Geochemical Processes and Migration of Substances

1.1. What is Soil Geochemistry?

Soil geochemistry is the science of the distribution and behaviour of chemical elements in the soil profile, their migration and accumulation under the influence of natural and anthropogenic factors (Mukha et al., 2003). It is, if you will, the “passport office” of chemical elements in the soil: where they are, where they move, in what forms, and why.

Soil is not just an accumulation of mineral particles. It is a complex polycomponent multiphase system consisting of four physical phases (Mukha et al., 2003):

• Solid phase – mineral skeleton and organic matter
• Liquid phase – soil solution
• Gas phase – soil air
• Living phase – soil organisms

It is the interaction of these phases that creates a unique environment where chemical elements do not simply lie still but actively move, participate in reactions, and change their forms.

1.2. Soil as a Bio-Abiotic System

Vernadsky called soil a bio‑abiotic body – that is, a body where organic (living) and inorganic (abiotic) matter are in inseparable unity and constant interaction (Mukha et al., 2003). This is fundamentally important for understanding migration, because it is living organisms that are the main “conductors” of this process. They extract elements from the soil to build their bodies, and after death they return them, but in an altered form.

1.3. Cycles of Matter

Now let us imagine the movement of elements on a broader scale. In nature, there are three main cycles of matter (Mukha et al., 2003):

1. Geological (large) cycle – the formation of rocks, their weathering, transport and redeposition. This is a process spanning millions of years, during which elements move from the Earth’s interior to the surface layers and back.

2. Biological (small) cycle – the movement of elements in the system “plants → soil → plants”. Plants extract elements from the soil, create biomass; after death, organic residues decompose and elements return to the soil. Solar energy is the main driving force of this process.

3. Anthropogenic cycle – the movement of substances as a result of human activity: mining, production, consumption, waste generation, environmental pollution. Today, this cycle often exceeds natural processes in intensity (Mukha et al., 2003).

For soil science, the small biological cycle is of key importance. It is this cycle that creates the main difference between soil and parent material – the accumulation of organic matter, humus, and biogenic elements.

1.4. Why Do Chemical Elements Migrate?

Let us return to our main question. There are several fundamental reasons:

The first reason is thermodynamic disequilibrium. Soil is an open system that constantly exchanges matter and energy with the environment. The minerals that make up the soil were formed at high temperatures and pressures deep within the Earth. On the surface, they find themselves in completely different conditions and tend to reach a more stable state (Birkeland, 1984; Buol et al., 2011). We call this process weathering and soil formation.

The second reason is water. Water is a universal solvent. It not only transports substances but also serves as a medium for chemical reactions. It is with water that ions enter the soil and “attack” minerals, forcing elements to go into solution (Weil and Brady, 2017).

The third reason is living organisms. Plant roots excrete organic acids that dissolve minerals. Microorganisms oxidise and reduce elements, converting them from one form to another. Biota creates a unique geochemical environment that does not exist in lifeless rocks (White, 2006).

The fourth reason is gradients. Elements always move from where there is a lot of them to where there is little (diffusion). They move from high‑potential regions to low‑potential regions. They move upward through capillaries, downward with rainwater, sideways along slopes. Soil is a medium of continuous gradients, which ensures the constant movement of substances.

Thus, soil stability is the stability of processes, not the repose of elements. Soil is in a state of dynamic equilibrium, where the input and output of substances, their transformation and movement are mutually balanced.

1.5. Geochemistry and Soil Formation

Now it is important to understand how geochemical processes are related to the formation of the most important property of soil – fertility.

Chemical elements, leaving the parent material, undergo a complex journey. Some are removed beyond the soil profile (this is the leaching pathway), others accumulate in certain horizons, still others become part of newly formed minerals and organic matter. It is this redistribution of elements that creates profile differentiation – the division of soil into horizons that differ in colour, structure, and chemical composition (Birkeland, 1984).

During soil formation, some elements (silicon, calcium, magnesium, sodium, potassium) are relatively easily leached from the upper part of the profile, while others (iron, aluminium, humic substances) may accumulate in the middle and lower horizons. This selective redistribution of elements determines the appearance of different soil types (Buol et al., 2011).

1.6. Driving Forces of Geochemical Processes

Now that we understand why elements move, let us briefly list the main forces that set this movement in action:

• Gravity – causes water and the substances dissolved in it to move downward, into the soil profile.
• Capillary forces – raise water with dissolved substances from groundwater upwards.
• Temperature gradients – induce movement of water in the vapour phase, transport of substances with heat flows.
• Concentration gradients (diffusion) – equalise concentrations, causing substances to move from high‑content areas to low‑content areas.
• Potential gradients (matric, osmotic, hydraulic) – control water movement, and with it the movement of dissolved substances.
• Electrochemical gradients – cause ions to move towards or away from charged surfaces of colloidal particles (White, 2006).
• Biological activity – roots, microorganisms, and soil animals actively redistribute substances in the soil.

2.1. Classification of Chemical Elements in Soil

In soil science, elements are conventionally divided according to their content in the solid phase (Mukha et al., 2003; Foth, 1990). This classification is not only quantitative but also functional: the content of an element often determines its role in soil processes.

Macroelements – elements whose content in soil exceeds 0.01% (or 100 mg/kg). These include:

• silicon (Si),
• aluminium (Al),
• iron (Fe),
• calcium (Ca),
• magnesium (Mg),
• sodium (Na),
• potassium (K),
• as well as carbon (C), oxygen (O), hydrogen (H), nitrogen (N), phosphorus (P), sulfur (S).

The first seven are the main elements of the mineral part of soil. Their total content in the Earth’s crust is about 98% (Mukha et al., 2003). They form the mineral skeleton of the soil and determine its main physicochemical properties. Nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium are plant nutrients, but from the soil science point of view they are interesting because of how they transform from mineral forms to plant‑available forms and how they migrate in the profile (Foth, 1990).

Microelements – elements whose content in soil ranges from 0.001% to 0.0001% (10–1 mg/kg). These are manganese (Mn), copper (Cu), zinc (Zn), cobalt (Co), boron (B), molybdenum (Mo), iodine (I), fluorine (F), selenium (Se), and others (Mukha et al., 2003; Foth, 1990). Their content is small, but they are vitally necessary for plants, animals and microorganisms. In soil science, microelements are valuable because their behaviour (mobility, forms of occurrence) is very sensitive to changes in pH and redox conditions. Therefore, they serve as indicators of geochemical processes.

Ultramicroelements – content less than 0.0001% (less than 1 mg/kg). These include gold, silver, mercury, uranium, radium and other rare elements. They usually do not play a significant role in soil formation, except in cases of technogenic pollution.

However, classification by content is not the only one. For us, a more important one is by biological role and mobility.

2.2. Biophilic and Non‑Biophilic Elements

Biophilic elements are elements actively involved in the biological cycle. They are part of living organisms, extracted from soil by plants and returned with plant litter (Foth, 1990; Mukha et al., 2003). They include:

• Organogens: C, H, O, N, P, S – form the basis of organic matter.
• Cationic elements: K, Ca, Mg, Fe, Mn, Cu, Zn, Mo, Co, B (as borates) – necessary for enzymatic systems and structural functions.

For soil science, it is important that biophilic elements are those actively moving in the small cycle. Their migration is largely controlled by biota.

Non‑biophilic elements – do not enter living organisms in appreciable amounts or are not essential. For example, titanium (Ti), zirconium (Zr), most heavy metals (lead, cadmium, chromium). They are usually immobile and accumulate in soil, or, conversely, may migrate with technogenic flows.

2.3. Macroelements and Their Role in Soil Formation

Now let us consider the main macroelements from the soil science perspective, emphasising their behaviour in the soil profile.

2.4. Microelements and Their Significance

Microelements, although present in trace amounts, play a key role in the enzymatic systems of plants and microorganisms. Their behaviour in soil is extremely sensitive to geochemical conditions, so they are excellent indicators of soil processes (Foth, 1990; Weil and Brady, 2017).

Let us consider the most important microelements in the soil science context.

Manganese (Mn). The behaviour of manganese is similar to that of iron, but it reduces at higher Eh (see Section 5). Therefore, in soils with periodic waterlogging, manganese is often more mobile than iron, and its concretions may form under less pronounced reducing conditions (Weil and Brady, 2017; Foth, 1990). Manganese forms black oxides (pyrolusite) and is a strong oxidiser.

Copper (Cu), zinc (Zn), cobalt (Co). These elements are usually bound to organic matter and oxides of iron and manganese. Their mobility sharply increases in acidic conditions and in the presence of organic chelating agents (fulvic acids). In alkaline soils, they are often unavailable to plants (micronutrient deficiency) (Foth, 1990; Weil and Brady, 2017).

Boron (B). Boron occurs in the soil solution as undissociated boric acid H₃BO₃ or borate ion B(OH)₄⁻. It is readily soluble and easily leached under humid conditions. In arid conditions it can accumulate to toxic concentrations (Foth, 1990; Weil and Brady, 2017).

Molybdenum (Mo). Unlike most microelements, molybdenum is more available in alkaline soils, where it occurs as molybdate ion MoO₄²⁻. In acidic soils it is strongly sorbed by iron and aluminium oxides (Foth, 1990). This is a rare case where a microelement is more mobile in alkaline conditions.

Heavy metals (Pb, Cd, Hg, Cr, As). They are not essential for plants and are toxic at elevated concentrations. In soils they are often in bound forms – adsorbed on clay minerals, organic matter, oxides, or as sparingly soluble precipitates (sulfides, carbonates, phosphates). Their mobility is usually low, but may increase under acidification, reducing conditions, or in the presence of organic chelating agents. This is important for understanding the behaviour of contaminants in soil (Weil and Brady, 2017; Foth, 1990).

2.5. Link between Macro‑ and Microelements and Pedogenic Processes

Now it is important to combine this knowledge. Element migration is not random wandering but a strictly regular process determined by a set of factors. Different pedogenic processes are characterised by different combinations of migrating and accumulating elements (Birkeland, 1984; Scheffer et al., 2018).

Podzolisation:

• Eluvial horizon (A₂, Ae): removal of Fe, Al, Mn, organic substances, partly clay.
• Illuvial horizon (Bh, Bs): accumulation of Fe, Al, organic matter, sometimes Mn.
• Microelements: podzols are often depleted in Cu, Zn, Co, Mo (leached with organic complexes).

Sod process (Chernozems):

• Accumulation of Ca, Mg, K, P, S, microelements in the humus horizon.
• Leaching of readily soluble salts from the upper part, formation of a carbonate horizon (Bk) in the middle part.

Gley process:

• Reduction of Fe and Mn → their dissolution and removal → formation of bluish‑grey colours, concretions of Fe and Mn at the redox boundary.
• Accumulation of organic matter (slowed decomposition) → increase in carbon, nitrogen, sulfur content.

Solonetz process:

• Accumulation of Na⁺ in the exchange complex, colloid dispersion, deterioration of structure.
• Downward migration of Na⁺ with formation of an illuvial solonetz horizon.

2.6. Silicon and Aluminium as Structure‑Forming Elements

Silicon and aluminium deserve special mention. They are not just macroelements – they are structure‑forming elements of the soil mass. Their ratio and forms determine the type of clay minerals:

• In slightly weathered soils (young, arid), 2:1 clay minerals predominate (montmorillonite, illite, vermiculite), with a high Si:Al ratio (about 2:1).
• In strongly weathered soils (humid tropics, podzols), desilication occurs – removal of silicon and accumulation of aluminium and iron. This leads to the formation of kaolinite (Si:Al = 1:1), gibbsite, hematite (Birkeland, 1984; Buol et al., 2011).

Thus, the migration of silicon and aluminium is not just a chemical process, but a criterion of the stage of soil formation. From the Si/Al ratio, from the content of mobile forms of these elements, we can judge the age of the soil and the direction of weathering.

2.7. Knowledge of Elements for Understanding Migration

So, we see that each element has its own “character” and its own history in the soil. Some elements (Na, Ca, Mg, Cl, S) readily leave the soil with water, others (Fe, Al, P, heavy metals) usually stay behind. But even they can migrate when pH, Eh change, or in the presence of organic ligands.

For the soil scientist it is important to know:

1. In which forms the element occurs in the soil (mineral, exchangeable, dissolved, organic).

2. Which factors affect the transition from one form to another (pH, Eh, organic matter, biota).

3. Where and at what speed the element can move (downward, upward, sideways).

4. Where and under what conditions it will accumulate (at geochemical barriers).

This knowledge will form the basis for our understanding of geochemical barriers, to which we will turn at the end of the lecture.

3.1. Principle: Forms Determine Mobility

For soil science, the following rule is fundamental: the mobility of an element is determined by its form, not only by its chemical nature. The same element can be:

• Immobile – if it is in the crystal lattice of a primary mineral. Such an element does not participate in migration until the mineral is destroyed. For example, potassium in the crystal lattice of feldspar – immobile.

• Weakly mobile – if it is part of a secondary mineral or in exchangeable form (adsorbed on colloid surfaces). Such an element can be involved in migration when conditions change (pH change, cation replacement). For example, potassium in exchangeable form on clay particles – already available to plants, but not yet leaving the soil.

• Highly mobile – if it is in the soil solution as ions or molecules. Such an element actively moves with water and can be removed from the profile (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

In addition, an element may occur in organic form (as part of humus, organic acids, chelates) – and then its mobility is determined by the properties of the organic molecules, not only the element itself (Foth, 1990; Scheffer et al., 2018).

Thus, the soil scientist must always ask: in what form is the element? This is what determines whether it will migrate.

3.2. Mineral Form

The mineral form is the most common form of occurrence of elements in the solid phase of soil. It includes all elements that are part of minerals – both primary and secondary.

3.3. Organic Form

The organic form consists of elements that are part of soil organic matter (humus) and living organisms (Foth, 1990; Scheffer et al., 2018; Mukha et al., 2003). It is especially important for biophilic elements: carbon, nitrogen, phosphorus, sulfur, and also for microelements that form complexes with organic substances.

3.4. Organo‑Mineral Form

Special attention should be paid to the organo‑mineral form (Scheffer et al., 2018; Weil and Brady, 2017). This is a form in which organic substances are bound to mineral particles – clay minerals, oxides of iron and aluminium.

