Soil Profile and Diagnostic Horizons

1.1. What is a Soil Profile?

Imagine cutting the soil vertically, like a cake, and looking at the cross-section. What you would see is the soil profile. In soil science, we give this concept a strict definition.

Soil profile is a vertical sequence of genetically related layers (horizons) that reflects the result of long-term effects of soil-forming factors on the parent material (Birkeland, 1984; Weil, 2017).

The profile is not just a "layered cake." It is a three-dimensional structure that develops over time. Each horizon in the profile is a "witness" to a specific stage of soil formation. By reading the profile, we can reconstruct the soil's history: how the climate changed, what vegetation was present, how long formation took, and even which processes predominated.

Key idea of the lecture: The soil profile is an "archive" of the soil's history. Everything that has happened to the soil has left its trace in its horizons.

1.2. Soil Profile and Soil Pit: An Important Distinction

In soil science practice, it is necessary to distinguish between two concepts:

A soil pit is our way of "looking into" the profile. It is in the pit that we study horizons, measure their thickness, describe color, structure, and other characteristics. A good pit should be deep enough to expose all genetic horizons down to the unaltered parent material (Buol et al., 2011; White, 2006).

1.3. The Pedon and Its Significance

When we study the soil profile, we are dealing with a specific area of the soil cover. To systematize the study, soil scientists introduced the concept of the pedon (Buol et al., 2011; Weil, 2017).

Pedon is the smallest three-dimensional volume of soil that retains all the characteristic features of that soil and is sufficient for studying its horizons.

Pedon dimensions vary:

• Area: from 1 to 10 m²
• Depth: up to 2 m (or to the parent material if it is shallower)

Imagine sticking a shovel into the ground and cutting out a cube of soil that encompasses all the horizons. That is the pedon. Why is this important? Because the pedon is the minimum unit for description and classification. All laboratory analyses are performed on samples taken from each horizon of the pedon.

Interesting fact: Several adjacent pedons with similar properties are combined into a larger unit — the polypedon, which corresponds to the actual soil boundary on a map.

1.4. Soil Cover and Its Heterogeneity

If we mentally "cut" the pedon, we get a profile. If we look at the Earth's surface from above, we see the soil cover — a continuous mosaic of different pedons.

The soil cover is never homogeneous. Even over a small area, you will find different soils: on the top of a hill — one type, on the slope — another, in the lowland — a third. This regular spatial change in soils is associated with changes in soil-forming factors (relief, moisture, parent material). Milne (1936) called such a regular sequence of soils on a slope a catena (Huang et al., 2012; Birkeland, 1984).

Catena is a series of soils replacing each other from the watershed to the foot of the slope, developed on a common parent material but under different hydrological conditions.

Example of a catena:

• Watershed (good drainage) → Slope (moderate drainage) → Slope foot (poor drainage)
• Soddy-podzolic → Gray forest → Meadow

1.5. Why is the Profile the Key to Soil History?

The soil profile is the main object of study in genetic soil science. Why?

1. The entire history of soil formation is encoded in the profile. Each horizon is the result of a specific process that operated with certain intensity at a certain time.

2. The profile reflects the balance of four fundamental soil-forming processes. Simonson (1959) identified these processes, and they underlie the formation of any profile (Weil, 2017):

3. From the shape and structure of the profile, we can judge the age of the soil. The more complex and thicker the profile, and the more horizons it contains, the older the soil generally is (all other things being equal).

4. The profile is a "record" of climatic fluctuations. If we find a buried humus horizon covered by loess in a single pit, this is evidence of changing climatic conditions (warming or cooling).

Example. In the Chernozem profile, we see a thick dark humus horizon, below which, at a depth of 1–2 m, carbonate neoformations appear. This tells us:

• The climate was humid enough to support abundant herbaceous vegetation.
• There was enough precipitation to leach carbonates from the upper part of the profile, but not enough to remove them beyond its limits.
• The soil formed over many thousands of years.

1.6. The Soil Profile as an Object of Study

When studying the soil profile, the soil scientist addresses three groups of questions:

1. Descriptive: Which horizons are present? What are their thickness, color, structure, and particle-size distribution?

2. Genetic: Which processes led to the formation of these horizons? In what order did they occur?

3. Functional: How do the properties of the profile affect fertility, water regime, and resistance to stress?

We obtain answers to these questions by describing the morphological characteristics of the horizons (color, structure, particle-size distribution, neoformations, inclusions). This is what we will address in the following lectures.

Key Takeaways from this Section:

1. Soil profile is a vertical sequence of genetic horizons reflecting the history of soil development.

2. Soil pit is the wall of a pit where we observe the profile.

3. Pedon is the minimum three-dimensional unit of soil sufficient for studying all its horizons.

4. Catena is a regular sequence of soils on a slope, reflecting changes in hydrological conditions.

5. The profile is formed under the influence of four processes: transformation, translocation, accumulation, and eluviation.

6. The profile is an "archive" of soil history, and the ability to "read" it is a key skill for a soil scientist.

2.1. What is a Genetic Horizon?

When we look at the soil profile, we see that it consists of layers that differ in color, structure, density, organic matter content, and other characteristics. These layers are called genetic horizons. Their appearance is the result of soil-forming processes.

Genetic horizon is a layer of soil that:

• differs from adjacent layers in morphological, physical, chemical, or biological properties;
• formed as a result of one or more soil-forming processes;
• has a genetic connection with other horizons in the given profile (Birkeland, 1984; Buol et al., 2011).

In other words, a horizon is not just a mechanical layer of deposition (as in geology). It is the result of soil formation: something has accumulated here, something has been leached out, something has been transformed. Each horizon is a "witness" to specific processes.

