Field Morphology of Soil Profiles

Why is colour so important?

Colour serves as a kind of diagnostic indicator. Colour itself does not directly affect fertility, but it gives us clues to understanding:

1. Organic matter content and quality: The darker the soil, the higher its humus content generally is. This is one of the main rules for upper horizons. However, as with any rule, there are exceptions (e.g., black colour can be caused by manganese oxides or iron sulphides in waterlogged soils).

2. Water‑air regime and drainage status: Colour allows us to judge how long the soil has been under excessive moisture. Red, bright yellow, and brown tones indicate good aeration and oxidised iron. Grey, bluish, or greenish colours are a “cry for help” from the soil, signalling that iron is in the reduced ferrous form due to prolonged waterlogging and oxygen deficiency (Weil & Brady, 2017).

3. The nature of soil‑forming processes: For example, lightening (“bleaching”) of upper horizons is often associated with the removal (eluviation) of iron and clay particles—a sign of podzolic or lessivage processes. Intense red colour in deep horizons in the tropics is evidence of powerful ferralitisation (White, 2006).

The Munsell System: A Precise Language of Colour

Subjective descriptions like “brownish‑grey” or “greyish‑brown” are fine for everyday communication, but in science they are absolutely unacceptable. One person may call a colour “brownish‑grey” while another calls it “greyish‑brown”. To ensure accuracy, repeatability, and uniformity of descriptions, soil scientists worldwide use the objective Munsell colour system (Munsell, 1912; Buol et al., 2011).

In this system, colour is broken down into three independent characteristics, each with a numerical value:

• Hue: This is the dominant wavelength of light. In other words, it is the colour itself: red, yellow, blue, etc. In the Munsell Soil Color Book, hues are denoted by letters and numbers, e.g., 10YR for yellow‑red, 5YR for reddish‑yellow, 5Y for yellow.
• Value: This is the relative lightness of the colour, its position on a scale from absolute black (0) to pure white (10). It indicates how dark or light the colour is. For example, value 3 is a dark colour, while value 7 is light. Value is strongly affected by soil moisture: moist soil is always darker (Foth, 1990).
• Chroma: This is a measure of colour purity, its intensity, its difference from a grey of the same value. The chroma scale starts at 0 (neutral grey) and can go up to 8 or more. The higher the number, the purer and more vivid the colour.

The Munsell notation is written as: Hue Value/Chroma. For example, 10YR 6/3 is a colour with hue 10YR, value 6, and chroma 3. From the colour chart, we see this is “pale brown”. And 10YR 3/2 is a much darker and less intense colour—“very dark greyish brown” (Weil & Brady, 2017).

Important practical note: Colour is always described for two moisture states: moist and air‑dry (Buol et al., 2011). Moisture lowers the value, making the colour darker, and can affect chroma. The standard is to describe the colour on a fresh break of an aggregate in its natural moist state and, separately, on an air‑dry sample.

Colour–Process Relationships

Let us now decipher what these colour codes tell us.

Organic matter (humus)

Humus is the main “darkener” of soil. The more humus, the lower the Value (darker) and often the lower the Chroma (less saturated), because organic matter masks the brightness of minerals. Rich Chernozems have very low Value (2–3) and low Chroma (1–2) when moist.

Iron—the main “painter” of soils

It is iron compounds that give soils their famous red, yellow, orange, and brown hues. The key role here is played by the redox status of the soil (Scheffer et al., 2018).

• Oxidised iron (Fe³⁺): In well‑drained, aerated soils, iron is present as insoluble oxides and hydroxides. Haematite (Fe₂O₃) gives bright red tones (Hue 5YR and redder). Goethite (FeOOH) is responsible for yellow‑brown and yellow tones (Hue 7.5YR and 10YR). The combination of these minerals creates a rich palette of brown and reddish‑brown colours. Bright, saturated colours (high Chroma) indicate stable oxidising conditions.
• Reduced iron (Fe²⁺): Under prolonged waterlogging (anaerobic conditions), iron is reduced to the mobile ferrous form Fe²⁺. It is either leached from the profile or forms grey, greenish‑grey, and bluish (gley) colours with very low Chroma (≤2). If organic matter is also present, black iron sulphides (FeS) may form, giving the horizon an almost black colour, but this is not humus—it is the result of gleying (White, 2006).