How does this happen? Humic substances have the ability to adsorb on the surface of mineral particles. This occurs through:

• Cationic bridges – cations (Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺) bind negatively charged organic molecules to negatively charged surfaces of clay minerals (Weil and Brady, 2017; White, 2006).
• Ligand exchange – organic anions (e.g., carboxyl groups) can replace OH⁻ groups on the surface of iron and aluminium oxides, forming strong bonds (White, 2006; Scheffer et al., 2018).
• Hydrogen bonds – weaker bonds between polar groups of organic molecules and surface OH groups of minerals (White, 2006).

Why is this important for migration?

1. Organo‑mineral complexes protect organic matter from mineralisation. Organic molecules adsorbed on minerals are less accessible to microorganisms. Therefore, in organo‑mineral form, carbon can be preserved for hundreds and thousands of years (Scheffer et al., 2018).

2. Organo‑mineral complexes change the surface charge of minerals. Upon adsorption of organic molecules, the negative charge increases, which enhances cation retention (increases CEC). This is an important mechanism, especially in acidic soils (Foth, 1990; White, 2006).

3. Organo‑mineral complexes affect soil structure. They glue mineral particles into aggregates. And aggregation determines pore space, water permeability, and conditions for migration of water and dissolved substances (Scheffer et al., 2018; Weil and Brady, 2017).

4. Organo‑mineral complexes are “traps” for microelements and heavy metals. Metals can bind both to the organic part and to the mineral surface, and in the organo‑mineral complex they are especially strongly fixed (Weil and Brady, 2017; Foth, 1990).

The organo‑mineral form is a compromise between mobility and immobility. It lies “between” the two poles: free ions in solution (full mobility) and minerals in the solid phase (full immobility). When conditions change (pH, redox regime, biological activity), organo‑mineral complexes can break down, releasing elements for further migration. Conversely, they can form, fixing mobile elements (Birkeland, 1984; Scheffer et al., 2018).

3.5. Soil Solution

And finally, we have reached the most mobile form – the soil solution (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017). This is the water that is in soil pores and contains dissolved mineral and organic substances.

Composition of the soil solution:

• Inorganic ions: Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, H⁺, Al³⁺, Fe²⁺, Mn²⁺ – cations; HCO₃⁻, SO₄²⁻, Cl⁻, NO₃⁻, H₂PO₄⁻ – anions.
• Organic compounds: dissolved organic carbon (DOC), low‑molecular‑weight organic acids (citric, malic, oxalic), amino acids, sugars, fulvic acids.
• Neutral molecules: H₄SiO₄ (silicic acid), H₃BO₃ (boric acid).

Key property of the soil solution – its dynamism. Its composition constantly changes depending on:

• Season – spring and autumn it is more diluted, summer more concentrated (Huang et al., 2012).
• Moisture – after rain, concentration drops, during dry periods it increases (Weil and Brady, 2017).
• Biological activity – roots excrete organic acids, microorganisms change pH and Eh (Huang et al., 2012; Foth, 1990).
• Fertiliser and ameliorant application – sharply changes the concentration of individual ions (Foth, 1990).

Role of the soil solution in migration:

1. The solution is the medium in which chemical reactions occur. All weathering, precipitation, complexation reactions take place in the solution.

2. The solution is the transport medium. Ions and molecules move together with water. This is the main pathway of migration.

3. The solution is the source of plant nutrition. From the solution, roots absorb ions. The concentration in the solution determines the availability of elements (Foth, 1990; Weil and Brady, 2017).

However, not all ions in solution are equally mobile. Some (Cl⁻, NO₃⁻, Na⁺) move with water almost without delay. Others (Ca²⁺, Mg²⁺, K⁺) may adsorb on colloids and move with delay (due to exchange processes). Still others (Fe³⁺, Al³⁺) may rapidly hydrolyse and precipitate if pH changes (Huang et al., 2012).

3.6. Exchangeable Form

Now we come to a special form – the exchangeable form (White, 2006; Weil and Brady, 2017; Foth, 1990). This is a form in which ions are on the surface of colloidal particles (clay minerals, humus) in an adsorbed state. This is not a solid form (ions are not in the crystal lattice), but not a solution (ions are not free in solution). It is a boundary state.

How does it work?

The surface of colloidal particles (clay minerals, organic matter) carries an electrical charge – almost always negative (Weil and Brady, 2017; White, 2006). This charge arises from:

• Isomorphic substitutions in the crystal lattice of clay minerals (Al³⁺ instead of Si⁴⁺, etc.) – this is a permanent charge.
• Dissociation of functional groups on the surface of organic molecules (—COOH, —OH) – this is a pH‑dependent charge.

This negative charge attracts positively charged ions from solution – cations: Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, H⁺, Al³⁺. They are held on the surface electrostatically, but are not part of the structure. They can exchange with other cations from solution – this is cation exchange (Weil and Brady, 2017; White, 2006; Foth, 1990).

Features of the exchangeable form:

1. This is a “store” of available elements. Although elements in exchangeable form are not in solution, they can quickly pass into solution when the solution composition changes (e.g., upon fertiliser application or root uptake).

2. This is a “buffer” for the soil solution. The exchange complex can absorb excess cations from solution or, conversely, release them, maintaining a relatively stable solution composition. This is an important mechanism of soil buffering (Foth, 1990; Weil and Brady, 2017).

3. The exchangeable form is the result of migration. Ions that enter the solution can adsorb on colloids and be on the way of further migration. Conversely, ions from the exchangeable form can desorb and continue moving.

Factors affecting exchange capacity:

• Quantity and type of colloids. The more clay minerals (especially 2:1 type) and humus, the higher the exchange capacity (Foth, 1990; Weil and Brady, 2017).
• pH. In acidic soils, part of the exchange sites is occupied by H⁺ and Al³⁺ (exchangeable acidity), and the availability of Ca, Mg, K decreases (Foth, 1990; Bloom and Skyllberg, 2012).
• Solution composition. The dominant cations in solution determine which ions will be on exchange sites (mass law: the higher the ion concentration in solution, the greater its share on the exchange complex) (White, 2006; Weil and Brady, 2017).

3.7. Transitions Between Forms

So, we have considered the main forms of occurrence of elements. Now the most important conclusion: these forms are not isolated – they constantly transform into each other (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

Main transitions:

Mineral → solution: weathering, dissolution. Elements are released from minerals (primary or secondary) and pass into the soil solution. This is the first step of migration.

Solution → exchangeable: adsorption of ions on colloid surfaces. This retains elements in the root zone but makes them available to plants.

Exchangeable → solution: desorption of ions under the action of changes in solution composition or competition with other ions. This allows elements to continue migration or reach plant roots.

Solution → precipitate: precipitation from solution as carbonates, sulfates, hydroxides or secondary minerals. This stops migration (geochemical barrier).

Organic ↔ mineral: immobilisation (incorporation into organic matter) or mineralisation (release from organic matter). Microorganisms are the main “conductors” of this transition (Foth, 1990; Scheffer et al., 2018).

Solution → organic (complex): chelation. Organic acids bind metal ions in solution, creating soluble complexes. This is a bypass route of migration that allows metals to move under conditions where their ionic form cannot (Birkeland, 1984; Weil and Brady, 2017).

Organo‑mineral form: a “trap” for organic and mineral substances. This form is especially important for long‑term stabilisation of carbon and microelements (Scheffer et al., 2018).

What does this mean for migration?

Each transition is either a step forward (towards mobility) or a step backward (towards fixation). Migration is a sequence of such transitions. Ions move from mineral to solution, then to exchange sites, then back to solution, then to precipitate or organic complex – and so on. This is the essence of the geochemical cycle of elements in soil (Birkeland, 1984).

3.8. Final Summary

Now let us summarise the forms of elements:

1. Mineral form: elements are fixed in the crystal lattice of primary or secondary minerals. This is the most immobile form. Transition to other forms occurs only upon weathering or dissolution (Birkeland, 1984; Buol et al., 2011).

2. Organic form: elements in humus and living organisms. This form can be both mobile (organic acids, chelates) and relatively immobile (humic acids protected from decomposition). Transitions are controlled by biota (Foth, 1990; Scheffer et al., 2018).

3. Organo‑mineral form: binding of organic and mineral particles. This is a very stable form that accumulates carbon and microelements for a long time (Scheffer et al., 2018; Weil and Brady, 2017).

4. Soil solution: ions and molecules in water. The most mobile form. Migration occurs here (Huang et al., 2012; Weil and Brady, 2017).

5. Exchangeable form: ions on colloid surfaces. A transitional form between solution and minerals. This is a reserve of available elements and a buffer for the solution (White, 2006; Weil and Brady, 2017).

6. All forms are linked by transitions. Migration is a constant transition of an element from one form to another. The movement of elements is not chaotic wandering, but a regular process determined by environmental conditions (Birkeland, 1984).

4.1. What is the Soil Solution? Definition and Functions

Soil solution is the aqueous phase of soil containing dissolved mineral and organic substances, as well as fine colloidal particles (Huang et al., 2012; Weil and Brady, 2017). It is not just water that is in the soil, but a complex multicomponent solution whose composition constantly changes under the influence of physical, chemical and biological processes.

The soil solution performs several fundamental functions:

• Transport function – the solution carries ions and molecules from one part of the profile to another, ensuring migration. Without the solution, there would be no leaching or accumulation – the soil would be static.

• Nutritive function – from the solution, plants and microorganisms absorb the elements they need. It is the solution that is the source of nutrition, not the solid particles themselves, which contain elements in unavailable forms (Foth, 1990; Weil and Brady, 2017).

• Reaction medium – the solution is a medium for chemical reactions: weathering, precipitation, complexation, oxidation, reduction. Without water, these reactions either do not occur or proceed extremely slowly (Huang et al., 2012; Birkeland, 1984).

• Buffering function – the solution participates in maintaining the stability of soil properties (pH, ion concentration), interacting with the solid phase and the exchange complex (Foth, 1990; White, 2006).

• Informational function – the soil solution is a “window” into soil processes (Huang et al., 2012). From the solution composition one can judge which processes are occurring in the soil: leaching, accumulation, pollution.

Thus, the soil solution is not a passive component of soil, but an active medium where processes determining the fate of chemical elements take place (Foth, 1990; Huang et al., 2012).

4.2. Composition of the Soil Solution

The composition of the soil solution is extremely diverse and includes both inorganic and organic components (Huang et al., 2012; Weil and Brady, 2017). Let us consider them systematically.

4.3. Concentration and Ionic Strength

An important characteristic of the soil solution is ionic strength (I) – a measure of the total concentration of all ions in the solution (White, 2006; Huang et al., 2012; Foth, 1990).

where Cᵢ is the concentration of ion i in mol/L, and zᵢ is its charge.

Ionic strength determines the activity of ions – their “effective” concentration, which participates in chemical reactions (White, 2006; Huang et al., 2012). Activity (aᵢ) is related to concentration through the activity coefficient (γᵢ):

In dilute solutions, γᵢ is close to 1, and activity is approximately equal to concentration. But in concentrated solutions (saline soils) or in the presence of multivalent ions (Al³⁺, Fe³⁺), γᵢ can be greatly reduced, and activity becomes significantly lower than concentration. This is important: when calculating precipitation, adsorption or toxicity, one must consider activity, not concentration (White, 2006; Huang et al., 2012).

Usually, the ionic strength of soil solutions is 0.001–0.05 mol/L in non‑saline soils and can reach 0.5 mol/L and above in saline soils (Huang et al., 2012; Foth, 1990).

4.4. Factors Determining the Composition of the Soil Solution

The composition of the soil solution is the result of complex interaction of many factors (Huang et al., 2012; Birkeland, 1984; Weil and Brady, 2017). Let us list the main ones.

4.5. The Soil Solution as a Migration Medium

Now let us move to the key question: how does the solution ensure the migration of elements? Here we link the material on the solution with our main topic (Huang et al., 2012; Weil and Brady, 2017; Birkeland, 1984).

Element migration in soil occurs predominantly in the liquid phase. The solid phase is the “store” from which elements are taken and to which they return, but the path between “stores” passes through the solution.

Main transport mechanisms in solution:

1. Mass flow (convection)

This is the transport of ions and molecules together with moving water (Huang et al., 2012; Weil and Brady, 2017). When water percolates downward (infiltration), it carries dissolved substances – this is the main mechanism of element removal from the upper part of the profile. When water rises by capillarity (capillary rise), it can bring elements from groundwater upward. The intensity of convection is determined by the soil water regime: leaching regime promotes removal, stagnant regime promotes accumulation (Birkeland, 1984; Scheffer et al., 2018).

2. Diffusion

This is the movement of ions from areas of high concentration to areas of low concentration. Diffusion is especially important under conditions of slow water movement (dense layers, aggregates) (Weil and Brady, 2017; Huang et al., 2012). For example, in the centre of a soil aggregate, where water hardly enters, ions can migrate by diffusion. Diffusion is also important in the root zone: roots absorb ions, creating a concentration gradient, and new ions are drawn to the root by diffusion (Foth, 1990).

3. Hydrodynamic dispersion

This is the “smearing” of the front of a dissolved substance when water moves in a porous medium because water in different pores moves at different speeds (Weil and Brady, 2017; Huang et al., 2012). As a result, the front of the dissolved substance does not remain sharp but gradually spreads out. This is important for the spread of pollutants and fertilisers.

4. Colloid‑mediated transport

Some substances can be transported not in dissolved form but as colloidal particles – clay minerals, humic substances, oxides (Weil and Brady, 2017; White, 2006). This is especially important for relatively immobile elements that adsorb on colloids and are transported with them.

4.6. The Soil Solution and Geochemical Barriers

Geochemical barriers, which we will discuss in more detail at the end of the lecture, are zones where the solution changes its properties so much that elements precipitate or adsorb (Birkeland, 1984; Mukha et al., 2003). The soil solution is the “carrier” that delivers elements to the barrier.

Examples:

• Oxidative barrier: a reducing solution rich in Fe²⁺ and Mn²⁺ enters an oxidising zone (e.g., at groundwater discharge). Fe²⁺ and Mn²⁺ are oxidised to Fe³⁺ and Mn⁴⁺ and precipitate as hydroxides. The concentration of these ions in solution drops sharply. This is how iron and manganese concretions and bog ores are formed (Weil and Brady, 2017; Birkeland, 1984).