It is important to understand: horizons are not parallel to the bedding surfaces of the parent material (although they are often roughly parallel to the soil surface). If we see layers in a pit that are parallel to each other and unrelated to the surface, these are more likely geological layers, not soil horizons. Soil horizons form from the top down and follow the surface topography (Birkeland, 1984).

2.2. Notation System for Genetic Horizons

In soil science, a unified system of letter symbols for genetic horizons is adopted. The main (master) horizons are denoted by uppercase Latin letters: O, A, E, B, C, R.

Let's examine each one. For clarity, let's present a classic profile of a temperate forest soil:

• O — organic litter
• A — humus-accumulative horizon
• E — eluvial (leached) horizon
• B — illuvial (accumulated) horizon
• C — parent material (slightly altered)
• R — underlying bedrock

2.3. Transitional Horizons

Between the main horizons, there are often transitional zones where the properties of two horizons are mixed. They are denoted by combinations of letters:

• AB — transitional, where A properties predominate, but B features appear.
• BA — transitional, where B properties predominate, but A features appear.
• EB — transition between E and B.
• BC — transition between B and C.

If the transition is sharp rather than gradual, a slash is used: E/B means that the horizon contains separate areas of E and B (Birkeland, 1984; Weil, 2017).

2.4. Horizon Numbering

To designate subhorizons within a single genetic type, Arabic numerals are used: B1, B2, B3, etc., starting from the top down. For example:

• A1 — upper part of the humus horizon
• A2 — lower part of the humus horizon (lighter)
• Bt1 — upper illuvial-clayey subhorizon
• Bt2 — lower illuvial-clayey subhorizon

Numerals are also used when the profile contains several layers of parent material of different composition (lithological discontinuities). Then a numeral is placed before the letter, denoting the layer (counting from the top): 2Btb means that the illuvial-clayey horizon is developed in the second layer of parent material (Birkeland, 1984; Buol et al., 2011).

2.5. How Do Genetic Horizons Help "Read" Soil History?

Now, knowing the notation system, we can answer the main question: how can we understand the soil's history from its horizons?

1. The sequence of horizons already tells us a great deal. If we see a profile O-A-E-Bt-C, this is a typical podzolic process: there are eluvial and illuvial horizons, indicating a leaching regime and removal of substances.

2. The thickness of horizons is an indicator of the duration of the process. A thick A-horizon of Chernozem speaks of millennia of humus accumulation in steppe conditions.

3. The presence of neoformations (clay cutans, carbonate veins, ferruginous concretions) indicates specific processes (illuviation, carbonatization, gleying). For example, clay coatings on the surface of peds are direct evidence of clay translocation (Birkeland, 1984; Weil, 2017).

4. Buried horizons — if we find an old A or Bt horizon at depth, covered by new material, this indicates an interruption of soil formation (this could be climatic changes, erosion, and sedimentation). Such horizons are denoted with the suffix b (e.g., Ab or Btb) (Birkeland, 1984).

5. Comparison of profiles in a catena — by observing how horizons change along the slope, we can reconstruct processes of lateral migration of substances (hydromorphism, erosion, accumulation).

Example of profile reading. Profile: Ah – Ael – Bt1 – Bt2 – BC – C

• Ah — humus layer (organic accumulation).
• Ael — light gray, structureless — eluvial, from which clay and iron have been leached.
• Bt1, Bt2 — brown, heavy loamy, with clay cutans — illuvial accumulation of clay.
• BC — C — transition to parent material.

Conclusion: This is a Soddy-podzolic soil, formed under forest in a leaching regime. The process is eluvial-illuvial redistribution of clay and oxides. The soil is relatively old (has a distinct Bt).

2.6. Relationship between Genetic Horizons and Diagnostic Horizons

It is important to distinguish between genetic horizons (which we have just described) and diagnostic horizons used in classification systems (e.g., in WRB or Soil Taxonomy). Genetic horizons are field designations; diagnostic horizons are strictly defined criteria (by color, clay content, cation exchange capacity, etc.) that serve for classification (Weil, 2017).

For example, the mollic epipedon (diagnostic) may include several genetic horizons (A, AB) and even part of Bt if they are dark and base-saturated. The argic horizon is a diagnostic horizon that usually corresponds to the genetic Bt but with clear quantitative criteria for clay increase. In the next part of the lecture, we will examine WRB diagnostic horizons in detail.

Key Takeaways from this Section:

1. Genetic horizon is a layer of soil that differs in properties and was formed as a result of soil-forming processes.

2. The main genetic horizons are: O (organic), A (humus-accumulative), E (eluvial), B (illuvial), C (parent material), R (bedrock).

3. Suffixes are used for clarification: t (clay), h (humus), s (iron/aluminum oxides), k (carbonates), y (gypsum), z (salts), g (gleying), w (weak alteration), and others.

4. Transitional horizons are denoted by two letters (AB, EB, etc.); numerals are used for subhorizons (B1, B2).

5. Buried horizons — suffix b — indicate a break in soil formation.

6. The system of genetic horizons allows us to "read" soil history: from the combination, thickness, and properties of horizons, we can reconstruct the predominant processes and environmental conditions.

3.1. From Genetic Horizons to Diagnostic Horizons: Why is This Important?

In the previous section, we became familiar with the system of genetic horizons (O, A, E, B, C, R) and their suffixes. This is a convenient language for field description of the soil profile. However, this system has an important drawback: it is not strict and quantitative.

Imagine: one soil scientist calls a horizon Bt because they see clay coatings on the surface of peds. Another calls the same horizon Bw because they believe the clay increase is insufficient for diagnosis. As a result, two specialists may classify the same soil differently. To eliminate subjectivism, international classifications (especially WRB and Soil Taxonomy) use diagnostic horizons (Birkeland, 1984; Buol et al., 2011; Weil, 2017).

