The process can be represented schematically as follows (Mukha et al., 2003):

[SAC]Ca²⁺ + 2H⁺ → [SAC]2H⁺ + Ca²⁺.

During alkalinization (addition of base): Conversely, if alkali (OH⁻) enters the soil, OH⁻ ions will bind with hydrogen ions from the soil solution, reducing their concentration. In response, the SAC releases H⁺ ions into the solution, compensating for their loss. As a result, the pH of the solution changes only slightly:

[SAC]2H⁺ + 2OH⁻ → [SAC]Ca²⁺ + 2H₂O (assuming calcium is present in the solution) (Mukha et al., 2003).

Factors affecting buffering:

The buffering capacity of a soil is higher, the higher its CEC, the richer it is in humus, and the more clay minerals it contains, especially from the smectite and vermiculite groups (Mukha et al., 2003; Weil and Brady, 2017). Light sandy soils with low CEC have extremely weak buffering, and their reaction can change dramatically even with small doses of fertilizers (Mukha et al., 2003).

4. Formation and Regulation of Acidity and Alkalinity

Soil acidity or alkalinity is not a static property. It results from a complex equilibrium between ions in the soil solution and exchangeable ions on the SAC surface (Eash et al., 2016; Mukha et al., 2003).

• Actual acidity: This is the acidity of the soil solution, determined by the concentration of free H⁺ ions (pH). It depends on the presence of soluble acids (e.g., carbonic, organic acids) and hydrolytically acidic salts in the solution.
• Potential acidity: This is the "reserve" of acidity associated with exchangeable H⁺ and Al³⁺ cations in the SAC. When these ions are displaced from the SAC by other cations (e.g., Ca²⁺ during liming), they enter the solution and acidify it. Potential acidity is divided into exchangeable (displaced by a neutral salt, e.g., KCl) and hydrolytic (displaced by a hydrolytically alkaline salt, e.g., sodium acetate, and reflects the total reserve of acidity). Hydrolytic acidity is used to calculate lime requirements (Mukha et al., 2003).

Agronomic significance: The composition of exchangeable cations in the SAC directly determines the soil reaction, and thus the availability of nutrients, the activity of microorganisms, and aluminum toxicity (Eash et al., 2016). Soils with a predominance of hydrogen and aluminum ions in the SAC are acidic; with a predominance of calcium and magnesium — neutral or slightly alkaline; with a large amount of sodium — alkaline and solonetzic (Mukha et al., 2003).

5. Aggregation and Soil Structure

Although soil structure is a physical parameter, its formation directly depends on chemical processes related to the SAC (Weil and Brady, 2017). The SAC acts as a "cement," binding individual sand and silt particles into larger aggregates (crumbs).

How does it work?

Coagulation of colloids: Fine colloidal particles, being in a sol state (suspension), can clump together, transitioning into a gel state (precipitate). This process is called coagulation (Mukha et al., 2003).

Role of cations: The nature of coagulation is decisively influenced by the cations in the diffuse layer of the colloids.

• Calcium (Ca²⁺) and iron (Fe³⁺) are strong coagulators. They bring colloidal particles closer together, promote their clumping, and the formation of a stable, water-resistant structure. Therefore, chernozems saturated with calcium have an excellent granular structure (Mukha et al., 2003; Weil and Brady, 2017).
• Sodium (Na⁺) acts in the opposite way. It promotes peptization — the transition of gel to sol, dispersion of colloids. This destroys the structure, making the soil sticky, viscous, prone to slaking, and crust formation. This is characteristic of solonetz soils, where there is a lot of sodium in the SAC (Weil and Brady, 2017; White, 2006).

Agronomic significance: Good structure is the basis of physical fertility. It ensures optimal water-air regime, ease of tillage, and resistance to erosion. By regulating the composition of exchangeable cations, we can influence soil structure (e.g., through gypsum application to solonetz soils or liming acidic soils) (Mukha et al., 2003).

6. Regulation of Element Migration

The SAC is the main "filter" and "barrier" regulating the migration of chemical elements through the soil profile and the landscape.

Slowing the migration of nutrients: Thanks to ion exchange, most nutrient cations (Ca, Mg, K) and many anions (especially phosphates) are retained in the soil profile, not leaching into groundwater. This allows plants to use them effectively.

Regulating pollutant migration: The SAC can also retain toxic elements.