• Alkaline barrier: an acidic or neutral solution containing Ca²⁺ and Mg²⁺ enters a zone with carbonates or high pH. Ca²⁺ and Mg²⁺ bind with CO₃²⁻ and precipitate as carbonates. This is how carbonate horizons are formed (Birkeland, 1984; Foth, 1990).

• Adsorption barrier: a solution containing heavy metals or phosphates passes through a layer with high clay mineral or humus content. Ions adsorb on colloid surfaces and are fixed (Weil and Brady, 2017; White, 2006).

• Organic barrier (complexation): a solution with high content of organic acids can bind metals into soluble complexes, preventing their precipitation. This is a different type of barrier – it does not precipitate but, on the contrary, keeps them in solution (Huang et al., 2012; Scheffer et al., 2018).

Thus, the composition of the soil solution is a “trace” of geochemical barriers. Where barriers work, the concentrations of some elements in solution decrease, others increase. By analysing the solution composition in different horizons, we can “read” the history of element migration in the soil (Birkeland, 1984; Huang et al., 2012).

4.7. Methods for Studying the Soil Solution

Although this is not the central topic of our lecture, it is worth briefly mentioning how the soil solution is studied (Huang et al., 2012; Weil and Brady, 2017). After all, to understand migration, one must be able to “look into” the solution.

Main methods:

• Field sampling – using lysimeters (vacuum or non‑pressure) to collect gravitational water from specific horizons.
• Laboratory methods – preparation of a water extract (soil:water ratio 1:1, 1:2 or 1:5). This gives an approximate idea of the solution composition, but does not always reflect natural conditions.
• Displacement methods – displacing the solution from a soil column with an immiscible liquid (e.g., centrifugation with organic solvent).
• Analysis – ion concentrations are determined by ion chromatography, atomic absorption spectroscopy, mass spectrometry, pH‑metry and potentiometry.

It is important to understand that any method alters the solution. Therefore, results are interpreted taking into account the sampling method and conditions (Huang et al., 2012; Weil and Brady, 2017).

4.8. The Soil Solution as an Indicator of Soil Processes

The composition of the soil solution is a valuable source of information about soil processes (Huang et al., 2012; Birkeland, 1984; Foth, 1990). It allows:

• Assess leaching. If Ca²⁺, Mg²⁺, Na⁺, K⁺ dominate in the solution and pH is low, active base leaching is occurring.
• Diagnose salinisation. High concentrations of Cl⁻, SO₄²⁻, Na⁺ indicate salinisation.
• Detect pollution. Presence of heavy metals, nitrates, pesticides in solution is a sign of anthropogenic impact.
• Assess nutritional status. Low concentrations of NO₃⁻, PO₄³⁻, K⁺ may indicate element deficiency.
• Judge the type of pedogenesis. In podzolic soils, organic acids and Al are present in solution; in chernozems – Ca²⁺ and HCO₃⁻; in solonetzes – Na⁺ and Cl⁻ (Birkeland, 1984; Foth, 1990).

4.9. Significance of the Soil Solution for Agriculture and Ecology

For the agronomist, the soil solution is a key object of management (Foth, 1990; Weil and Brady, 2017). It is the ion concentration in the solution that determines the availability of nutrients to plants. If the concentration is too low – plants starve. If too high – toxicity or osmotic shock (salinisation) may occur. By managing the solution composition through fertilisers and amelioration, we manage plant nutrition.

For the ecologist, the soil solution is the pathway for pollutant migration to groundwater (Weil and Brady, 2017; Huang et al., 2012). If heavy metals or pesticides pass into solution, they can reach groundwater. Therefore, knowledge of the solution composition and factors affecting it is necessary for predicting pollution.

Finally, for the soil geneticist, the soil solution is the “living history” of the soil. From the solution composition one can reconstruct past processes and predict future soil development (Birkeland, 1984; Scheffer et al., 2018).

4.10. Conclusion

So, the soil solution is not just water with impurities. It is a highly dynamic, multicomponent medium that links all soil phases into a single system. Without the solution, there would be no migration. Without migration, there would be no pedogenesis. Without pedogenesis, there would be no fertility.

We have come to an important conclusion: the soil solution is the “blood” of the soil, circulating through the profile, delivering nutrition to roots, carrying away weathering products, forming horizons and geochemical barriers. And to understand soil, one must understand its solution.

In the next section we will continue our journey and consider redox processes – those very “engines” that force elements to change from one form to another and determine the direction of their migration.

5.1. What are Oxidation and Reduction? Fundamental Concepts

Let us start with basic definitions (Weil and Brady, 2017; White, 2006; Foth, 1990).

Oxidation is the process of losing electrons by an atom, ion or molecule. Upon oxidation, the oxidation state of the element increases.

Reduction is the process of gaining electrons. Upon reduction, the oxidation state of the element decreases.

The most important principle: oxidation and reduction always go together. If one element gives up electrons (oxidises), another must accept them (reduces). Therefore, we speak of oxidation‑reduction (redox) reactions (Weil and Brady, 2017; White, 2006).

Consider the classic example – the transition of iron from the divalent to the trivalent form:

Here the Fe²⁺ ion loses one electron and becomes Fe³⁺. But the electron cannot exist by itself – it must be accepted by someone. Most often in soil, the “acceptor” of electrons is oxygen:

Together, these two half‑reactions give the overall oxidation of iron by oxygen:

This is the classic reaction that occurs when iron rusts or when divalent iron is oxidised in soil under good aeration. Oxygen acts as the oxidant (electron acceptor), and iron as the reductant (electron donor) (Weil and Brady, 2017; White, 2006; Foth, 1990).

What is important to remember:

1. In any redox reaction, there is an electron donor (reductant, which is oxidised) and an electron acceptor (oxidant, which is reduced).

2. Electrons cannot exist freely in solution – they are always transferred from one substance to another.

3. The transition of an element from one form to another changes its properties: solubility, toxicity, migration ability (Weil and Brady, 2017; White, 2006; Birkeland, 1984).

5.2. Role of Oxygen and Microorganisms in Redox Processes

In soil, the main oxidant is oxygen (O₂) (Weil and Brady, 2017; White, 2006; Foth, 1990). Why? Because oxygen has high electronegativity – it is “greedy” for electrons and readily accepts them, being reduced to water or hydroxide ions.

In well‑aerated soils (forest, steppe, arable at normal moisture):

• Oxygen constantly enters from the atmosphere.
• Most elements are in oxidised forms: Fe³⁺, Mn⁴⁺, SO₄²⁻, NO₃⁻.
• Reactions proceed actively, with participation of microorganisms and enzymes (aerobic respiration, nitrification, sulfide oxidation).
• The soil has a high redox potential (Eh). We will discuss this later.

In conditions of oxygen deficiency (waterlogged, stagnant soils, wetlands, rice paddies):

• Oxygen supply from the atmosphere is hindered (water fills pores, diffusion of O₂ is 10 000 times slower than in air) (Weil and Brady, 2017).
• Oxygen is rapidly consumed by root and microbial respiration.
• Other substances act as “traps” for electrons: nitrates (NO₃⁻), iron and manganese oxides (Fe³⁺, Mn⁴⁺), sulfates (SO₄²⁻), organic matter, carbon dioxide (CO₂).
• Elements pass into reduced forms: Fe²⁺, Mn²⁺, N₂, NH₄⁺, H₂S, CH₄ (Weil and Brady, 2017; White, 2006; Foth, 1990).
• The soil has a low or even negative Eh.

But it is important to emphasise: redox reactions in soil are not just chemistry. They are biochemistry (White, 2006; Foth, 1990; Scheffer et al., 2018). Microorganisms act as catalysts of these processes. Why? Because for them, oxidation and reduction of inorganic compounds is a way to obtain energy.

Recall from soil biology (Scheffer et al., 2018; White, 2006):

• Aerobic microorganisms use oxygen as the final electron acceptor in the respiratory chain. This gives them a lot of energy.
• Facultative anaerobes can switch to other acceptors (nitrates, sulfates) when oxygen is absent.
• Obligate anaerobes use only inorganic acceptors (e.g., sulfate‑reducing bacteria reduce SO₄²⁻ to H₂S).
• Chemolithotrophs (e.g., nitrifiers, iron bacteria) obtain energy by oxidising inorganic substances (NH₄⁺ → NO₂⁻, Fe²⁺ → Fe³⁺) (White, 2006; Foth, 1990).

It is microorganisms that ensure the sequential reduction of various compounds as Eh falls. Without them, these reactions would proceed at a snail’s pace (Weil and Brady, 2017; White, 2006).

5.3. Redox Potential (Eh) – a Quantitative Measure of Redox Conditions

For quantitative characterisation of redox conditions in soil, the redox potential (Eh) is used – measured in volts or millivolts (Weil and Brady, 2017; White, 2006; Foth, 1990). Eh is the electrical potential arising on an inert (platinum) electrode placed in the soil, relative to a standard hydrogen electrode.

What does Eh indicate?

• High positive Eh (usually > +0.3 V) – oxidising conditions. Oxygen is abundant, elements are in oxidised forms.
• Low positive or negative Eh (< +0.2 V) – reducing conditions. Oxygen is scarce or absent, elements are reduced (Weil and Brady, 2017; White, 2006; Foth, 1990).

Range of Eh in soils:

• Well‑aerated soils: Eh ≈ +0.4 – +0.7 V (up to +0.8 V in very acidic soils).
• Soils with periodic waterlogging: Eh ≈ +0.3 – +0.1 V.
• Permanently waterlogged (gley) soils: Eh ≈ 0 – –0.3 V (Weil and Brady, 2017; White, 2006).

It is important to note that Eh strongly depends on pH (Weil and Brady, 2017; White, 2006). Almost all redox reactions in soil involve protons (H⁺). The lower the pH (more acidic), the higher the Eh at which reduction can occur. Therefore, in soil science, combined consideration of Eh and pH is often used, for example in the form of Eh‑pH diagrams (Weil and Brady, 2017; White, 2006). On such diagrams, stability fields of different forms of elements are shown as a function of Eh and pH. This is a very convenient tool for predicting element behaviour.

In soil science, the concept of pe is also used – the negative logarithm of electron activity (analogous to pH for electrons) (White, 2006; Foth, 1990). The relationship between Eh and pe is simple:

The higher the pe, the lower the electron activity and the more oxidising the conditions. But Eh is more commonly used because it is easier to measure (White, 2006; Weil and Brady, 2017).

5.4. Sequence of Reduction – the Main Key to Understanding Redox Processes

Now we come to the most important point for soil science: the sequence of reduction of elements as Eh falls (Weil and Brady, 2017; White, 2006; Foth, 1990). This sequence is strictly determined by thermodynamics and reflects the ability of substances to accept electrons (the higher the standard Eh of the half‑reaction, the more easily the substance is reduced).

As the soil becomes increasingly waterlogged and oxygen is depleted, Eh sequentially passes through a series of “steps”, at each of which a particular group of compounds is reduced (Weil and Brady, 2017; White, 2006; Foth, 1990; Birkeland, 1984).

Here is this sequence (at about pH 6.5–7):

1. Reduction of oxygen (O₂ → H₂O). This is the first and most energetically favourable reaction. As long as oxygen is present, other substances are not reduced. This process occurs at Eh ≈ +0.38 – +0.32 V. We call this stage aerobic. Oxygen is consumed in respiration (oxidation of organic matter) (Weil and Brady, 2017; White, 2006).

2. Reduction of nitrates (NO₃⁻ → N₂, NO, N₂O). Once oxygen is depleted, facultative anaerobes begin to use nitrates as electron acceptors. This is denitrification. The process occurs at Eh ≈ +0.28 – +0.22 V. As a result, nitrogen is lost from the soil as gases (N₂, N₂O). This is an important process, reducing nitrogen nutrition of plants and contributing to the greenhouse effect (Weil and Brady, 2017; White, 2006; Foth, 1990).

3. Reduction of manganese (Mn⁴⁺ → Mn²⁺). When nitrates are exhausted, it is the turn of manganese oxides (MnO₂, Mn₂O₃). Mn⁴⁺ is reduced to Mn²⁺, which is readily soluble and mobile. This leads to the removal of manganese from reducing zones and the formation of concretions at oxidative barriers. The process occurs at Eh ≈ +0.22 – +0.18 V (Weil and Brady, 2017; White, 2006; Birkeland, 1984).

4. Reduction of iron (Fe³⁺ → Fe²⁺). This is the key stage for soil science. Iron hydroxides and oxides are reduced to Fe²⁺, which passes into solution. This is the gley process (reductive gleying). Iron becomes mobile and can be leached from some horizons and accumulate in others, giving the soil characteristic bluish‑grey, greenish tones. The process occurs at Eh ≈ +0.11 – +0.08 V. It is the reduction of iron that gives the characteristic colour to gley horizons (Weil and Brady, 2017; White, 2006; Birkeland, 1984).

5. Reduction of sulfates (SO₄²⁻ → H₂S, S²⁻). When almost all Fe³⁺ is reduced, sulfates are next. Sulfate‑reducing bacteria reduce SO₄²⁻ to hydrogen sulfide (H₂S) or sulfide ion (S²⁻). H₂S is a toxic gas with the smell of rotten eggs. In the presence of Fe²⁺, black iron sulfide (FeS) is formed, which gives the soil a black colour. The process occurs at Eh ≈ –0.14 – –0.17 V (Weil and Brady, 2017; White, 2006; Foth, 1990).

6. Reduction of carbon dioxide (CO₂ → CH₄). This is the deepest stage of reduction. Methanogenic archaea reduce CO₂ to methane (CH₄) – marsh gas. This occurs at very low Eh (< –0.2 V) under conditions of complete water stagnation and absence of other acceptors. Methane is a powerful greenhouse gas (Weil and Brady, 2017; White, 2006; Foth, 1990).

Crucial conclusion: each stage of reduction “switches on” only after the previous acceptor is exhausted. Therefore, from the Eh value, we can determine which processes are occurring in the soil at a given moment (Weil and Brady, 2017; White, 2006; Birkeland, 1984).