Diagnostic horizon is a layer of soil that has strictly defined, measurable, and reproducible properties (thickness, organic carbon content, clay content, oxide content, cation exchange capacity, base saturation, pH, etc.). If a horizon meets these criteria, it can be unambiguously identified regardless of who is doing the description.

The main difference (Birkeland, 1984; Weil, 2017):

Diagnostic horizons are divided into two broad groups:

1. Surface (epipedons) — formed in the upper part of the profile (including A and partially B, if darkened by humus).

2. Subsurface — formed below the epipedon (mainly B and sometimes C) (Weil, 2017; IUSS Working Group WRB, 2015).

Knowledge of diagnostic horizons is critically important because they determine which WRB Reference Soil Group (and which Soil Taxonomy order) the soil will be assigned to.

3.2. Overview of WRB Diagnostic Horizons

Let's examine the key diagnostic horizons used in the World Reference Base for Soil Resources (WRB) (IUSS Working Group WRB, 2015; Scheffer et al., 2018; Huang et al., 2012).

3.3. How Do Diagnostic Horizons Help "Read" Soil History?

Diagnostic horizons are not just "labels" for classification. They are strictly linked to processes and environmental conditions. If we know which diagnostic horizons are present in the profile, we can confidently reconstruct:

1. Climatic regime:

• Spodic — acidic leaching regime (cool, humid climate).
• Oxic — hot, humid climate with long-term weathering (tropics).
• Calcic / Gypsic / Salic — arid or semiarid climate, where precipitation is insufficient to leach salts.

2. Drainage regime:

• Ferric / Plinthic — seasonal waterlogging with alternating oxidizing and reducing conditions.
• Natric — often associated with poor drainage and sodium accumulation.

3. Age (developmental stage):

• Cambic → initial stage of weathering.
• Argic / Natric → more advanced stage (clay illuviation present).
• Oxic → very old stage (deep, multi-million-year weathering).

3.4. Relationship with Other Classification Systems

It is important to understand that WRB diagnostic horizons are not the only system. Soil Taxonomy uses similar, but sometimes differently named and differently criterial, horizons:

Soil Taxonomy also uses epipedons and subsurface horizons, but the criteria for thickness, percentages, and measurement methods may differ slightly. When working with international materials, always check which system is being used (Weil, 2017; Soil Survey Staff, 2014).

Key Takeaways from this Section:

1. Diagnostic horizons are strictly defined (quantitatively) layers of soil used for classification in WRB and Soil Taxonomy.

2. They are divided into surface (epipedons) — Mollic, Umbric, Ochric, Histic, Melanic, etc. — and subsurface — Argic, Cambic, Spodic, Oxic, Calcic, Gypsic, Salic, Natric, Duric, Ferric, Plinthic.

3. The presence or absence of certain diagnostic horizons is the main criterion for assigning soil to WRB Reference Soil Groups (e.g., presence of Spodic → Podzol; presence of Oxic → Ferralsol; presence of Mollic + Calcic → Chernozem or Kastanozem).

4. Diagnostic horizons reflect key soil-forming processes and environmental conditions, thus serving as reliable indicators for reconstructing soil history.

5. Different classification systems (WRB and Soil Taxonomy) may have different horizon names and criteria, but the basic concepts are shared.

4.1. Why Do Transitional Horizons Occur?

In a real soil profile, boundaries between horizons are rarely sharp. More often, we observe a gradual transition from one horizon to another. This is natural: soil-forming processes do not cease abruptly at a certain depth. For example, humus accumulation in the A-horizon gradually weakens with depth, while weathering processes in the B-horizon intensify. This results in a zone of mixing of properties of adjacent horizons.

These transitional zones, having a thickness from a few centimeters to tens of centimeters, are called transitional horizons (Birkeland, 1984; Weil, 2017).

It is important to distinguish transitional horizons from lithological layers. Geological layers (e.g., interlayers of sand and clay in alluvium) have sharp boundaries and are not related to soil-forming processes. Transitional horizons result from the gradual change in the intensity of soil formation with depth.

Why are transitional horizons important?

1. They "smooth out" sharp jumps in properties, showing exactly how the change of processes occurs.

2. They contain information about the rate of soil formation: the thicker and more gradual the transition, the slower the processes proceeded or the older the soil.

3. They help to correctly determine the boundaries of diagnostic horizons (especially in WRB, where it is important at what depth argic, calcic, spodic, etc., begin).

4.2. Types of Transitional Horizons

Depending on how exactly the properties are mixed, transitional horizons are divided into two main types (Birkeland, 1984; Weil, 2017; Soil Survey Staff, 2014).

4.3. How Transitional Horizons Are Designated in Field Practice

In field description, transitional horizons are distinguished when their thickness is at least 5 cm (and usually 10–20 cm) (Birkeland, 1984; Weil, 2017). It is important to:

1. Assess which horizon dominates (this is the first letter).

2. Describe the boundaries (sharp, clear, gradual, diffuse).

3. For A/B-type — note the depth and shape of the "tongues" (narrow, wide, wedge-shaped).

Example of field description:

• Ah 0–20 cm — dark gray, humus, granular
• AB 20–35 cm — transitional, darker than Bt, but clay coatings already visible
• Bt1 35–65 cm — brown, clayey, with cutans
• Bt2 65–100 cm — lighter, clayey, with rare cutans
• BC 100–120 cm — transition to rock, rock fragments visible
• C 120+ cm — unaltered parent material

Here AB is a transition with A dominance (humus properties are still strong), and BC is a transition with B dominance (weathering features are still preserved, but parent material is already noticeable).