• Heavy metals (Pb, Cd, Cu, Zn) are strongly bound to humus and Fe/Mn oxides, often forming inner-sphere complexes. This prevents them from entering groundwater and plants (Weil and Brady, 2017). The nature of this binding depends on the type of reactive surface.
• Radionuclides, e.g., ¹³⁷Cs, can be fixed in the interlayer spaces of vermiculite and illite, limiting their uptake by plants (Weil and Brady, 2017).
• Organic pollutants (pesticides) can be adsorbed on humus, which also limits their migration and determines their bioavailability.

• "Negative adsorption" (anion exchange) for pollutant anions: At the same time, for anions that are weakly retained (e.g., nitrates NO₃⁻, chlorides Cl⁻), the SAC is not a barrier. They easily migrate with the water flow. This explains why nitrate contamination of groundwater is a common problem (Weil and Brady, 2017).

Thus, the SAC is the main regulator of geochemical element migration, acting as a "pump" retaining some elements in the root zone and as a "filter" allowing others to pass through.

7. Conclusions from Part Four

We see that the SAC is truly a multifunctional system regulating the key chemical and physicochemical properties of the soil:

1. Ion exchange (CEC/AEC) — provides the soil's ability to retain and gradually release nutrients.

2. Buffering — protects the soil environment from sharp pH fluctuations, making it resistant to external influences.

3. Acidity/alkalinity — determined by the composition of exchangeable cations in the SAC and directly affects nutrient availability and microbial activity.

4. Aggregation — promotes the formation of water-stable structure, improving the physical properties of the soil.

5. Element migration — controls the movement of both nutrients and pollutants through the soil profile and the environment.

All these processes are closely intertwined and managed by a single center — the Soil Absorbing Complex, which is based on the charged reactive surfaces of colloidal particles.

In the next, concluding part of our lecture, we will discuss how all this knowledge connects to the central concept of agronomy — soil fertility — and what role the SAC plays in it.

Fertility as a Result of SAC Operation

Fertility is a fundamental, integral property of the soil. In its most general sense, soil fertility is understood as the soil's ability to provide plants with the factors and conditions necessary for their life activities, including nutrient, water-air, temperature, and redox regimes (Mukha et al., 2003). It is not a static quantity but a dynamic characteristic formed through the complex interaction of natural and anthropogenic factors.

The key point we must realize is: The Soil Absorbing Complex is the central, system-forming link determining the level of fertility. It is the SAC that is the "lever" through which nature, and subsequently humans, manage fertility. Without reactive surfaces, without their ability for ion exchange, buffering, and structure formation, the soil would be an inert medium, and it would be impossible to speak of fertility in the modern sense (Weil and Brady, 2017).

To understand this connection, we need to examine how the functions of the SAC, which we discussed in the previous part, manifest at the level of soil fertility.

2. The SAC as the Basis of the Plant Nutrient Regime

The nutrient regime is the soil's ability to meet plant needs for nutrients throughout the growing season. This is one of the most important components of fertility, directly dependent on the properties of the SAC.

Cation Exchange Capacity (CEC) as a Measure of Potential Fertility.

A quantitative measure of this ability is the cation exchange capacity (CEC) (Mukha et al., 2003; Weil and Brady, 2017). Soils with a high CEC (chernozems, rich in humus and smectitic clays) possess high potential fertility. They can retain in exchangeable form a large reserve of nutrient cations: calcium, magnesium, potassium, ammonium. These elements are not leached but stored in the SAC "reservoir," gradually being released into the soil solution as needed by plants (Eash et al., 2016).

Conversely, soils with low CEC (sandy, poor in humus, or dominated by kaolinite) have low potential fertility. Their "reservoir" has small capacity, and nutrient cations are quickly lost with percolating water, making the soil poor. The CEC, along with the composition of exchangeable cations, is a critical indicator used for assessing soil fertility and developing strategies for their cultivation (Weil and Brady, 2017).

Composition of Exchangeable Cations: The Quality of Fertility.

But fertility is determined not only by quantity (CEC) but also by the quality of the retained ions. The composition of exchangeable cations in the SAC is the soil's "chemical portrait," directly influencing its fertility.