Gleyic (low‑chroma) colours are a diagnostic feature of hydromorphism and are used to identify soils such as Gleysols and to define the water regime class (Weil & Brady, 2017).

Other colouring agents

• Calcium carbonates (CaCO₃) and salts: High contents of these substances, especially as efflorescences and crusts on the surface, give soils whitish, pale, or almost white colours. This is typical of soils in arid and semi‑arid regions (Scheffer et al., 2018).
• Manganese (Mn): Manganese oxides have black, brown, or dark purple colours. They often form dotted black concretions or coatings that can noticeably affect the overall background of the horizon.
• Quartz and other light minerals: Eluvial horizons from which iron and clay have been leached often have light grey or nearly white colours (Hue 10YR or 2.5Y, high Value, low Chroma), reflecting the colour of the sand and silt grains of quartz and feldspars themselves.

Practical conclusion: Colour is the “passport” of the soil. Using the Munsell chart, we can precisely determine its parameters, and by knowing the relationship between colour and components (humus, iron) and processes (oxidation, reduction), we “read” the history of formation and current conditions of the soil. In the next section, we move from the soil’s “appearance” to its “skeleton”—the structure of soil aggregates.

Why is structure the “health” of the soil?

The importance of structure for agriculture and ecosystem functions cannot be overestimated. In essence, good structure is good physical condition of the soil (Foth, 1990).

1. Regulates water‑air regime: Aggregates create two pore systems:

• Inter‑aggregate pores—large, “macropores”. Through these, water quickly infiltrates during rain, and in dry weather air circulates here. They are the “highways” for water and roots.
• Intra‑aggregate pores—small, “micropores”. In these, water is held against gravity, creating a moisture reserve for plants.
• thout structure, in a structureless clay soil, water practically does not infiltrate, forming a crust and puddles on the surface; in a structureless sandy soil, conversely, water and nutrients quickly leach downwards. Good structure creates an optimal balance between water‑holding and drainage capacity (Scheffer et al., 2018).

2. Provides root respiration: Plant roots, like us, need oxygen. A well‑structured soil contains enough large pores filled with air, allowing roots to breathe and actively absorb nutrients. It is often said: “In loose soil, roots grow like crazy.”

3. Reduces erosion hazard: Aggregates are resistant to water washing and wind blowing. If the soil has good structure, even a heavy rain will not wash away its surface, because large aggregates resist raindrop impact well.

Types of Structure: “The ABC of Aggregates”

In the field, structure is described according to three main parameters: type (shape of aggregate), size (diameter), and degree of expression (or grade) (Foth, 1990; Buol et al., 2011).

I. Type (shape of aggregates): This is the main characteristic. Four main types are distinguished (Scheffer et al., 2018; Weil & Brady, 2017):

1. Platy: Aggregates are flat plates, oriented predominantly horizontally. They resemble a stack of thin plates or the pages of a book. Often arises from compaction or in eluvial horizons where particles become aligned under gravity. An indication of possible reduced water permeability.

2. Prismatic and Columnar: Aggregates are elongated vertically. If the tops of the prisms are flat, it is prismatic structure. If the tops are rounded like domes, it is columnar structure. Columnar structure is a diagnostic feature of solonetzic horizons with high exchangeable sodium content.

3. Blocky: Aggregates have a more or less cubical shape. If the angular corners and edges are sharp, the structure is called angular blocky. If the edges and corners are rounded, it is subangular blocky. This is a very common type of structure in illuvial (B) horizons where clay accumulates and cements particles.

4. Granular and Crumb: These are the most valuable types for agriculture. Aggregates have rounded, irregular shapes resembling nuts (granular) or pellets, grains, crumbs (crumb). Granular structure is the “gold standard” for arable horizons. It provides an ideal water‑air regime. It forms under the influence of plant roots, earthworm activity, and humus accumulation, which binds particles into water‑stable aggregates.