4.4. What Do Transitional Horizons Tell Us About Soil History?

1. Thickness of transition zones (especially AB and BC) indicates the rate and duration of soil formation:

• In young soils, transition zones are often narrow (a few cm) because processes have just begun.
• In mature soils, transitions can be thick (20–40 cm) and gradual, indicating long-term, equilibrium development.

2. Presence of E/B or A/B tongues indicates uneven water movement (preferential flow through cracks, macropores). This may indicate:

• Periodic freezing-thawing (cryoturbation), forming tongues.
• Active bioturbation (worms, rodents mixing material).
• Texture conducive to uneven filtration (e.g., sand in loam).

3. Gradual vs. sharp transitions:

• Gradual transition (thick AB, EB) indicates that the process forming the horizon acted for a long time and uniformly.
• Sharp transition (narrow, clear boundary between A and Bt) indicates a change in conditions — for example, a sharp change in water regime, or rapid sediment accumulation, or severe erosion exposing a deeper horizon.

4. Presence of a buried transitional horizon (e.g., 2ABb) is evidence that soil formation was interrupted (sedimentation) and then resumed.

4.5. Transitional Horizons and WRB Diagnostic Horizons

In WRB, transitional horizons are not diagnostic (i.e., they do not have strict criteria for classification). However, their description is mandatory for correct identification of diagnostic horizons.

For example, to diagnose an argic horizon, it is necessary to ensure that the clay increase occurs specifically in Bt, and not due to lithological discontinuity. The transitional EB helps to see where the depletion zone (E) begins and where eluviation ends.

In addition, WRB has a special concept — abrupt textural change, which is used as a diagnostic property (e.g., for Planosols). This is precisely the case when a transitional horizon is absent or very narrow, and the boundary between A/E and Bt is sharp (Birkeland, 1984; IUSS Working Group WRB, 2015).

4.6. Special Cases: Bisequal Profiles

Sometimes, two sequences of horizons formed at different times or under different conditions occur in one profile. This is called a bisequal profile (Birkeland, 1984; Weil, 2017). For example:

• O — forest litter
• E — eluvial
• Bh — humus-illuvial
• E' — second eluvial horizon (deeper)
• Bt — illuvial-clayey
• C

Here, between Bh and Bt, there is a transitional E' (denoted by a prime). This means that the soil had two stages of soil formation: first, Bt formed, then conditions changed, and Bh began to form, "cutting into" the old E. Transitional horizons in such profiles are especially important because they show the "junction" of different epochs of soil formation.

Key Takeaways from this Section:

1. Transitional horizons are zones of gradual mixing of properties of adjacent horizons, arising from the gradual attenuation or intensification of soil-forming processes with depth.

2. They are denoted by two letters: AB (A dominates), BA (B dominates), EB, BE, BC, CB.

3. If separate fragments (tongues) of another horizon are visible in a horizon, a slash is used: A/B, E/B, B/C.

4. The thickness and nature of transition zones provide information about the duration, rate, and uniformity of soil formation.

5. Sharp boundaries often indicate changes in conditions or lithological discontinuity.

6. Transitional horizons are not diagnostic in WRB, but their description is necessary for correct determination of diagnostic horizons.

7. In complex (bisequal) profiles, transitional horizons help to distinguish different stages of soil formation.

5.1. What are Soil Neoformations?

When we describe the soil profile, we see not only horizons of different colors and compositions. Within the horizons, we often encounter structures that are not part of the main soil mass — they stand out in color, density, shape, and chemical composition. These are soil neoformations (in English literature — pedofeatures).

Soil neoformations are isolated morphological elements that formed within the soil as a result of specific soil-forming processes (Birkeland, 1984; Buol et al., 2011; Weil, 2017).

Neoformations are "documentary evidence" of processes that have occurred or are occurring in the soil. If horizons are the "chapters" of soil history, then neoformations are the "illustrations" to these chapters — specific "traces" of transformation and redistribution of substances.

It is important to distinguish neoformations from inclusions (see the next section):

5.2. Classification of Soil Neoformations

There are several approaches to classifying neoformations. We will use the system proposed by Brewer (1964) and developed in works on soil micromorphology (Buol et al., 2011; Huang et al., 2012). According to this system, four main groups of neoformations are distinguished.

5.3. Neoformations and Major Soil-Forming Processes

Now let's link neoformations to specific processes that shape the soil profile.

5.4. Macromorphology vs. Micromorphology of Neoformations

It is important to distinguish between two levels of studying neoformations:

1. Macromorphology — study of neoformations with the naked eye or with a hand lens. We see the color, shape, size, orientation of neoformations, and their position in the profile (Birkeland, 1984; Buol et al., 2011).

2. Micromorphology — study of neoformations in thin sections under a microscope (petrographic or electron). We can see the internal structure, orientation of clay particles, nature of contact with the matrix, and microscopic layered structures (Buol et al., 2011; Birkeland, 1984).

Micromorphology allows us to:

• Distinguish illuvial argillans (with parallel orientation of particles) from stress argillans (formed during clay shrinkage/swelling).
• See the sequence of superimposition of different neoformations (e.g., carbonate coating on a clay cutan — this means that clay illuviation occurred first, then carbonatization).
• Detect signs of bioturbation (mixing, ruptures of cutans).

Example: In a thin section of a Bt-horizon, we see clay cutans covered on top by a thin layer of carbonate. This means: first, clay accumulation (illuviation) occurred; then the climate became drier, and carbonates began to precipitate over the clay coatings. This is direct evidence of climatic change in the history of this soil.

5.5. Neoformations and Soil Age

Neoformations are among the most important chronological indicators in soil science.