Predominance of calcium (Ca²⁺) and magnesium (Mg²⁺): This is a sign of high cultivation and fertility. These cations not only serve as nutrients. As we already know, they are strong coagulators, promoting the formation of a stable structure. Moreover, calcium blocks the uptake of toxic heavy metal ions and radionuclides by plants and creates a physiologically balanced soil solution favorable for roots and microorganisms (Mukha et al., 2003). Saturation of the SAC with calcium is the key to soil health.

Predominance of hydrogen (H⁺) and aluminum (Al³⁺): This is characteristic of acidic, base-unsaturated soils. Such soils possess a whole range of negative properties for fertility:

1. Aluminum toxicity: Mobile forms of aluminum appearing in acidic environments (pH < 5) are toxic to the roots of many crop plants, inhibiting their growth and development (Eash et al., 2016; Mukha et al., 2003).

2. Reduced nutrient availability: In acidic conditions, phosphorus is fixed into less available forms, and molybdenum becomes less available. This leads to nutrient deficiencies, even if the total element content in the soil is high.

3. Structural deterioration: Lack of calcium and excess hydrogen promote colloid dispersion and structure destruction.

4. Microflora inhibition: Acidic conditions suppress the development of beneficial microorganisms, including nitrogen fixers and nitrifiers.

Thus, the composition of the SAC is directly related to effective fertility — the part of potential fertility that is actually available to plants under specific conditions (Mukha et al., 2003). A high content of hydrogen and aluminum represents "locked" fertility that needs to be "unlocked" through chemical amelioration (liming), replacing these ions with calcium.

3. The SAC and Physical Fertility (Water-Air Regime)

We have already mentioned that the SAC influences aggregation. This is a direct link to physical fertility.

• Water-stable structure is the result of colloid coagulation under the action of calcium and other polyvalent cations. An agronomically valuable structure (granular, crumbly) is the key to an optimal water-air regime (Weil and Brady, 2017). In such soil, there are both large pores (for air and rapid drainage of excess water) and small pores (for water retention), creating ideal conditions for root system development and aerobic microorganisms.
• When the SAC is saturated with sodium, colloids peptize, and the structure is destroyed. The soil becomes structureless, slakes, forms a dense crust upon drying, which sharply worsens water permeability and aeration, and thus physical fertility (Mukha et al., 2003).

Therefore, the chemical state of the SAC directly determines whether the soil will be "loose" and air-permeable or "slaking" and cloddy.

4. The SAC and Buffering: Stability of Fertility

Buffering is the most important characteristic ensuring the stability of fertility. Thanks to buffering, the soil can withstand adverse influences, such as acidification from nitrogen fertilizers or acid rain, or alkalinization from irrigation water.

• A high CEC, a large reserve of humus and calcium in the SAC, is the key to high buffering capacity. Such soil retains an optimal reaction medium longer, and its fertility is more resistant to anthropogenic pressures (Mukha et al., 2003; Weil and Brady, 2017).
• Sandy, low-humus soils with low CEC have low buffering. Their pH is easily "shifted," which can quickly lead to fertility degradation.

5. Conclusions from Part Five

So, we can formulate the key conclusion of our entire introductory lecture:

Soil fertility is an integral, systemic property of the soil, which is a result of the functioning of the Soil Absorbing Complex.

The connection between the SAC and fertility is manifested through:

1. Nutrient regime: The CEC determines the capacity of the nutrient cation "reservoir," and the composition of exchangeable cations (Ca²⁺ or H⁺/Al³⁺) determines their availability and the presence of toxic factors.

2. Physical properties: The SAC's ability to coagulate under the influence of calcium forms a water-stable structure, ensuring an optimal water-air regime.

3. Buffering: The SAC ensures the stability of fertility against external influences, protecting the soil from drastic changes in properties.

4. Element migration: The SAC regulates the movement of both nutrients and pollutants, performing an ecological function.

Ultimately, potential fertility, established by nature (the set of clay minerals, humus content), and economic fertility, created by human labor (changing the SAC composition through amelioration), are inextricably linked through the same object — the Soil Absorbing Complex. Understanding its structure and the laws governing its operation is the key to competent, scientifically based management of soil fertility.

This concludes our introductory lecture. In the following modules, we will analyze in detail each of the processes we discussed today: ion exchange, acidity, buffering, the role of organic matter and clay minerals. We will move from the general to the specific, deepening our knowledge of how this amazing natural mechanism — the Soil Absorbing Complex — works.