II. Size of structural peds: Depending on the structure type, several size classes are distinguished. Usually in field descriptions we use words like “fine”, “medium”, “coarse”, “very coarse”, and “cloddy”. For example, for granular structure, fine is 1–2 mm, coarse is 5–10 mm. For blocky structure, the sizes are different: fine <5 mm, medium 5–10 mm, coarse 10–20 mm. Cloddy structure (>10 cm) is always a sign of degradation, e.g., after tillage when the soil is too wet or too dry (Weil & Brady, 2017).

III. Degree of expression (grade): This parameter indicates how well the aggregates are formed and visible in the profile.

• Weak: Aggregates are barely visible; when extracted from the pit, the soil mainly breaks into shapeless lumps.
• Moderate: Aggregates are well visible but not very strong. When squeezed in the hand, most aggregates retain their shape.
• Strong: Aggregates are clearly expressed, well distinguishable, and strong. When removed from the pit, the soil easily separates along planes of weakness into individual aggregates that are hard to crush with fingers.

Recording structure in the field notebook is always done in strict order: grade, size, type. For example: “moderate medium subangular blocky” or “strong fine granular”.

Why does structure form? Relationship to processes

Aggregate formation is a complex process, the result of physical, chemical, and biological factors.

1. Biological factor: This is the most powerful agent of aggregation. Plant roots penetrate the soil, breaking it into crumbs. Root exudates and decomposition products of organic matter (polysaccharides, humates) act as “glue”, binding sand grains and clay particles. Particularly important are mycorrhizal fungi and earthworms. Earthworms, passing soil through their gut, create very strong, water‑stable aggregates (casts) (Weil & Brady, 2017; Foth, 1990).

2. Physico‑chemical factor: Wetting‑drying and freezing‑thawing cycles create micro‑stresses in the soil mass, leading to cracking and splitting into peds. This process is especially active in soils with high contents of swelling clays (smectites) (Scheffer et al., 2018).

3. Chemical factor: Iron (Fe₂O₃) and aluminium (Al₂O₃) oxides play a cementing role. In tropical red soils, they cement particles into strong microaggregates—pseudo‑sand, giving the soil the physical properties of sand despite high clay content.

Practical conclusion: In the field notebook, always describe structure, even if it is weak or absent. “Weak fine platy” or “structureless—single grain” are also diagnostic features. Structure is not just a form; it is an indicator of biological activity, organic matter content, and soil stability against erosion and compaction. It is how the soil is “organised” for plant life.

In the next part of our lecture, we will consider an equally important feature—consistence, which includes assessment of density, porosity, and cracks. If structure is the “skeleton”, then consistence is the “posture” of that skeleton.

How to assess consistence in the field?

In the field, consistence is assessed subjectively, through the feeling of resistance during digging and pressing.

1. By resistance to the spade (for the whole profile):

• Very loose (friable): The spade enters easily, almost without effort. The soil is loose, like sand.
• Loose: The spade enters with slight effort. The soil crumbles easily.
• Firm (dense): The spade enters with noticeable effort. The soil is hard to dig.
• Very firm (compact/cemented): The spade enters with great difficulty, often only a few centimetres. With strong pressure, it penetrates with difficulty.

2. By resistance to crushing of an aggregate (for individual horizons): With a knife or fingers, try to squeeze or crumble a lump of soil in the hand.

• Very loose: The lump crumbles at the slightest touch.
• Loose: The lump easily crumbles with slight pressure.
• Firm: The lump is difficult to crush with fingers but possible.
• Very firm: The lump does not yield to finger pressure; it can only be broken by a blow.