1. The number and thickness of neoformations correlate with age. The older the soil, the more clay cutans, carbonate concretions, and ferruginous nodules.

2. Stages of development (e.g., for carbonate neoformations). In arid soils, carbonate neoformations go through stages (after Gile et al., 1966; Birkeland, 1984):

• Stage I: thin carbonate coatings on the lower side of rock fragments.
• Stage II: continuous coatings, carbonate veins (pseudomycelium) appear.
• Stage III: carbonates fill 50–90% of the horizon, concretions appear.
• Stage IV and above: a continuous carbonate horizon (K-horizon, petrocalcic pan) forms.

The higher the stage — the older the soil (all other things being equal).

3. Repeated superimposition of processes. If we see clay cutans overlain by carbonate coatings in one profile, this indicates that first there was a humid climate (clay illuviation), then a drier one (carbonatization). Such "traces of changing conditions" are the most valuable material for paleoclimate reconstruction.

5.6. Neoformations as Diagnostic Criteria in WRB

Many WRB diagnostic horizons are defined precisely through neoformations:

Therefore, when describing a profile, the soil scientist must record neoformations — their type, quantity, size, shape, and distribution across horizons.

Key Takeaways from this Section:

1. Soil neoformations are isolated structures within horizons that formed as a result of specific soil-forming processes.

2. Main groups of neoformations: cutans (coatings), pedotubules (tubular structures), glaebules (concretions, nodules), and amorphous formations.

3. Cutans (especially argillans) are the main morphological feature of illuvial clay accumulation (lessivage).

4. Glaebules (Fe, Mn, CaCO₃, CaSO₄ concretions) indicate redistribution and accumulation of chemical compounds.

5. Neoformations allow us to reconstruct the sequence of processes over time (e.g., first clay illuviation, then carbonatization).

6. The degree of development of neoformations (stages) is an important indicator of soil age.

7. Many WRB diagnostic horizons are defined through the presence of specific neoformations.

8. Micromorphological study complements macromorphology and reveals details not visible to the naked eye.

6.1. What are Soil Inclusions?

Continuing our "archaeological" analogy: if soil neoformations are "illustrations" of processes occurring within the soil, then soil inclusions are "artifacts" that entered the soil from outside and are not related to the soil formation itself.

Soil inclusions are isolated bodies or particles that are present in the soil mass but did not form as a result of soil-forming processes and have no genetic connection with the soil (Birkeland, 1984; Buol et al., 2011; Weil, 2017).

In other words, inclusions are "accidental" or "introduced" objects that ended up in the soil profile as a result of:

• geological processes (sedimentation, weathering);
• biological processes (activity of plants and animals);
• anthropogenic processes (human activity).

It is important to emphasize the main difference we already mentioned in the previous section (Birkeland, 1984; Weil, 2017):

6.2. Why are Inclusions Important for Understanding Soil History?

Despite the fact that inclusions are not "products" of soil formation, they are invaluable witnesses for reconstructing the history of the soil and landscape. They allow us to answer questions that the horizons themselves cannot answer:

1. Where did the material come from? Inclusions of rock fragments can indicate the source of parent material or processes of erosion and transport.

2. What was the vegetation? Roots, seeds, pollen, phytoliths — all are "passports" of former vegetation.

3. Which animals lived here? Mollusk shells, animal bones, burrows — traces of fauna.

4. When did events occur? Anthropogenic artifacts (ceramics, tools), radiocarbon dating of organic remains — tie to absolute time.

5. Were there interruptions in soil formation? Presence of stone lines or gravel lenses within the profile — evidence of erosion pauses or sedimentation episodes.

Thus, inclusions are "material evidence" that complements the picture of soil history we build from horizons and neoformations.

6.3. Classification of Soil Inclusions

In soil science, several broad groups of inclusions are distinguished by origin (Birkeland, 1984; Buol et al., 2011; Weil, 2017).

6.4. How Do Inclusions Help "Read" Soil History? (Examples)

6.5. How to Describe Inclusions in the Field?

When describing a profile, inclusions are recorded in the field notebook (after Birkeland, 1984; Buol et al., 2011; Weil, 2017):

1. Type of inclusion (rock fragments, roots, artifacts, etc.).

2. Size (diameter, length) — accurate to mm or cm.

3. Quantity (as a percentage of the horizon volume or by scale: single, rare, frequent, abundant).

4. Shape (rounded, angular, splintery, cylindrical, etc.).

5. Distribution (uniform, clustered, lenses, layers).

6. Origin (if it can be determined in the field — eluvium, alluvium, colluvium, biogenic, anthropogenic).

7. Relation to horizons (which horizon it occurs in, whether it is confined to horizon boundaries).

Example entry in field description:

Horizon Bt1 (35-65 cm):

• argillans: abundant, brown, on ped surfaces and in pores
• carbonate concretions: rare, 0.5-1 cm, white, in the lower part of the horizon
• inclusions: single quartzite fragments (2-5 cm), angular, mostly in the lower third of the horizon
• roots: rare (2-3 per 1 dm²), fine (< 2 mm), mostly along cracks

Such a description allows not only to characterize the horizon but also to record those "pieces of evidence" (inclusions) that will later help in reconstructing the profile's history.

Key Takeaways from this Section:

1. Soil inclusions are objects that entered the soil from outside and are not related to soil-forming processes (rock fragments, roots, artifacts, shells, charcoal).

2. Unlike neoformations, inclusions have external origin and do not carry information about processes within the soil, but they provide information about landscape history, climatic changes, biological activity, and human activity.