5.5. Sequence of Reduction and Element Migration

Now let us link the reduction sequence to migration (Birkeland, 1984; Weil and Brady, 2017; Foth, 1990).

At the oxygen reduction stage (aerobic conditions): most elements are immobile. Fe and Mn are in the form of insoluble oxides. Nitrogen is in the form of nitrates (mobile) and organic form. Sulfur is in the form of sulfates (mobile).

At the denitrification stage: nitrates are reduced to gaseous nitrogen. Nitrogen is lost from the soil – this is loss of an element, not its movement in the profile. But importantly: denitrification can occur in anaerobic microzones inside aggregates even in well‑aerated soil (White, 2006; Weil and Brady, 2017).

At the Mn and Fe reduction stage: manganese and iron become mobile and can migrate with the soil solution. This leads to:

• Removal of Fe and Mn from eluvial horizons (podzols, gley horizons).
• Accumulation of Fe and Mn at oxidative barriers (e.g., at the soil surface, at the boundary with the aerated zone) in the form of concretions, ortsteins, bog ores (Birkeland, 1984; Weil and Brady, 2017).
• Formation of bluish‑grey and greenish tones (gley horizons), where iron is removed and the soil acquires the colour of the original minerals (quartz, silicates) (Birkeland, 1984; Scheffer et al., 2018).

At the sulfate reduction stage: H₂S is formed, which can bind with Fe²⁺ to form black FeS. This leads to blackening of reduced horizons (e.g., in wetland soils, muds). H₂S is also toxic to plant roots (Weil and Brady, 2017; White, 2006; Foth, 1990).

At the methanogenesis stage: CH₄ is formed, which can be released into the atmosphere. This is an important contribution of soils to the greenhouse effect (Weil and Brady, 2017; White, 2006).

5.6. Gley Process – a Classic Example of the Influence of Reduction on Soil Formation

The gley process (gleying) is one of the most important types of soil formation under waterlogged conditions (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017). It occurs where the soil is for a long time under stagnant wetness and experiences oxygen deficiency.

How the gley process proceeds:

1. Soil becomes saturated with water, oxygen access drops sharply.

2. Microorganisms begin to use Fe and Mn oxides as electron acceptors.

3. Fe³⁺ and Mn⁴⁺ are reduced to Fe²⁺ and Mn²⁺, passing into solution.

4. Mobile Fe²⁺ and Mn²⁺ can:

• Be removed beyond the profile – then the horizon loses iron and acquires a bluish‑grey or greenish‑grey colour (this is a gley horizon). In such a horizon, silicates (quartz, feldspars) predominate, giving the grey colour (Birkeland, 1984; Scheffer et al., 2018).
• Migrate within the horizon – and accumulate as concretions or spots at the boundary with the oxidising zone (e.g., on aggregate surfaces, around roots).
• Accumulate at the oxidative barrier – for example, at groundwater discharge, where Fe²⁺ is oxidised to Fe³⁺ and precipitates as hydroxides, forming bog ores or ortsteins (Birkeland, 1984; Weil and Brady, 2017).

Morphological features of gley processes:

• Bluish‑grey, greenish‑grey, bluish‑grey tones (result of removal of Fe and Mn and predominance of silicates).
• Presence of rusty‑brown, ochreous spots and concretions (accumulation of Fe and Mn at local oxidative barriers).
• Often – presence of black sulfide inclusions (FeS) in strongly reduced horizons (Birkeland, 1984; Scheffer et al., 2018).

Gley process is characteristic of:

• Wetland and bog soils.
• Gley horizons in soils with shallow groundwater (gleysols, gley podzols).
• Podzolic soils (gley‑podzols) under excessive moisture.
• Rice soils (under prolonged flooding) (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

5.7. Podzolisation Process and the Role of Reduction

The podzolisation process, which we mentioned in the section on complexation, is also closely related to redox conditions (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017). In podzolic soils, especially under conditions of periodic waterlogging (gleyed podzols), reduction of Fe and Mn promotes their removal from the upper part of the profile.

But in classical podzols, organic acids (complexation) play the main role, not purely reductive processes. However, in gleyed podzols, reduction enhances the mobility of Fe and Mn, making podzolisation more intense (Birkeland, 1984; Scheffer et al., 2018).

5.8. Redox Processes and Microelements: Toxicity and Availability

Redox processes strongly affect the behaviour of microelements, especially those with variable valence (Foth, 1990; Weil and Brady, 2017; Huang et al., 2012).

• Manganese (Mn): Under reducing conditions, Mn²⁺ is readily soluble and can accumulate to toxic concentrations for plants (especially in acidic waterlogged soils). Under oxidising conditions, Mn⁴⁺ is part of insoluble oxides and is unavailable (Foth, 1990; Weil and Brady, 2017).

• Iron (Fe): Similar to manganese: under reducing conditions, Fe²⁺ is mobile and available to plants, but in acidic soils may be toxic. Under oxidising conditions, Fe³⁺ is part of poorly available hydroxides (Foth, 1990; Weil and Brady, 2017).

• Copper (Cu), zinc (Zn), cobalt (Co): Their mobility also increases under reducing conditions, as they can form complexes with organic acids and hydrogen sulfide (Foth, 1990; Weil and Brady, 2017).

• Arsenic (As): Under oxidising conditions, As occurs as arsenate (As⁵⁺), which is strongly sorbed on Fe oxides. Under reducing conditions, As is reduced to arsenite (As³⁺), which is more mobile and toxic. This is important for contaminated soils (Weil and Brady, 2017; Huang et al., 2012).

• Chromium (Cr): Under oxidising conditions, Cr⁶⁺ (chromate) is very mobile and toxic. Under reducing conditions, Cr⁶⁺ is reduced to Cr³⁺, which is immobile and significantly less toxic. This is used for remediation of contaminated soils (Weil and Brady, 2017; Huang et al., 2012).

5.9. Redox Processes in Agronomy and Ecology

Knowledge of redox processes has great practical significance (Weil and Brady, 2017; Foth, 1990; Birkeland, 1984; White, 2006).

For the agronomist:

• Assessment of plant conditions. Reducing conditions (low Eh) lead to oxygen deficiency in the root zone, which inhibits root respiration. Many crops (wheat, maize, potato) do not tolerate prolonged waterlogging (Weil and Brady, 2017; Foth, 1990).
• Management of nitrogen nutrition. Under waterlogging, denitrification is activated and nitrogen is lost from soil. Therefore, under excessive moisture, nitrogen fertilisers should be applied cautiously, in ammonium form (NH₄⁺) (Foth, 1990; Weil and Brady, 2017).
• Diagnosis of toxicity. In acidic waterlogged soils, toxicity of Mn²⁺ and Al³⁺ is possible. Reducing conditions promote their accumulation in solution. Liming and drainage help reduce toxicity (Foth, 1990; Weil and Brady, 2017).
• Drainage and amelioration. Drying of waterlogged soils leads to an increase in Eh, oxidation of Fe²⁺ and Mn²⁺ and their precipitation. This improves soil structure, but may cause acidification (release of H⁺ upon oxidation) and reduce availability of microelements (Birkeland, 1984; Weil and Brady, 2017).

For the ecologist:

• Heavy metal pollution. Under reducing conditions, many heavy metals (Cd, Pb, Zn, Cu, Hg) can form sulfides and become immobile. Upon drainage or Eh change, they may become mobile again and contaminate groundwater (Weil and Brady, 2017; Huang et al., 2012).
• Greenhouse gases. Denitrification (N₂O) and methanogenesis (CH₄) are important sources of greenhouse gases from soils. Management of water regime can reduce emissions of these gases (Weil and Brady, 2017; White, 2006).
• Wastewater treatment. Reducing conditions are used in biological treatment methods to remove nitrates (denitrification) and sulfates (sulfate reduction) (Weil and Brady, 2017; Huang et al., 2012).

5.10. Final Summary of the Section

Let us summarise redox processes (Weil and Brady, 2017; White, 2006; Foth, 1990; Birkeland, 1984).

1. Oxidation – loss of electrons, reduction – gain. These processes always go together (redox reactions). The main oxidant in soil is oxygen.

2. Redox potential (Eh) – quantitative characteristic of redox conditions. High Eh – oxidising conditions, low – reducing.

3. The sequence of reduction is strictly defined: O₂ → NO₃⁻ → Mn⁴⁺ → Fe³⁺ → SO₄²⁻ → CO₂. Each subsequent electron acceptor “switches on” only after the previous one is exhausted.

4. Reduction of Fe and Mn – a key process for soil science. It causes the gley process – removal of iron and manganese from the soil, change of colour to bluish‑grey, formation of concretions at oxidative barriers. The gley process is the basis for the formation of gley horizons, wetland and podzolic soils.

5. Redox processes determine the mobility, availability and toxicity of many elements: Fe, Mn, N, S, As, Cr, microelements. They affect nitrogen nutrition (denitrification), formation of greenhouse gases (N₂O, CH₄), and remediation of contaminated soils and waters.

6. In agronomy, knowledge of redox conditions is necessary for managing drainage, liming, fertiliser application, preventing toxicity and nitrogen losses. In ecology – for predicting pollutant behaviour and managing greenhouse gas emissions.

So, we have covered redox processes. Now you understand that redox processes are the “engine” of migration for many elements. They convert elements from immobile to mobile forms and back, determining the direction of their movement in the soil profile. Without redox processes, there would be no gley soils, no podzols, and many other genetic soil types.

In the next section we will move on to acidity – another key factor determining element migration. If redox processes “turn on” and “turn off” mobility, then pH controls in which exact form the element will be – soluble or insoluble, available or toxic.

6.1. What is Soil Acidity? Definition and Essence

Soil acidity is a property of soil caused by the presence in the soil solution and on the surface of colloidal particles of hydrogen ions (H⁺) and aluminium (Al³⁺) in exchangeable and hydrolysed forms (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017). Simply put, acidity is a measure of how “acidic” the soil is.

The quantitative measure of acidity is pH – the negative decimal logarithm of the activity of hydrogen ions in the soil solution (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017):

Where [H⁺] is the activity of hydrogen ions in mol/L.

• pH = 7 – neutral medium (concentrations of H⁺ and OH⁻ are equal).
• pH < 7 – acidic medium (H⁺ greater than OH⁻).
• pH > 7 – alkaline medium (OH⁻ greater than H⁺).

Most soils have pH in the range from 3 to 10 (Foth, 1990; Bloom and Skyllberg, 2012). Extreme values (pH < 3 or pH > 10) are rare and usually associated with strong anthropogenic impact (acid sulfate soils, soda solonchaks).

It is important to understand that pH is a logarithmic scale. This means that a change in pH by 1 unit corresponds to a tenfold change in H⁺ concentration. For example, in soil with pH 4, the concentration of H⁺ is 10 times higher than at pH 5, and 100 times higher than at pH 6 (Foth, 1990; Weil and Brady, 2017). This is very important for understanding the scale of changes during acidification or liming.

6.2. Active and Exchangeable Acidity – Two Sides of the Same Coin

In soil, acidity manifests in two forms, which are closely related (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

Active acidity – the acidity of the soil solution due to free H⁺ ions in the solution. It is determined by measuring the pH of a water or salt suspension of soil. Active acidity is the one that plant roots and microorganisms directly encounter. It determines the habitat for living organisms and the rate of chemical reactions in solution (Foth, 1990; Bloom and Skyllberg, 2012).

Exchangeable acidity – acidity associated with exchangeable cations H⁺ and Al³⁺ on the surface of colloidal particles (clay minerals and humus). These ions are not in solution, but can be displaced by other cations (e.g., upon addition of a neutral salt). Exchangeable acidity determines the potential ability of the soil to acidify the solution (Foth, 1990; Bloom and Skyllberg, 2012; White, 2006).

Relationship between active and exchangeable acidity:

Active and exchangeable acidity are in dynamic equilibrium. When plants absorb cations (Ca²⁺, Mg²⁺, K⁺) from solution, they release H⁺. This increases active acidity. In response, part of the H⁺ adsorbs on colloids, displacing other cations into solution. Exchangeable acidity increases. Upon liming (addition of CaCO₃), we first neutralise active acidity, then exchangeable, by replacing H⁺ and Al³⁺ with Ca²⁺ (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

How is pH measured?

In the laboratory, pH is measured in a water suspension of soil (pH(H₂O)) or in a suspension with 0.01 M CaCl₂ (pH(CaCl₂)). pH(CaCl₂) is usually 0.3–0.7 units lower than pH(H₂O), because Ca²⁺ displaces H⁺ and Al³⁺ from the exchange complex into solution. pH(CaCl₂) is more stable and better reflects the actual acidity of the soil solution (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017). Recently, many laboratories have switched to measuring pH in 0.01 M CaCl₂, especially in acidic soils.

6.3. Sources of Acidity – Where Does H⁺ Come From?

Acidity in soils can arise from several sources (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017; Birkeland, 1984).

6.4. Aluminium as the Main Source of Acidity in Acidic Soils

In acidic soils (pH < 5.5), the main source of acidity is not H⁺ but aluminium (Foth, 1990; White, 2006; Bloom and Skyllberg, 2012; Weil and Brady, 2017). This is a key point that must be well remembered!

How does it work?

1. In an acidic environment (pH < 5.5), clay minerals and aluminium oxides begin to break down, and Al³⁺ ions are released into solution.

2. In aqueous solution, Al³⁺ immediately hydrolyses, binding OH⁻ and releasing H⁺ (Foth, 1990; Bloom and Skyllberg, 2012; White, 2006):

Each aluminium ion can yield up to three hydrogen ions! Therefore, acidic soils are essentially “aluminium” soils. It is aluminium, not free H⁺, that is the main cause of low pH and buffering of acidic soils (White, 2006; Foth, 1990; Bloom and Skyllberg, 2012).

What does this mean for migration and toxicity?

• Al³⁺ and its hydroxy complexes (AlOH²⁺, Al(OH)₂⁺) adsorb on colloid surfaces, blocking exchange sites and displacing Ca²⁺, Mg²⁺, K⁺. This reduces the availability of these elements to plants (Foth, 1990; Weil and Brady, 2017).
• Free Al³⁺ in solution is toxic to plant roots: it inhibits cell growth, disrupts water and nutrient uptake. This is one of the main reasons for plant inhibition on acidic soils (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).
• Upon liming (addition of CaCO₃ or Ca(OH)₂), Al³⁺ precipitates as insoluble Al(OH)₃, and toxicity disappears. Therefore, liming of acidic soils is not just neutralisation of H⁺, but precipitation of aluminium (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

How to diagnose aluminium acidity?