3. Lithogenic inclusions (rock fragments, stone lines) — indicate parent material, erosion, breaks in sedimentation.

4. Biogenic inclusions (roots, pollen, phytoliths, shells, bones, burrows) — paleoecological and paleofaunal indicators.

5. Anthropogenic inclusions (ceramics, tools, charcoal, construction debris) — markers of human activity, allow dating events and tracing land-use history.

6. Inclusions complement the information obtained from genetic horizons and neoformations, forming a complete picture of the history of soil formation.

7. Proper and detailed description of inclusions in the field is a mandatory part of professional soil description.

7.1. What are Soil Morphological Characteristics?

When a soil scientist goes into the field and begins to describe a pit, they encounter a huge amount of visual information: colors, aggregate shapes, density, porosity, root and stone distribution. To turn these observations into systematized data, morphological characteristics are used.

Soil morphological characteristics are physical properties that can be determined directly in the field (or with simple tools: a lens, a knife, the Munsell color chart) without complex laboratory analyses (Birkeland, 1984; Buol et al., 2011; Weil, 2017).

These characteristics are the "calling card" of each horizon. From them we:

• identify genetic horizons;
• diagnose processes (eluviation, illuviation, gleying);
• classify the soil (diagnostic horizons are often determined through color, structure, texture);
• compare soils with each other.

Important: Morphological characteristics are not just "color and structure." They are a system of objective, measurable or standardized described parameters that provide the basis for all subsequent interpretations (Birkeland, 1984; Buol et al., 2011).

7.2. Main Morphological Characteristics

Let's examine them sequentially, from the most obvious to the more detailed.

7.3. Field Description of the Profile: Algorithm

In soil science practice, there is a standard algorithm for pit description (Birkeland, 1984; Buol et al., 2011; Weil, 2017). We briefly list the main steps.

1. Site selection for the pit (representative area, digging to parent material, cleaning the wall).

2. Photographing the profile with a scale ruler.

3. Dividing into horizons (by visual changes in color, structure, texture).

4. Description of each horizon sequentially from top to bottom:

• Horizon index (e.g., Ah, AB, Bt1, Bt2, BC, C).
• Upper and lower boundary depth (cm from the surface).
• Color (moist and dry according to Munsell).
• Texture (field determination).
• Structure (type, size, grade).
• Consistence (dry, moist, stickiness, plasticity).
• Porosity (quantity, size, shape).
• Neoformations (type, quantity, size, shape, distribution).
• Inclusions (type, quantity, size, shape, distribution).
• Root distribution (quantity, size, depth).
• Nature of boundary (distinctness, form) with the underlying horizon.
• Special notes (pH by field method, reaction to HCl, presence of carbonates, gypsum, salts, gleying, etc.).
• Sample collection for laboratory analyses (from each horizon).

7.4. Morphological Characteristics and WRB Diagnostic Horizons

Many WRB diagnostic horizons are determined through morphological characteristics (often in combination with laboratory data) (IUSS Working Group WRB, 2015; Weil, 2017).

Thus, morphological description is the first and necessary step for subsequent classification. Without it, laboratory analyses "hang in the air."

7.5. Practical Tips for Field Description

• Always use standard terms (they are given in soil description guidelines).
• Work with a cleaned wall (fresh cut, free of contamination and sloughing).
• For color determination, use the Munsell chart at the same time of day (preferably at noon, in diffuse light, without direct sunlight).
• Determine texture on a moist but not wet sample (rub between fingers).
• Assess structure on dry and moist material (structure appears differently at different moisture levels).
• Record reaction to 10% HCl (effervescence) — a sign of carbonates.
• Use a lens (×10–×20) for detailed examination of cutans and microaggregates.
• Photograph each horizon close-up with scale.
• Record everything in the field notebook immediately — memory can fail, especially when describing multiple pits in a day.

Key Takeaways from this Section:

1. Morphological characteristics are physical properties of soil determined in the field: color, structure, texture, consistence, porosity, neoformations, inclusions, nature of boundaries.

2. Color is measured by the Munsell scale (Hue/Value/Chroma) and provides information about humus content, Fe/Mn oxides, carbonates, and redox conditions.

3. Structure is described by type, size, and grade, and indicates aggregation processes (biogenic, physico-chemical).

4. Texture (particle-size distribution) is the basis for diagnosing argic/argillic horizons and for assessing water-physical properties.

5. Consistence (density, stickiness, plasticity) is related to texture and compaction processes.

6. Porosity reflects structural state and biological activity.

7. Neoformations and inclusions are the most important "traces" of processes and soil history (covered in Sections 5–6).

8. Boundaries (distinctness, form) are indicators of uniformity and intensity of processes.

9. Standard field description includes sequential characterization of each horizon by all listed features.

10. Many WRB diagnostic horizons can be preliminarily identified by morphological features, but final diagnosis requires laboratory confirmation.

8.1. From Theory to Practice

In the previous sections, we studied the "alphabet" of soil science: genetic and diagnostic horizons, transitional zones, neoformations, inclusions, and morphological characteristics. Now it is time to answer the practical question: how exactly does a soil scientist describe a pit in the field?

Field pit description is a key skill for a soil scientist. It is in the field that the foundation is laid for all subsequent interpretations: genetic, classificatory, agronomic, ecological. An error in the field cannot be corrected by any amount of laboratory analyses (Birkeland, 1984). Therefore, pit description is both an art (the ability to see and interpret) and a strict methodology (a standard algorithm of actions).

8.2. Preparation for Description: Site Selection and Digging the Pit

8.3. Algorithm for Field Description of a Pit

Let's describe the sequence of actions of a soil scientist at a freshly dug pit.