• By high content of exchangeable Al³⁺ (determined by extraction with 1 M KCl).
• By low pH (usually < 5.0–5.5).
• By the ratio of pH(H₂O) and pH(KCl): in soils where Al³⁺ dominates, pH(KCl) is usually significantly lower than pH(H₂O) (difference > 0.5–1.0) (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

6.5. Buffering Capacity – Why Does pH Not Change Sharply?

One of the most important properties of soil is buffering capacity (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017). This is the ability of soil to resist changes in pH when acids or bases are added. Buffering is the “strength reserve” of soil against acidification or alkalisation.

What provides buffering?

1. Presence of exchangeable cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺, Al³⁺). When acid is added to the solution, H⁺ displaces Ca²⁺ and Mg²⁺ from exchange sites, binding to colloids. This reduces the concentration of H⁺ in the solution and prevents a sharp drop in pH. When a base is added, OH⁻ neutralises H⁺ from the exchange complex, and Ca²⁺ passes into solution – pH remains stable (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

2. Presence of carbonates (CaCO₃). In carbonate soils, buffering is provided by the reaction: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂↑. As long as CaCO₃ is present, pH does not fall below 7–8, because any H⁺ is immediately neutralised by carbonate (Birkeland, 1984; Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

3. Presence of clay minerals (especially 2:1 type). They have high exchange capacity and can hold large amounts of cations, increasing buffering (Foth, 1990; Weil and Brady, 2017).

4. Presence of organic matter. Humus has very high exchange capacity (2–5 times higher than clay minerals) and can hold large amounts of H⁺ and Al³⁺ (Foth, 1990; Scheffer et al., 2018; Bloom and Skyllberg, 2012).

5. Hydrolysis of aluminium. As we already discussed, Al³⁺ can release or absorb H⁺ depending on pH, creating additional buffering in the acidic range (Foth, 1990; Bloom and Skyllberg, 2012; White, 2006).

Which soils have high buffering?

• Soils with high clay content (especially 2:1 type) – chernozems, chestnut soils, grey soils.
• Soils with high organic matter content – chernozems, sod soils, peat soils.
• Soils containing carbonates (pH > 7) – carbonate chernozems, grey soils, rendzinas.

Which soils have low buffering?

• Sandy soils with low clay and humus content – podzols, sod‑podzolic, grey forest soils.
• Soils dominated by 1:1 clay minerals (kaolinite) and oxides – red soils, ferralitic soils.

Low buffering means that such soils acidify quickly under anthropogenic impact (acid rain, fertilisers) and require frequent liming (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

6.6. Effect of pH on Element Migration

Now we come to the most important point for our topic: how pH affects element migration (Birkeland, 1984; Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017). This is a key point that links acidity to geochemical processes.

In acidic media (pH < 5.5–6.0):

• Fe, Al, Mn pass into solution and become mobile. Fe³⁺ and Al³⁺ hydrolyse but remain in solution as hydroxo complexes.
• This leads to: 1) Removal of Fe and Al from the upper part of the profile (podzolisation). 2) Accumulation of Fe and Al in illuvial horizons as hydroxides (Bs horizon) (Birkeland, 1984; Foth, 1990; Scheffer et al., 2018). 3) Formation of Fe and Mn concretions at oxidative barriers. 4) Toxicity of Al³⁺ and Mn²⁺ to plants (Foth, 1990; Weil and Brady, 2017).
• Microelements (Cu, Zn, Co, Mo) become more available, but may reach toxic concentrations (Foth, 1990; Weil and Brady, 2017).
• Phosphates become less soluble (binding with Fe and Al). This leads to phosphorus starvation of plants on acidic soils (Foth, 1990; Weil and Brady, 2017).
• Calcium, magnesium, potassium are leached and removed from the profile (Birkeland, 1984; Foth, 1990).
• Organic matter becomes more mobile (formation of soluble complexes). This enhances podzolisation (Birkeland, 1984; Scheffer et al., 2018).

In neutral media (pH 6.0–7.5):

• Most elements are in forms optimal for plant nutrition.
• Fe and Al are immobile, in the form of hydroxides (insoluble).
• Migration is moderate, elements are not leached too intensively.
• Favorable conditions for microorganisms (nitrification, organic decomposition).
• Phosphorus is most available (minimal fixation) (Foth, 1990; Weil and Brady, 2017).

In alkaline media (pH > 7.5):

• Fe, Mn, Cu, Zn become immobile (form insoluble hydroxides, carbonates, phosphates). Plants suffer from deficiency (chlorosis – yellowing of leaves due to Fe deficiency) (Foth, 1990; Weil and Brady, 2017).
• Ca and Mg can accumulate as carbonates (CaCO₃, CaMg(CO₃)₂), forming carbonate horizons (Bk, K) (Birkeland, 1984; Foth, 1990).
• Phosphorus binds with calcium, forming poorly available calcium phosphates (Foth, 1990; Weil and Brady, 2017).
• Mobility of organic matter decreases, humus is stabilised (Foth, 1990; Scheffer et al., 2018).
• Molybdenum becomes more available (unlike most microelements) (Foth, 1990; Weil and Brady, 2017).

Thus, the same element behaves completely differently depending on pH. This is why pH is the “conductor” of migration – it determines in which form elements will be and whether they will move (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

6.7. Acidity and Geochemical Barriers

As we have already said, geochemical barriers are zones where conditions change sharply and elements precipitate or adsorb (Birkeland, 1984; Mukha et al., 2003). Acidity plays a key role in the formation of barriers.

Alkaline barrier (carbonate):

This is one of the most common barriers, especially in semi‑arid and arid areas (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

Where it occurs: Where acidic or neutral waters containing Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺, heavy metals encounter carbonates (CaCO₃, CaMg(CO₃)₂) or enter a zone with high pH (e.g., upon evaporation, upon contact with alkaline rocks) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

What happens: Upon increase in pH (usually > 7–8), many elements precipitate as carbonates, hydroxides or basic salts (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

Main reactions:

• Ca²⁺ + CO₃²⁻ → CaCO₃↓ (calcite, aragonite)
• Mg²⁺ + CO₃²⁻ → MgCO₃↓ (magnesite) – to a lesser extent, since MgCO₃ is more soluble
• Fe³⁺ + 3OH⁻ → Fe(OH)₃↓ (iron hydroxide)
• Al³⁺ + 3OH⁻ → Al(OH)₃↓ (gibbsite)
• PO₄³⁻ + Ca²⁺ → Ca₃(PO₄)₂↓ (calcium phosphate) (Foth, 1990; Weil and Brady, 2017)

Morphological manifestations:

• Carbonate horizons (Bk, K) with pseudomycelium (white thread‑like segregations), concretions (“crane stones”), sometimes continuous carbonate crust (calcrete, caliche) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).
• Accumulation of Fe and Al hydroxides as brown, ochreous, red spots and concretions, if the barrier coincides with an oxidative one (Birkeland, 1984; Weil and Brady, 2017).
• Formation of phosphate concretions in carbonate soils (Birkeland, 1984; Foth, 1990).

Soil processes and soil types:

• Formation of carbonate horizons in chernozems, chestnut soils, grey soils, brown semi‑desert soils (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).
• Formation of carbonate crusts (calcretes) in arid regions (Birkeland, 1984; Foth, 1990).
• Accumulation of calcium phosphates in carbonate soils (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

6.7. Acidity and Geochemical Barriers

As we have already discussed, geochemical barriers are zones where conditions change sharply, causing elements to precipitate out of solution or become adsorbed (Birkeland, 1984; Mukha et al., 2003). Acidity plays a key role in the formation of these barriers.

Alkaline Barrier (Carbonate):

This is one of the most common barriers. It occurs where acidic or neutral waters encounter carbonates (CaCO₃, MgCO₃) or enter a zone with high pH (Birkeland, 1984; Foth, 1990). Upon an increase in pH (e.g., upon contact with carbonates), many elements precipitate:

• Ca²⁺ + CO₃²⁻ → CaCO₃↓ (calcite)
• Mg²⁺ + CO₃²⁻ → MgCO₃↓ (magnesite)
• Fe³⁺ + OH⁻ → Fe(OH)₃↓
• Al³⁺ + OH⁻ → Al(OH)₃↓
• PO₄³⁻ + Ca²⁺ → Ca₃(PO₄)₂↓ (calcium phosphate)

It is the alkaline barrier that explains:

• The formation of carbonate horizons in soils of semi-arid and arid regions.
• The accumulation of carbonates, gypsum, and readily soluble salts in the lower part of the profile.
• The accumulation of calcium phosphates in carbonate soils (Birkeland, 1984; Foth, 1990).

Acidic Barrier:

This occurs during the transition from an alkaline or neutral environment to an acidic one. Upon a decrease in pH, the following may precipitate:

• Silica (SiO₂) — at pH < 9, silicic acid polymerizes and precipitates.
• Humic substances — upon acidification, humic acids coagulate.
• Some trace elements (Cu, Zn) — at very low pH, they may precipitate as sulfides (Birkeland, 1984; Foth, 1990).

Adsorption Barrier (pH-dependent):

The adsorption of cations on negatively charged colloids increases with rising pH (as the negative charge increases). The adsorption of anions (phosphates, sulfates) on positively charged surfaces (Fe, Al oxides in acidic environments) increases with decreasing pH (Foth, 1990; White, 2006; Weil and Brady, 2017).

6.8. Practical Significance of Acidity in Agronomy and Ecology

Knowledge of acidity and the ability to manage it is the foundation of modern agriculture and ecological monitoring (Foth, 1990; Weil and Brady, 2017; Bloom and Skyllberg, 2012).

For the Agronomist:

1. Crop Selection. Different plants have different tolerances to acidity. Alfalfa, sugar beet, and wheat prefer neutral soils; potatoes, rye, and lupine tolerate acidic soils. Knowing the pH helps in selecting the right crops (Foth, 1990; Weil and Brady, 2017).

2. Liming. Acidic soils require the application of lime (CaCO₃, Ca(OH)₂, CaMg(CO₃)₂). This raises pH, reduces Al and Mn toxicity, improves the availability of Ca, Mg, P, and activates microorganisms. The lime rate is determined by pH and the soil's buffering capacity (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

3. Fertilizer Selection. On acidic soils, it is not recommended to use physiologically acidic fertilizers (ammonium, potassium). Fertilizers with a neutral or alkaline reaction (calcium ammonium nitrate, phosphate rock) are preferred (Foth, 1990; Weil and Brady, 2017).

4. Diagnosing Deficiencies. Low pH is often accompanied by deficiencies of Ca, Mg, Mo and excess of Al, Mn (toxicity). pH analysis helps diagnose these disorders (Foth, 1990; Weil and Brady, 2017).

For the Ecologist:

1. Heavy Metal Pollution. In acidic soils, heavy metals (Pb, Cd, Zn, Cu) are more mobile and can migrate into groundwater or be taken up by plants. Therefore, the risk of pollution is higher in acidic soils (Weil and Brady, 2017; Huang et al., 2012).

2. Acid Rain. In regions with acidic precipitation, soils with low buffering capacity acidify faster, leading to the degradation of forest ecosystems and acidification of waters (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

3. Managing pH for Soil Remediation. Raising pH (liming) can reduce the mobility of heavy metals and their uptake by plants. Lowering pH (applying sulfur) can be used to mobilize elements during phytoremediation (Weil and Brady, 2017; Huang et al., 2012).

6.9. Summary for the Section

Let's summarize the key points on acidity (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017; Birkeland, 1984; White, 2006).

1. Soil acidity is a property due to the presence of H⁺ and Al³⁺ in the solution and on the exchange complex. The measure of acidity is pH.

2. We distinguish between active (in solution) and exchangeable (on colloids) acidity. They are in dynamic equilibrium.

3. The main source of acidity in acidic soils (pH < 5.5) is aluminum. Al³⁺ hydrolyzes, releasing H⁺, and is itself toxic to plants. Therefore, liming is essentially the precipitation of aluminum.

4. Sources of acidification: natural (CO₂, organic acids, Al hydrolysis, leaching of bases) and anthropogenic (acid rain, physiologically acidic fertilizers, intensification of mineralization).

5. Buffering capacity is the soil's ability to resist changes in pH. It is provided by exchangeable cations, carbonates, clay minerals, and humus. High buffering capacity is found in Chernozems and carbonate soils. Low buffering capacity is found in sandy Podzols.

6. pH determines the migration of elements:

• Acidic environment: Fe, Al, Mn, and micronutrients are mobile; Ca, Mg, K are leached; P availability decreases.
• Neutral environment: optimal availability of most elements; migration is moderate.
• Alkaline environment: Fe, Mn, Cu, Zn are immobile; Ca, Mg, and calcium phosphates accumulate.

7. Practical significance: pH determines crop selection, the need for liming, fertilizer choice, and the risk of toxicity and pollution.

Thus, acidity is the "conductor" that governs the migration of elements. It determines in which form the elements will exist, whether they will move within the profile or become fixed, and whether they will be available to plants or toxic to them. Therefore, knowledge of pH and the factors that determine it is essential for any agronomist, ecologist, and soil scientist.

In the next section, we will move on to complexation — another crucial mechanism that allows elements to migrate even under conditions where their ionic form is insoluble. If pH and Eh are the "conductor" and the "engine," then complexation is the "cunning detour" that allows elements to move around barriers.

7.1. What is a Complex? Basic Concepts

Let's start with a definition (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017).

A complex (or coordination compound) is a chemical particle consisting of a central atom or ion (usually a metal) bonded to a specific number of surrounding molecules or ions — ligands (Huang et al., 2012; Weil and Brady, 2017). The central ion is called the complexing agent, and the surrounding ligands form the coordination sphere.

The simplest and most ubiquitous example is the hydration of metal ions in aqueous solution. The metal ion is surrounded by water molecules, which act as ligands. For example, the Fe³⁺ ion in aqueous solution exists as an aqua complex:

Here, six water molecules are coordinated around the central Fe³⁺ ion (Huang et al., 2012; Foth, 1990). This is a hydrated complex. Water molecules are the most common ligands in soil solutions.