8.4. Special Considerations for Description in Different Conditions

8.5. Common Errors in Field Description

Experienced soil scientists know that errors are easily made in the field. Here are the most frequent ones (Birkeland, 1984; Buol et al., 2011):

1. Too hasty horizon delineation — without detailed study of the entire wall. Sometimes horizons are not parallel to the surface (e.g., paleosols, cryoturbation, erosion). The entire wall must be examined, not just one vertical line.

2. Ignoring transitional horizons — they are skipped, and the profile appears "stepped," while the real picture (gradual transitions) is lost. This is especially critical for correct diagnosis of argic and cambic horizons.

3. Incorrect color determination — due to poor lighting, incorrect use of the Munsell chart, or attempting to "guess" the color without comparing with chips. Color must be determined in diffuse daylight (not in shadow, not in direct sunlight, not under artificial light). Always check against the chart chips.

4. Incorrect texture determination "by feel" — especially for beginners. Compare with reference samples or follow a strict methodology (rubbing between fingers, rolling into a ribbon).

5. Underestimation of neoformations — they are not noticed or not described, although they carry key information. Use a lens and always check ped surfaces for cutans.

6. Confusing neoformations with inclusions — for example, ferruginous concretions (neoformations) are confused with pebbles (inclusions). Important: if the object is related to processes within the soil (Fe/Mn concretions, carbonate nodules) — it is a neoformation. If it is a rock fragment different from the matrix, with no signs of soil formation — it is an inclusion.

7. Incorrect assessment of boundaries — not measuring the thickness of the transitional zone, not recording the boundary form. Boundaries must be measured (with a tape measure), not assessed "by eye."

8. Taking mixed samples — when the sample captures two horizons or a transitional zone. This makes laboratory data uninterpretable. Take the sample from the middle of a homogeneous horizon.

8.6. Field Form: Example Entry

Below is an example of filling out a field form for one horizon.

PIT No. 2024-01

Location: Moscow Oblast, Serpukhov District, block 12, south-facing slope, 5° gradient.

Date: 15.05.2024

Author: Ivanov I.I.

Vegetation: mixed forest (birch, spruce, wood sorrel, fern).

Parent material: mantle loam over moraine.

8.7. Why is Field Description So Important?

1. Unrepeatability. Field description is the only moment when we can observe the soil in situ (in its natural, undisturbed state). Laboratory analyses are always "secondary" information obtained on disturbed samples.

2. Connection with the landscape. Only in the field can we see the connection of the soil with relief, vegetation, and hydrology. We see how the profile changes along the slope (catena), how groundwater influences gleying.

3. Efficiency. A correctly performed field description can save a huge number of laboratory analyses. For example, the diagnosis of a mollic/umbric epipedon can often be made by color, thickness, and structure, without expensive analyses for organic carbon and base saturation (although they will still be needed for classification, but with high confidence).

4. Interpretation. Without a good field description, even the most accurate laboratory data "hang in the air." It is unclear which horizon they belong to, which processes they are associated with, and how to interpret them genetically.

Key Takeaways from this Section:

1. Pit description is a systematic procedure that includes site selection, digging, cleaning, sequential description of each horizon by a standard set of features, and sampling.

2. Main stages of description: general inspection and photography, preliminary horizon delineation, detailed description of each horizon (color, texture, structure, consistence, porosity, neoformations, inclusions, roots, boundaries), refinement of transitional zones, sampling, documentation.

3. Field form is a standardized form into which all observations are entered. It must be filled out completely and carefully, as it serves as the basis for all subsequent reports and interpretations.

4. Errors in the field (incorrect color determination, texture, ignoring transitional horizons) cannot be corrected in the laboratory. Therefore, field work requires attentiveness, knowledge of methodologies, and experience.

5. Field description is the only way to see the soil in situ, in all its complexity and connection with the landscape.

9.1. The Soil Profile as a Historical Document

We have come to the main question of the lecture: how can we understand the history of soil formation from a pit?

Throughout the previous sections, we have gradually mastered the "tools" of the soil scientist-historian. Now it is time to bring everything together and show how these tools work in combination.

Imagine that the soil profile is a multi-layered archive, where each horizon is a "folder" with documents, neoformations are "photographs" of processes, and inclusions are "artifacts" from outside. The soil scientist's task is to read this archive, decipher the sequence of events, and reconstruct the history of soil and landscape development (Birkeland, 1984; Weil, 2017; Huang et al., 2012).

Reconstruction of soil formation is not a speculative exercise. It has direct practical significance:

• Prediction of soil behavior under climate change or land-use change.
• Assessment of soil resilience to erosion, degradation, and pollution.
• Search for mineral resources (placer deposits, bauxites, iron ores) — many ore deposits are associated with ancient weathering mantles.
• Reconstruction of paleoclimate and paleolandscapes for understanding global climate change.
• Archaeology and paleoecology — reconstruction of the living environment of ancient people and their economic activities.

9.2. Basic Principles of Reconstruction

Before moving to specific examples, let's formulate the fundamental principles on which the reconstruction of soil formation from the profile is based (Birkeland, 1984; Buol et al., 2011; Weil, 2017).

9.3. Time Indicators in the Soil Profile

Which features allow us to estimate the relative age of the soil and the duration of individual stages?

9.4. Indicators of Paleoclimate and Paleoenvironments

The soil profile is a climatic archive. From its features, we can reconstruct the climatic conditions of past epochs (Birkeland, 1984; Weil, 2017; Huang et al., 2012).

Important: Climatic interpretations should always consider the catena. The same horizon (e.g., gleying) may be caused by either climate (excess precipitation) or local poor drainage (relief). Therefore, paleoclimatic conclusions are made on the basis of comparison of several profiles in a region, not just one (Birkeland, 1984; Weil, 2017).

9.5. Indicators of Changing Conditions (Polycyclicity)

How can we recognize that there were changes in conditions (climate, vegetation, hydrology) in the soil's history?

9.6. Step-by-Step Algorithm for Reconstructing Soil History

Now let's bring everything together and propose an algorithm of actions for reconstructing soil history from the profile.

9.7. Reconstruction Example: Analysis of a Specific Profile

Let's apply the algorithm to the profile we partially described in Section 8.

Profile: Ah – Ael – Bt1 – Bt2 – BC – C

Field description (summary):

Steps 1-2. Processes:

• Ah — humus accumulation.
• Ael — eluviation (removal of clay, Fe/Al, lightening).
• Bt1/Bt2 — clay illuviation (argillans, clay increase).
• BC — weak weathering, transition to parent material.

Step 3. Sequence:

1. Weathering of parent material began (C → BC).

2. Bt formed through clay illuviation (requires a long leaching regime). First Bt2 (less pronounced), then Bt1 (more extensive cutans).

3. Later (or simultaneously?) the eluvial zone — Ael — began to form (removal of clay from the upper part). This could be due to an intensification of the leaching regime or the onset of podzolization.

4. The upper Ah formed as a result of humus accumulation under forest vegetation (modern stage).

Step 4. Environmental conditions:

• Stage 1 (Bt formation): humid, moderately warm climate, sufficient for leaching and clay transport (but not for complete destruction of clay minerals). Forest vegetation.
• Stage 2 (Ael formation): possibly increased acidity (appearance of conifers?) or increased precipitation, leading to more active eluviation.
• Stage 3 (Ah formation): modern temperate climate, mixed forest, humus accumulation in the upper layer.

Step 5. Age (estimate):

• Formation of a thick Bt with cutans in loamy parent material requires at least 5–10 thousand years (post-glacial period).
• The eluvial Ael could have formed over the last 3–5 thousand years (under modern climate, but possibly with increased acidity).
• Ah — modern humus horizon (hundreds to thousands of years).

Step 6. History:

The soil began to form on mantle loam at the beginning of the Holocene (about 10–12 thousand years ago) under broad-leaved forest in a moderately humid climate. Intense weathering and leaching led to the formation of an illuvial clayey horizon (Bt). A gradual increase in acidity (possibly due to changes in forest species or increased humidity) led to the formation of an eluvial zone (Ael) and removal of clay from the upper part of the profile. The upper humus horizon (Ah) is currently forming under mixed forest with herbaceous vegetation. Thus, we see a classic Soddy-podzolic soil (in WRB — Albic Luvisol or Retisol, depending on the presence of E/B tongues). The profile reflects a long evolution from a brown forest soil (just Bw) through a lessived brown forest soil (Bw + Bt) to the modern Soddy-podzolic with distinct eluvial-illuvial differentiation.

9.8. Limitations and Challenges of Reconstruction

It is important to understand that reconstruction is an interpretation, not an absolute truth. There are several factors that can distort the picture or make it ambiguous:

1. Overprinting of processes (polygenesis). The same horizon may bear traces of several processes that occurred at different times. For example, Bt may be both illuvial and partly formed in situ. Separating these effects is difficult.

2. Destruction of traces. Later processes may erase or mask earlier ones (e.g., bioturbation destroys clay cutans). Then we may not see traces of the early stage and may underestimate the age of the soil.

3. Lithological heterogeneity. If the parent material was initially heterogeneous (e.g., layered alluvium), some changes in the profile may not be pedogenic but inherited from the rock. This can lead to false interpretation of processes.

4. Catena effects. A profile at one point on a slope may reflect not only vertical processes but also lateral ones (input of material from upslope areas). In such cases, horizons may be "foreign" — brought from other catena positions (Huang et al., 2012; Birkeland, 1984).

5. Insufficient information. Without laboratory analyses (mineralogy, micromorphology, absolute dating), interpretation remains hypothetical. Therefore, field description should always be supplemented by laboratory studies.

Key Takeaways from this Section (and the Entire Lecture):

1. The soil profile is an archive of soil history. Every horizon, neoformation, inclusion, and morphological feature is a "record" of certain processes and conditions.

2. Reconstruction is based on the principles of uniformitarianism, stage development, polygenesis, and lithological continuity.

3. Time indicators: thickness and differentiation of horizons, degree of neoformation development, depth of carbonates, presence of buried horizons.

4. Paleoclimate indicators: epipedon type (Mollic, Ochric, Spodic, Oxic), presence of calcic/gypsic/salic horizons, signs of gleying, plinthite/laterite.

5. Indicators of changing conditions: superposition of neoformations (cutans + carbonates), buried horizons, disturbed structures.

6. Reconstruction algorithm: description → identification of processes → determination of sequence → interpretation of conditions → age estimation → synthesis of history.

7. Reconstruction is a complex interpretive task, requiring consideration of all data, knowledge of processes, principles, and limitations. It is both an art and a science.

Conclusion to the Lecture

We have come a long way: from the concept of the soil profile as a vertical section to the complex system of genetic and diagnostic horizons, neoformations, inclusions, and morphological characteristics. We have learned to "read" the profile as a historical document and to reconstruct from it the stages of soil formation, changes in climate, vegetation, and hydrological conditions.

You now possess the basic toolkit of a genetic soil scientist. In the following lectures, we will apply this knowledge to study specific soil processes, soil types, and their classification. But the foundation — understanding the profile as an integrated system — has already been laid.

The main takeaway: Soil is not just "dirt underfoot." It is an extremely complex natural object that preserves the Earth's history over thousands and millions of years. The ability to "read" this history is the key to understanding the current state of soils and predicting their future.