However, ligands can also be other molecules and ions: inorganic (Cl^-, SO₄^2-, F⁻, HCO₃^-, H₂PO₄⁻) and organic (citrate, oxalate, fulvic acids) (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017). When a metal binds to an organic ligand, a metal-organic complex is formed.

Why is complexation so important for migration?

Because complexation can drastically change the properties of the metal ion:

• Solubility: many metals (Fe³⁺, Al³⁺) in their free ionic form readily hydrolyze and precipitate. As part of a complex, they can remain in solution and migrate (Huang et al., 2012; Weil and Brady, 2017).
• Charge: the metal in a complex may have a different effective charge, affecting its adsorption on colloids and its migration ability (White, 2006; Huang et al., 2012).
• Toxicity: the metal in a complex may be less toxic than the free ion (or, conversely, more toxic in some cases) (Weil and Brady, 2017; Huang et al., 2012).

7.2. Types of Complexes: Outer-Sphere and Inner-Sphere

In soil science and aqueous chemistry, two main types of complexes are distinguished — outer-sphere and inner-sphere (Huang et al., 2012; White, 2006; Weil and Brady, 2017).

Outer-Sphere Complexes (Ion Pairs):

These are complexes in which the ligand and the central ion are separated by one or more water molecules. The ligand does not enter the first coordination sphere of the metal but is held at a distance by electrostatic attraction (Huang et al., 2012; White, 2006). This is a relatively weak bond.

Examples of outer-sphere complexes in soil solutions:

• CaSO₄⁰ (calcium sulfate)
• MgSO₄⁰
• NaSO₄⁻
• CaHCO₃⁺
• CuCl⁺ (Huang et al., 2012; Foth, 1990)

Outer-sphere complexes are generally less stable than inner-sphere complexes and dissociate more readily. They are important for the transport of ions in solution but not for long-term migration (White, 2006; Huang et al., 2012).

Inner-Sphere Complexes (Chelates):

These are complexes in which the ligand directly bonds to the central ion, displacing water molecules from its coordination sphere (Huang et al., 2012; White, 2006; Weil and Brady, 2017). A direct coordination bond is formed between the ligand and the metal (usually through oxygen, nitrogen, or sulfur atoms). This is a significantly stronger bond than in outer-sphere complexes.

A special case of inner-sphere complexes is chelates (from the Greek chele — claw). These are complexes in which a single ligand (chelating agent) binds to the metal through two or more donor groups, forming a ring structure (Huang et al., 2012; Weil and Brady, 2017). Chelates are generally significantly more stable than simple inner-sphere complexes.

Examples of inner-sphere complexes and chelates in soils:

• Fe³⁺-citrate (iron bound to citric acid via several carboxyl groups)
• Al³⁺-oxalate (aluminum bound to oxalic acid)
• Cu²⁺-fulvate (copper bound to fulvic acid — a high-molecular-weight chelating agent)
• Fe³⁺-siderophores (specific chelators secreted by microorganisms) (Huang et al., 2012; Scheffer et al., 2018; Weil and Brady, 2017)

7.3. Ligands in Soil Solution: Inorganic and Organic

Numerous potential ligands are present in the soil solution. They can be divided into two groups: inorganic and organic (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017).

7.4. Mechanism of Complexation: How Does It Work?

Now, let's examine how complexation occurs at the molecular level (Huang et al., 2012; White, 2006; Weil and Brady, 2017).

The basic principle: the ligand "attacks" the metal ion, displacing water molecules from its coordination sphere.

Let's consider the example of the Fe³⁺ ion in aqueous solution. In the absence of organic ligands, Fe³⁺ exists as the aqua complex Fe(H₂O)₆³⁺. In this state, it is prone to hydrolysis and precipitation.

When an organic acid (e.g., citric acid) enters the solution, its carboxyl groups (—СООН) dissociate, forming negatively charged carboxylate ions (—СОО⁻). These anions are "attracted" to the positively charged Fe³⁺ and displace one or more water molecules from the coordination sphere (Huang et al., 2012; White, 2006; Weil and Brady, 2017):

As a result, an inner-sphere complex Fe³⁺-citrate is formed, in which the Fe³⁺ ion is directly bonded to the oxygen atoms of the carboxyl groups of citric acid. This complex is stable, soluble, and can migrate with the soil solution (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017).

The stability of the complex is determined by several factors (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017):

1. Nature of the metal: The higher the charge and the smaller the ionic radius, the stronger the complex (Irving-Williams series: Mg²⁺ < Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺). Fe³⁺ and Al³⁺ form particularly strong complexes (Huang et al., 2012; Weil and Brady, 2017).

2. Nature of the ligand: The more donor groups (carboxyl, phenolic, amino) and the more favorable their spatial arrangement for chelate ring formation, the stronger the complex. Fulvic acids and siderophores are among the strongest chelators (Huang et al., 2012; Scheffer et al., 2018; Weil and Brady, 2017).

3. pH: In an acidic environment, most organic ligands are protonated (—СООН, not —СОО⁻) and bind metals less effectively. In neutral and slightly alkaline environments, complexation is enhanced. However, at very high pH, metals may hydrolyze and compete with ligands (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017).

4. Competition with other ions: Ca²⁺, Mg²⁺ can compete with Fe³⁺ and Al³⁺ for organic ligands, reducing the stability of heavy metal complexes (Huang et al., 2012; Weil and Brady, 2017).

5. Ionic strength: High salt concentration can reduce ion activity and weaken complexation (Huang et al., 2012; White, 2006).

7.5. Significance of Complexation for Element Migration

Now we come to the most important part: how does complexation affect migration? (Huang et al., 2012; Birkeland, 1984; Weil and Brady, 2017; Scheffer et al., 2018).

The main effect: complexation converts elements into a soluble, mobile form, bypassing precipitation and adsorption barriers.

Let's consider a few classic examples.

7.6. Complexation and Geochemical Barriers

Complexation is closely linked to geochemical barriers (Birkeland, 1984; Mukha et al., 2003; Weil and Brady, 2017).

Complexation is an "anti-barrier" mechanism. It allows elements to overcome barriers that would have trapped them in their ionic form. For example, Fe and Al, as part of organic complexes, can pass through adsorption barriers (clay minerals) and oxidation barriers (precipitation of hydroxides).

However, the complexes themselves can create barriers elsewhere. For instance, in the Podzolization process, complexation facilitates the removal of Fe and Al from the upper part of the profile, but their accumulation occurs at a geochemical barrier in the lower part of the profile (change in pH, Eh, metal concentration, or destruction of complexes) (Birkeland, 1984; Scheffer et al., 2018).

The barrier for complex destruction occurs where:

• pH changes sharply (e.g., upon contact with carbonates).
• Eh changes sharply (e.g., upon entering an oxidizing zone).
• The concentration of competing ions (Ca²⁺, Mg²⁺) increases.
• Microbial decomposition of the organic ligand is intense (Huang et al., 2012; Weil and Brady, 2017; Birkeland, 1984).

In the zone of complex destruction, metals precipitate or are adsorbed, forming accumulation horizons (Birkeland, 1984; Scheffer et al., 2018).

7.7. Complexation and Element Toxicity

An important aspect of complexation is its effect on the toxicity of elements (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017; Bloom and Skyllberg, 2012).

Free metal ions (Fe³⁺, Al³⁺, Cu²⁺, Pb²⁺, Cd²⁺, Zn²⁺) are generally more toxic than their complexes. Toxicity is determined by the activity of the free ion in solution, not the total metal concentration. Therefore, complexation can reduce toxicity by binding toxic ions into low-activity complexes.

This is particularly important for:

• Aluminum (Al³⁺): In acidic soils, Al³⁺ is toxic to roots. Organic acids (especially fulvic acids) bind Al³⁺, reducing its toxicity. Therefore, in soils with high organic matter content, Al toxicity manifests at lower pH (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).
• Heavy metals (Cd, Pb, Cu, Zn): Their toxicity in soils is often determined by the activity of free ions. Organic matter (humus) binds heavy metals, reducing their toxicity to plants and microorganisms (Huang et al., 2012; Weil and Brady, 2017).
• Copper (Cu²⁺): Free Cu²⁺ is highly toxic to microorganisms (hence copper is used as a fungicide). In soils with high organic matter content, Cu is bound into complexes, and its toxicity is reduced (Huang et al., 2012; Foth, 1990; Weil and Brady, 2017).

7.8. Practical Significance of Complexation

Knowledge of complexation has great practical significance in agronomy and ecology (Foth, 1990; Weil and Brady, 2017; Huang et al., 2012).

In Agronomy:

1. Managing Micronutrients. Micronutrients (Fe, Mn, Zn, Cu) are often applied as chelate complexes (e.g., Fe-EDTA, Zn-EDTA). This prevents their fixation and increases their availability to plants (Foth, 1990; Weil and Brady, 2017).

2. Mobilizing Phosphorus. Application of organic fertilizers (manure, green manure, compost) increases the content of organic acids, which mobilize phosphorus from sparingly soluble phosphates (Foth, 1990; Weil and Brady, 2017).

3. Reducing Al Toxicity. Liming (raising pH) and application of organic matter reduce Al toxicity through complexation (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

In Ecology:

1. Remediation of Contaminated Soils. For the removal of heavy metals from soils, phytoextraction (hyperaccumulator plants) and chelation therapy (application of chelators that mobilize metals for subsequent extraction) are used (Weil and Brady, 2017; Huang et al., 2012).

2. Predicting Pollutant Migration. Knowing the complexation capacity of soils, one can predict whether heavy metals and radionuclides will be fixed in the soil or migrate to groundwater (Huang et al., 2012; Weil and Brady, 2017).

3. Wastewater Treatment. Chelating agents are used to bind and remove heavy metals from industrial effluents (Huang et al., 2012; Weil and Brady, 2017).

7.9. Summary for the Section

Let's summarize the key points on complexation (Huang et al., 2012; Birkeland, 1984; Foth, 1990; Weil and Brady, 2017; Scheffer et al., 2018; White, 2006).

1. A complex is a particle consisting of a central metal ion (complexing agent) and surrounding ligands (molecules or ions).

2. We distinguish between outer-sphere (ion pairs, weak) and inner-sphere (strong) complexes. Chelates are a special case of inner-sphere complexes with a ring structure.

3. Ligands in soil solutions include inorganic (OH⁻, CO₃²⁻, HCO₃^-, Cl^-, SO₄^2-, F⁻, H₂PO₄⁻) and organic (low-molecular-weight acids, fulvic acids, humic acids, siderophores). Organic ligands are generally more effective chelators.

4. Complexation increases the solubility and mobility of metals, allowing them to migrate under conditions where their ionic form is insoluble. This underlies the Podzolization process, the migration of micronutrients and heavy metals.

5. Complexation reduces the toxicity of metals by binding them into low-activity complexes. This is especially important for Al and heavy metals.

6. Geochemical barriers are often associated with the destruction of complexes (changes in pH, Eh, competition, microbial degradation of ligands), leading to metal precipitation.

7. Practical significance: use of chelate fertilizers, phosphorus mobilization, Al toxicity reduction, phytoremediation, and pollution forecasting.

Thus, complexation is the "cunning detour" for element migration. It allows metals to move in solution, bypassing barriers that would have trapped them. Without complexation, there would be no Podzols, no movement of many micronutrients in soils, and the toxicity of many metals would be significantly higher. Complexation is another tool with which nature manages the migration of elements in soil.

In the next section, we will move on to migration as an integral process — how all the mechanisms discussed (solutions, pH, Eh, complexation) combine into a unified picture of element movement within the soil profile. If we have already discussed the "engines" and "detours," we now need to understand the entire journey.

8.1. What is Migration? Definition and Types

Migration of chemical elements in soil is the movement of elements within the soil profile, between soil horizons, or beyond the soil under the influence of various physical, chemical, and biological factors (Birkeland, 1984; Mukha et al., 2003; Weil and Brady, 2017). Migration is not random wandering but a regular process determined by the combination of environmental conditions and the properties of the elements.

Migration can be classified according to various criteria.

By Direction of Movement:

1. Vertical migration — the movement of elements up or down the soil profile (Birkeland, 1984; Scheffer et al., 2018). This is the main type of migration that forms profile differentiation.

• Downward migration — with infiltrating water (precipitation, meltwater). This is the main pathway for the removal of elements from the upper part of the profile under humid conditions (Birkeland, 1984; Foth, 1990).
• Upward migration — with capillary rise of water from groundwater or with evaporative flux. This is the main pathway for the accumulation of salts, carbonates, and gypsum under arid conditions (Birkeland, 1984; Foth, 1990).

2. Horizontal (lateral) migration — the movement of elements within a single horizon or across the soil surface. It is particularly important:

• On slopes — during surface runoff and subsurface lateral flow (Birkeland, 1984; Scheffer et al., 2018).
• In floodplains — during river flooding.
• In wetland ecosystems — during lateral groundwater flow (Birkeland, 1984; Weil and Brady, 2017).

3. Radial migration — movement from plant roots to the soil and back (biogenic migration). This is part of the biological cycle (Foth, 1990; Weil and Brady, 2017).

By Form of Transport:

1. Ionic migration — the movement of ions in the soil solution. This is the main transport mode for most elements (Huang et al., 2012; Weil and Brady, 2017).

2. Colloidal migration — the transport of elements as part of colloidal particles (clay minerals, humic substances, oxides). Particularly important for Fe, Al, P, and heavy metals (White, 2006; Weil and Brady, 2017; Scheffer et al., 2018).

3. Migration as organic complexes — the transport of metals as metal-organic chelates. This is the "cunning detour" we discussed in Section 7 (Huang et al., 2012; Birkeland, 1984; Scheffer et al., 2018).

4. Gaseous migration — the transport of elements in gaseous form (CO₂, CH₄, N₂O, NH₃, H₂S). Important for carbon, nitrogen, and sulfur (Weil and Brady, 2017; White, 2006; Foth, 1990).

5. Mechanical migration — the transport of solid particles (erosion, deflation, bioturbation). Important for soil formation in general but is usually not considered chemical migration (Birkeland, 1984; Scheffer et al., 2018).

8.2. Driving Forces of Migration — What Makes Elements Move?

Migration is always the result of one or several driving forces acting together (Huang et al., 2012; Weil and Brady, 2017; Birkeland, 1984; White, 2006). Let's list them again, but now in the context of the soil profile.

1. Hydrodynamic Forces — Water Flow.

This is the main driving force of migration in soil. Water is the "carrier" that transports dissolved and suspended particles.

• Infiltration — downward movement of water under gravity. This is the main mechanism of downward migration. Its intensity depends on soil permeability and precipitation amount (Birkeland, 1984; Weil and Brady, 2017; Foth, 1990).
• Capillary rise — upward movement of water due to capillary forces. This is the main mechanism of upward migration in arid conditions and in soils with shallow groundwater (Birkeland, 1984; Foth, 1990).
• Horizontal flow — movement of water along slopes or in aquifers. This is the basis of lateral migration (Birkeland, 1984; Scheffer et al., 2018).

2. Gravitational Forces.

They act always and everywhere. Gravity causes water to move downward, and with it, dissolved substances. This is an integral part of the hydrodynamic flow (Weil and Brady, 2017; Birkeland, 1984).

3. Diffusion.

The movement of ions and molecules from a region of high concentration to a region of low concentration. Diffusion is particularly important under conditions of slow water movement (dense layers, interiors of aggregates) (Weil and Brady, 2017; Huang et al., 2012). It:

• Ensures the equalization of concentrations between the solution and the exchange complex.
• Draws ions towards plant roots in the rhizosphere.
• Facilitates the movement of ions within soil aggregates (Weil and Brady, 2017; Foth, 1990).

4. Electrochemical Forces (Adsorption and Desorption).

Adsorption of ions on the surface of colloids (clay minerals, humus, oxides) and their desorption is a mechanism that can retain or release elements (White, 2006; Weil and Brady, 2017; Foth, 1990). Adsorption is a "trap" that slows migration. Desorption is the "release" that allows the element to continue its journey.

5. Biological Factors.

• Plant roots actively absorb elements from the solution, creating a concentration gradient and enhancing diffusion and mass flow towards the roots (Foth, 1990; Weil and Brady, 2017).
• Microorganisms and soil animals mix the soil (bioturbation), altering conditions for migration (Scheffer et al., 2018; Foth, 1990).
• Root exudation of organic acids and siderophores is an active management of migration (mobilization of elements) (Huang et al., 2012; Weil and Brady, 2017).

6. Temperature Gradients.

They cause the movement of water in the vapor phase (thermal diffusion) and transport of dissolved substances with heat flows. This is particularly important in arid conditions (Weil and Brady, 2017; Huang et al., 2012; Foth, 1990).

7. Chemical Gradients (pH, Eh, Ligand Concentration).

Sharp changes in pH, Eh, or the concentration of organic substances create zones where elements precipitate or, conversely, go into solution. This is the basis of geochemical barriers (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

8.3. Eluvial and Illuvial Processes — The Basis of Profile Differentiation

Now we come to a key concept in soil science: eluviation and illuviation (Birkeland, 1984; Scheffer et al., 2018; Mukha et al., 2003; Weil and Brady, 2017). These two processes — "removal" and "accumulation" — are the result of migration and shape the soil profile.

Eluviation is the process of removal of substances from one horizon to the underlying ones. The horizon from which removal occurs is called eluvial (Birkeland, 1984; Scheffer et al., 2018; Mukha et al., 2003). In it, the content decreases of:

• Organic matter (humus) — during Podzolization.
• Clay particles — during lessivage (less than 0.01 mm).
• Iron, aluminum, and manganese oxides — during Podzolization and gleyzation.
• Carbonates — during leaching.
• Readily soluble salts — during washing.

Eluvial horizons typically have a light color (whitish, grayish, pale) due to the removal of colored compounds (Fe, humus) (Birkeland, 1984; Scheffer et al., 2018). Classic examples: A₂ (Podzol), Al (Paraburonozem), Ae (eluvial).

Illuviation is the process of accumulation of substances removed from the overlying horizons. The horizon in which accumulation occurs is called illuvial (Birkeland, 1984; Scheffer et al., 2018; Mukha et al., 2003). In it, the content increases of:

• Clay particles — during lessivage (clay accumulation) — Bt.
• Fe and Al oxides and organic matter — during Podzolization — Bh, Bs.
• Carbonates — during leaching (in the lower part of the profile) — Bk, K.
• Gypsum and readily soluble salts — under arid conditions — By, Bz.

Illuvial horizons often have a darker (Bh), reddish or brownish (Bs, Bt) color and a denser structure (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

The combination of eluvial and illuvial processes forms profile differentiation — the division of the soil into horizons differing in composition, color, and structure. This is the basis of soil classification (Birkeland, 1984; Scheffer et al., 2018).

8.4. Types of Migration by Composition of Removed Substances

Depending on which substances are removed from the upper part of the profile, several types of migration are distinguished (Birkeland, 1984; Scheffer et al., 2018; Foth, 1990; Weil and Brady, 2017).

1. Podzolic Type of Migration (Acidic, Complex).

Characteristic of Podzolic soils and Podzols. Removed are:

• Fe, Al, Mn, and partially Si as part of metal-organic complexes (formed by acidic organic substances from forest litter).
• Organic matter (humus) as soluble fulvic acids.
• Result: eluvial horizon (A₂, Ae) — light, depleted in Fe, Al, humus; illuvial horizon (Bh, Bs) — dark (humus) or brown-red (Fe, Al) (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

2. Lessivage Type of Migration (Solonetzic, Clay).

Characteristic of lessivated soils (Paraburonozems, Gray Forest soils). Removed is:

• The clay fraction (less than 0.001–0.002 mm) as a suspension and colloids.
• Along with clay, Fe, Al, and micronutrients adsorbed on clay particles are removed.
• Result: eluvial horizon (Al) — depleted in clay, lighter; illuvial horizon (Bt) — enriched in clay, denser, brownish-reddish (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

3. Carbonate Type of Migration (Alkaline, Bicarbonate).

Characteristic of soils in semi-arid and arid regions (Chernozems, Chestnut soils, Serozems). Removed are:

• Ca²⁺, Mg²⁺, K⁺, Na⁺ as bicarbonates.
• In the lower part of the profile, upon an increase in pH, these ions precipitate as carbonates (CaCO₃, CaMg(CO₃)₂) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).
• Result: formation of a carbonate horizon (Bk, K) — often with pseudomycelium, concretions ( "cranes" ), sometimes with a continuous carbonate layer (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

4. Saline (Halide) Type of Migration.

Characteristic of arid regions (Solonchaks). Removed are:

• Readily soluble salts (NaCl, Na₂SO₄, Na₂CO₃, CaCl₂, etc.).
• Under an evaporative barrier, they accumulate in the upper part of the profile, forming a saline horizon (Bz) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).
• Result: soil salinization, formation of Solonchaks.

5. Gley Type of Migration (Reductive).

Characteristic of waterlogged soils (Gley soils, Bog soils). Removed are:

• Fe²⁺, Mn²⁺ in their reduced, mobile form.
• In oxidizing zones (near the surface, around roots), they precipitate as hydroxides (Fe³⁺, Mn⁴⁺) (Weil and Brady, 2017; Birkeland, 1984; Foth, 1990).
• Result: eluvial horizon — bluish-gray, depleted in Fe and Mn; illuvial horizon — with concretions (ortsteins, ortzands) or a continuous layer of hydroxides (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

8.5. Factors Controlling Migration

The intensity and direction of migration are determined by a combination of several factors (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017; Scheffer et al., 2018; Huang et al., 2012).

1. Water Regime:

This is the main factor determining the direction and intensity of migration.

• Leaching regime (precipitation > evaporation): downward migration prevails (Ca, Mg, K, Na, Fe, Al, organic matter are leached). Characteristic of humid regions (Podzols, Sod-Podzolic, Brown Forest soils) (Birkeland, 1984; Foth, 1990).
• Non-leaching regime (precipitation ≈ evaporation): migration is limited, elements accumulate in the profile (carbonates, gypsum). Characteristic of semi-arid regions (Chernozems, Chestnut soils) (Birkeland, 1984; Foth, 1990).
• Effluent regime (evaporation > precipitation): upward migration prevails (capillary rise from groundwater). Readily soluble salts accumulate. Characteristic of arid regions (Solonchaks) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

2. Acidity (pH):

We have already discussed this in detail in Section 6. pH determines the form in which elements exist and, consequently, their mobility (Birkeland, 1984; Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

3. Redox Conditions (Eh):

Determine the valence state of Fe, Mn, S, N, As, Cr and their mobility (Weil and Brady, 2017; White, 2006; Foth, 1990). Reducing conditions increase the mobility of Fe and Mn; oxidizing conditions decrease it.

4. Organic Matter Content:

• Humus is a powerful adsorbent for cations. High humus content slows the migration of cations (Ca, Mg, K, NH₄, heavy metals) (Foth, 1990; Scheffer et al., 2018; Weil and Brady, 2017).
• Organic acids (fulvic acids) are powerful chelators that increase the mobility of Fe, Al, and heavy metals (Birkeland, 1984; Huang et al., 2012; Scheffer et al., 2018; Weil and Brady, 2017).

5. Particle Size Distribution and Permeability:

• Sandy soils: high permeability → intense downward migration, intense leaching (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).
• Clayey soils: low permeability → slow migration, possible water stagnation and gleyzation. High adsorption capacity (clay) → fixation of cations (Foth, 1990; Weil and Brady, 2017; Birkeland, 1984).
• Presence of dense layers (plow pan, illuvial horizon) creates a barrier to water movement and can cause local accumulation of elements (Birkeland, 1984; Scheffer et al., 2018).

6. Biological Activity:

• Plant roots actively extract elements and can move them upward (biological migration). This creates a "biogenic cycle" and can counteract the removal of elements (Foth, 1990; Weil and Brady, 2017; Birkeland, 1984).
• Microorganisms accelerate weathering, decomposition of organic matter, nitrification, denitrification — all of which affect migration (Foth, 1990; Weil and Brady, 2017; Scheffer et al., 2018).

8.6. Classification of Elements by Migration Ability

Elements differ greatly in their ability to migrate under different conditions. Several groups can be distinguished (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017; White, 2006).

Elements with high migration ability under most conditions:

• Cl, Br, I, B (as borates), S (as SO₄^2-), Na, Ca, Mg. They are easily leached under humid conditions and accumulate under arid ones (Birkeland, 1984; Foth, 1990).

Elements with medium migration ability:

• K, Si (as silicic acid), organic matter (as DOC), Sr, Ba. Their mobility strongly depends on pH and Eh (Birkeland, 1984; Foth, 1990).

Elements with low migration ability under most conditions:

• Fe, Al, Ti, Mn (under oxidizing conditions), P, Pb, Cd, Cu, Zn, Co, Ni (often as immobile precipitates or adsorbed on colloids). Their migration is possible only under specific conditions: low pH (Fe, Al, Mn, heavy metals), reducing conditions (Fe, Mn), complexation (Fe, Al, heavy metals) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017; Huang et al., 2012).

Elements whose migration depends on biota:

• C, N, P, S (in organic form). Their migration is determined by the biological cycle and can be intense even at low solubility of mineral forms (Foth, 1990; Weil and Brady, 2017; Scheffer et al., 2018).

Important: the migration ability of the same element can change by orders of magnitude with changing conditions. For example, Fe is practically immobile under oxidizing conditions but becomes very mobile under reducing and acidic conditions (Weil and Brady, 2017; White, 2006; Foth, 1990).

8.7. Migration and the Formation of Soil Horizons

Now let's see how migration leads to the formation of specific soil horizons (Birkeland, 1984; Scheffer et al., 2018; Mukha et al., 2003; Weil and Brady, 2017).

Humus horizon (A, Ah): formed by biogenic migration — the accumulation of organic matter in the upper part of the profile. Simultaneously, removal (eluviation) of Fe, Al, Ca, Mg may occur here if there is a leaching regime (Birkeland, 1984; Foth, 1990; Scheffer et al., 2018).

Eluvial horizon (A₂, Ae, Al): result of removal (eluviation) of Fe, Al, humus, clay. Usually light, whitish, sandy (Birkeland, 1984; Scheffer et al., 2018).

Illuvial horizon (Bh, Bs, Bt, Bk, By): result of accumulation (illuviation) of substances removed from the upper part. Different types of illuvial horizons reflect the different composition of the removed substances (Birkeland, 1984; Scheffer et al., 2018; Mukha et al., 2003; Weil and Brady, 2017):

• Bh — accumulation of humus (Podzolization).
• Bs — accumulation of Fe and Al (Podzolization).
• Bt — accumulation of clay (lessivage).
• Bk — accumulation of carbonates (calcic illuviation).
• By — accumulation of gypsum (gypsic illuviation).

Gley horizon (G): result of the gley process — reduction and removal of Fe and Mn under stagnant waterlogging conditions. Bluish-gray, greenish-gray (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

Carbonate horizon (Bk, K): result of carbonate migration — accumulation of CaCO₃ in the zone of insufficient leaching (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

Saline horizon (Bz, Cca): result of saline migration — accumulation of readily soluble salts under an evaporative barrier (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

8.8. Migration and Geochemical Barriers (Relationship with Section 9)

Migration and geochemical barriers are two sides of the same process (Birkeland, 1984; Mukha et al., 2003). Migration is movement. The barrier is the stop. Without migration, there would be no barriers (nothing to stop). Without barriers, migration would lead to the complete removal of all elements from the soil. Their interrelationship:

1. Migration "delivers" elements to the barrier. The solution containing ions or complexes moves through the profile until it encounters a zone where conditions change (pH, Eh, concentration, adsorption capacity) (Birkeland, 1984; Foth, 1990).

2. The barrier "stops" the elements, converting them into an immobile form (precipitation, adsorption). This leads to the accumulation of elements in a specific horizon (illuviation) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

3. After the stop at the barrier, migration can resume under changing conditions (e.g., acidification of the carbonate horizon, drainage of the gley horizon). This makes the soil a dynamic system (Birkeland, 1984; Weil and Brady, 2017).

8.9. Practical Significance of Knowledge about Migration

Understanding migration has enormous practical significance (Foth, 1990; Weil and Brady, 2017; Birkeland, 1984; Scheffer et al., 2018).

In Agronomy:

1. Diagnosing Fertility. Analysis of the soil solution composition and forms of elements allows assessment of the availability of nutrients to plants and the need for fertilizer application (Foth, 1990; Weil and Brady, 2017).

2. Predicting Element Losses. Knowing the water regime and migration ability of elements allows predicting losses of nitrogen (denitrification), leaching of calcium, magnesium, and potassium (Birkeland, 1984; Foth, 1990).

3. Managing Fertility. By regulating pH, moisture, applying organic matter and amendments, we manage migration and direct elements into the root zone (Foth, 1990; Weil and Brady, 2017; Birkeland, 1984).

In Ecology:

1. Assessing Pollution Risk. Knowing the migration ability of pollutants (heavy metals, pesticides), one can predict their entry into groundwater and plants (Weil and Brady, 2017; Huang et al., 2012).

2. Soil Remediation. Using knowledge of migration, methods for soil remediation can be developed (phytoremediation, washing, immobilization of pollutants) (Weil and Brady, 2017; Huang et al., 2012).

3. Soil Monitoring. Analysis of element migration (especially mobile forms) allows assessing the current state of soils and the degree of their anthropogenic change (Birkeland, 1984; Weil and Brady, 2017; Foth, 1990).

8.10. Summary for the Section

Let's summarize the key points on migration (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017; Scheffer et al., 2018; Huang et al., 2012; White, 2006; Mukha et al., 2003).

1. Migration is the movement of chemical elements within the soil profile or beyond. It can be vertical (downward or upward), horizontal (lateral), and biogenic (via plants).

2. Forms of migration: ionic (in solution), colloidal (as part of suspended particles), as organic complexes (chelates), gaseous, mechanical (erosion, bioturbation).

3. Driving forces of migration: hydrodynamic water flow (infiltration, capillary rise), gravity, diffusion, electrochemical forces (adsorption/desorption), biological factors (roots, microorganisms), and temperature and chemical gradients (pH, Eh).

4. Eluviation and illuviation are two opposing processes that are the result of migration. Eluviation is the removal of substances from a horizon. Illuviation is their accumulation in another horizon. The combination of these processes forms profile differentiation.

5. Types of migration differ by the composition of the removed substances: Podzolic (Fe, Al, humus), Lessivage (clay), Carbonate (Ca, Mg), Saline (readily soluble salts), Gley (Fe, Mn under reducing conditions).

6. The intensity and direction of migration are determined by the water regime, pH, Eh, organic matter content, particle size distribution, and biological activity.

7. The migration ability of elements varies greatly: some elements are easily leached (Cl, Na, Ca, Mg), others are immobile (Fe, Al, Ti, P, heavy metals), but their mobility can change drastically with changing conditions (pH, Eh, complexation).

8. Migration and geochemical barriers are inextricably linked: migration delivers elements to barriers, and barriers stop them, creating accumulation horizons.

9. Practical significance: fertility management (regulating the migration of nutrients), predicting and preventing pollution, soil remediation.

Thus, migration is the "bloodstream" of the soil, which unites all geochemical processes into a single system. It is the process that transforms the parent material into a soil with its characteristic profile, horizons, and fertility. Without migration, there would be no Podzols, no Chernozems, no Solonchaks — there would be no diversity of soils on Earth.

In the next, concluding section, we will examine geochemical barriers — those very "stops" that complete the migration path and lead to the accumulation of elements in specific horizons. It is the barriers that link the entire geochemistry to specific soil processes and the formation of soil types.

9.1. What is a Geochemical Barrier? Definition and Essence

A geochemical barrier is a zone within the soil profile or landscape where a sharp change in physicochemical conditions occurs, resulting in a sharp decrease in the migration rate of chemical elements and a sharp increase in their accumulation in the solid phase (Birkeland, 1984; Mukha et al., 2003; Weil and Brady, 2017). Simply put, it is a place where "substances stop" and accumulate.

The mechanism of barrier action is always the same: when conditions change, the element transitions from a mobile form to an immobile form (Birkeland, 1984; Foth, 1990). This can be:

• Precipitation — from solution to solid phase.
• Adsorption — fixation on the surface of colloids.
• Incorporation into secondary minerals — crystallization.
• Uptake by organisms — biogenic accumulation (Birkeland, 1984; Weil and Brady, 2017).

Why do barriers arise?

Barriers arise where the conditions of migration change sharply (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017):

• pH — transition from acidic to alkaline environment or vice versa.
• Eh — transition from oxidizing to reducing environment or vice versa.
• Ion concentration — saturation of the solution, leading to precipitation.
• Composition and activity of organic substances — appearance or disappearance of complexing agents.
• Particle size distribution — sharp change in pore size, affecting permeability and adsorption capacity.
• Temperature and moisture — evaporation, freezing, changes in solubility.

Changes in these conditions can occur:

• Vertically — as water moves down the profile (e.g., upon encountering carbonate rock or a dense layer).
• Horizontally — with lateral water movement (e.g., upon groundwater discharge at the surface).
• Spatially and temporally — with seasonal changes in moisture, temperature, groundwater level (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

9.2. Classification of Geochemical Barriers

Geochemical barriers can be classified according to various criteria. The most common classification is by the type of change in conditions (Birkeland, 1984; Mukha et al., 2003; Weil and Brady, 2017; Foth, 1990; White, 2006; Bloom and Skyllberg, 2012).

9.3. Combination of Barriers and Formation of Complex Horizons

In nature, barriers rarely act alone. More often they combine, creating complex accumulation zones (Birkeland, 1984; Weil and Brady, 2017; Scheffer et al., 2018).

Example 1 – podzolisation:

• In the upper part of the profile – acidic and complex barriers: organic acids bind Fe and Al in complexes, preventing their precipitation and promoting their migration (eluviation).
• In the lower part of the profile, where pH is higher and metal concentrations higher, complexes are destroyed – a barrier of complex destruction arises. Fe and Al hydrolyse and precipitate – oxidative and alkaline barrier (illuviation) (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

Example 2 – gley process:

• In the lower part of the profile (waterlogging zone) – reductive barrier: Fe³⁺ and Mn⁴⁺ are reduced to Fe²⁺ and Mn²⁺, pass into solution and migrate (eluviation of Fe and Mn).
• In the upper part of the profile (aeration zone) – oxidative barrier: Fe²⁺ and Mn²⁺ are oxidised to Fe³⁺ and Mn⁴⁺ and precipitate as hydroxides (illuviation) (Birkeland, 1984; Weil and Brady, 2017; Scheffer et al., 2018).

Example 3 – carbonate process in semi‑arid soils:

• In the upper part of the profile (leaching zone) – removal of Ca²⁺ and Mg²⁺ as bicarbonates.
• In the middle part of the profile (accumulation zone) – alkaline barrier: upon increase in pH (due to decrease in CO₂, evaporation), Ca²⁺ and Mg²⁺ precipitate as carbonates (illuviation) (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

9.4. Barriers and Pedogenesis: Link with Genetic Soil Types

Now let us summarise: how barriers “work” in different soil types (Birkeland, 1984; Scheffer et al., 2018; Mukha et al., 2003).

Podzolic soils: complex (eluviation) + acidic/oxidative (illuviation) barriers. Eluvial horizon (depleted in Fe, Al, humus) and illuvial (Bh, Bs – with accumulation of Fe, Al, humus) are formed (Birkeland, 1984; Scheffer et al., 2018; Weil and Brady, 2017).

Sod‑podzolic soils: biogenic barrier (humus horizon) + complex + acidic/oxidative. The humus horizon is less powerful than in chernozems, but the illuvial horizon (Bh, Bs) is pronounced (Birkeland, 1984; Scheffer et al., 2018).

Chernozems and chestnut soils: biogenic barrier (humus horizon) + alkaline (carbonate) barrier in the middle part of the profile. The humus horizon is powerful, dark; carbonate horizon (Bk) in the form of pseudomycelium or concretions (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

Grey soils (arid soils): alkaline (carbonate, gypsum) and evaporative barriers. Carbonates and gypsum accumulate in the upper part of the profile, readily soluble salts in the lower or on the surface (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

Solonchaks: evaporative barrier. Readily soluble salts accumulate at the surface (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017).

Gley soils: reductive (eluviation of Fe, Mn) + oxidative (illuviation of Fe, Mn) barriers. Bluish‑grey eluvial horizon and Fe/Mn concretions in the oxidising zone (Birkeland, 1984; Weil and Brady, 2017; Scheffer et al., 2018).

9.5. Practical Significance of Geochemical Barriers

Knowledge of geochemical barriers has great practical significance in agronomy, ecology and soil diagnostics (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017; Huang et al., 2012).

In agronomy:

1. Soil diagnostics. Barriers help identify soil types and assess their fertility. For example, the presence of a carbonate horizon indicates moisture deficiency and possible micronutrient deficiency (Birkeland, 1984; Foth, 1990).

2. Fertility management. Knowing barriers, we can manage element migration: for example, liming is the creation of an alkaline barrier for Al and Mn (precipitation), reducing their toxicity (Foth, 1990; Bloom and Skyllberg, 2012; Weil and Brady, 2017).

3. Fertiliser application. Barriers can either enhance or weaken fertiliser efficiency. For example, in carbonate soils, phosphorus is fixed as calcium phosphates (poorly available), so phosphorus fertilisers must be applied in larger doses or as chelates (Foth, 1990; Weil and Brady, 2017).

In ecology:

1. Pollution prediction. Barriers determine whether pollutants (heavy metals, pesticides) will be fixed in the soil or migrate to groundwater. For example, in acidic soils, heavy metals are more mobile (no barrier), while in neutral and alkaline soils they are fixed (alkaline and adsorption barriers) (Huang et al., 2012; Weil and Brady, 2017).

2. Soil remediation. Barriers can be used to fix pollutants (e.g., liming to precipitate heavy metals) or, conversely, to mobilise them (e.g., acidification for phytoextraction) (Weil and Brady, 2017; Huang et al., 2012).

3. Risk assessment. Barriers help assess the risk of pollutant entry into food chains (Huang et al., 2012; Weil and Brady, 2017).

In soil diagnostics:

1. Identification of soil horizons. Barriers create characteristic morphological features: carbonate horizons, concretions, gley horizons, illuvial horizons (Birkeland, 1984; Scheffer et al., 2018; Mukha et al., 2003).

2. Assessment of soil age and stage of development. Presence of well‑developed barriers (e.g., thick carbonate horizon, deep illuvial horizon) indicates long soil development (Birkeland, 1984; Foth, 1990).

3. Diagnosis of anthropogenic changes. Disturbance of barriers (e.g., removal of carbonates by acid rain, acidification) indicates soil degradation (Birkeland, 1984; Weil and Brady, 2017).

9.6. Barriers as “Memory” of Soil

Geochemical barriers are not just zones of accumulation. They are the “memory” of the soil (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017). They preserve information about past soil‑forming conditions:

• Carbonate horizons – “traces” of semi‑arid and arid conditions.
• Illuvial horizons – “traces” of podzolisation and lessivage processes.
• Gley horizons – “traces” of prolonged waterlogging.
• Saline horizons – “traces” of arid conditions and shallow groundwater.
• Humus horizons – “traces” of biological cycling and fertility (Birkeland, 1984; Scheffer et al., 2018).

Paleobarriers (relict barriers) can remain in soils after climate or conditions change. For example, carbonate horizons in soils that are now in humid conditions (paleocarbonates) – evidence of past arid phases (Birkeland, 1984; Foth, 1990).

Thus, geochemical barriers are not just stops along the migration path. They are the “archive” of soil history, which we can read from morphological, chemical and mineralogical features (Birkeland, 1984; Weil and Brady, 2017).

9.7. Final Summary of the Section

Let us summarise geochemical barriers (Birkeland, 1984; Foth, 1990; Weil and Brady, 2017; Scheffer et al., 2018; Mukha et al., 2003; White, 2006; Huang et al., 2012; Bloom and Skyllberg, 2012).

1. Geochemical barrier – a zone of sharp change in conditions where element migration slows down or stops, and they accumulate in the solid phase.

2. Classification of barriers: alkaline (carbonate), acidic, oxidative, reductive, adsorption (sorption), evaporative, biogeochemical, mechanical.

3. Barriers determine the formation of soil horizons: eluvial (removal) and illuvial (accumulation). The combination of barriers creates characteristic profiles of different soil types.

4. Barriers are the “memory” of soil. They preserve information about past soil‑forming conditions (climate, water regime, biological activity).

5. Practical significance: soil diagnostics, fertility management, pollution prediction and remediation, risk assessment of degradation.

9.8. Concluding Word: Barriers and Fertility

We have come a long way – from soil geochemistry through macro‑ and microelements, forms of occurrence, solutions, redox processes, acidity, complexation, migration – and finally arrived at geochemical barriers. And now we can answer the main question with which we started: Why do chemical elements not stay in place?

They do not stay in place because soil is a dynamic bio‑abiotic system where processes of weathering, dissolution, transport, precipitation, adsorption, complexation, oxidation and reduction constantly occur. Migration is the result of the action of many forces: water, gravity, diffusion, pH, Eh, biota. And this migration is not chaotic – it is controlled by geochemical barriers that stop elements in certain places, creating horizons and forming fertility.

It is geochemical barriers that ensure the accumulation of fertility in soil. Without barriers, elements would be completely removed from the soil, and it would remain barren. Barriers are “traps” that retain nutrients in the root zone, making them available to plants. Carbonate barriers accumulate calcium and phosphorus. The humus barrier accumulates nitrogen, phosphorus, sulfur and microelements. Adsorption barriers retain potassium, ammonium, microelements.

Thus, soil fertility is the result of the work of geochemical barriers. And by managing conditions (irrigation, drainage, liming, application of organic matter, fertilisers), we can influence barriers, direct migration and maintain soil fertility.

We have examined the entire path of chemical elements in soil – from their occurrence in the solid phase to their stopping at geochemical barriers. I hope that now you see soil not as a static body, but as a living, breathing, moving system where every atom is in constant motion.