Porosity: The “Respiratory System” of the Soil

Consistence is directly related to porosity—the total volume of pores in the soil (Foth, 1990; Weil & Brady, 2017). In a good soil, pores occupy about 50% of the volume. But equally important is how these pores are distributed by size:

• Macropores (large): Size > 0.08 mm (approximately). These are spaces between aggregates, worm channels, cracks, old root channels. They ensure rapid infiltration of water, aeration, and serve as pathways for root growth. Water does not stay in them (gravitational water drains down).
• Mesopores (medium): 0.03–0.08 mm. These are the most valuable pores for plants. They hold capillary water available to roots.
• Micropores (small): < 0.03 mm. These are pores within aggregates, between clay particles. They hold water very tightly, but often this moisture is unavailable to most plants (hygroscopic or “dead” reserve). However, these micropores determine the soil’s ability to retain moisture.

Field assessment of porosity:

• Fine‑porous: Many pores, but they are small (like in a sponge). Characteristic of dense, structureless clays.
• Medium‑porous: Pores of medium size, visible to the eye but not very large. Typical of well‑structured loams.
• Coarse‑porous: Pores visible to the naked eye, several millimetres in diameter (e.g., in loose sandy loams).
• Very coarse‑porous: Pores larger than 5–10 mm, e.g., old root channels.

Pore shape also matters:

• Rounded pores: Typical of well‑structured soils.
• Flat (crack‑like): Indicate compaction, platy structure.
• Tubular (channels): Traces of root and worm activity (biopores).

Cracks: A Special Case of Pores

Special attention should be given to cracks—large, often vertical breaks in the soil mass, visible to the naked eye. They arise from drying and shrinkage of clayey soils. These cracks:

• Have diagnostic value: Wide and deep cracks (>0.5 cm wide and >25 cm deep) are a diagnostic feature of Vertisols (soils with high content of swelling clays) (Weil & Brady, 2017; White, 2006).
• Play an important ecological role: On one hand, they serve as “emergency” pathways for rapid water entry into dry soil, allowing seeds to germinate faster. On the other hand, excessive cracking can lead to root breakage, exposure of the root collar, and erosion along the cracks.

Practical conclusion: Consistence is the “posture” of the soil. Loose, porous consistence is a sign of healthy, well‑aerated soil suitable for agriculture. Dense, compact consistence, on the contrary, signals degradation, compaction, and impaired water‑air regime. In the field notebook, always record consistence, e.g., “loose, medium‑porous” or “firm, fine‑porous, with vertical cracks up to 5 mm wide”.

In the next section, we move to the description of the elementary particles that make up the soil—its particle‑size distribution. If structure and consistence describe how particles are organised, then particle‑size distribution tells us what kind of particles (sand, silt, clay) the soil is made of.

Why is particle‑size distribution so important?

1. Determines water‑physical properties: It is the ratio of sand, silt, and clay that determines water‑holding capacity, permeability, moisture capacity, and aeration. Sandy soils are “permeable but forgetful”: water passes through them quickly, but they hold it poorly. Clayey soils are the opposite: they hold water and nutrients well, but they poorly transmit air and water.

2. Affects fertility: Clay particles have a huge specific surface area and a high negative charge, allowing them to retain nutrient cations (Ca²⁺, Mg²⁺, K⁺) on their surfaces. Sand particles have almost no charge, so sandy soils are poor in nutrients and require constant fertilisation.

3. Determines tillage technology: Clayey soils are “sticky”, difficult to till, and require special timing and methods to avoid creating clods and plough pans. Sandy soils are easy to work but dry out quickly and need frequent irrigation.

4. Affects erosion resistance: Silty (loamy) and fine‑sandy soils are most susceptible to wind and water erosion because their particles are easily lifted by wind and washed away by water.

Soil Separates (Size Fractions)

In world practice, including in Russia, the classification of soil particles by size developed by the USDA (U.S. Department of Agriculture) is adopted. It distinguishes three main fractions (Weil & Brady, 2017; Foth, 1990):

1. Sand: Size from 2.0 to 0.05 mm. Particles are visible to the naked eye, feel gritty, “sandpaper‑like” (Foth, 1990). They consist mainly of quartz and other primary minerals. Sand has no plasticity or stickiness. Soil containing much sand is called sandy.

2. Silt: Size from 0.05 to 0.002 mm (2 microns). Particles are not visible to the eye; they feel like flour or talc—smooth, “silky” (Foth, 1990; Weil & Brady, 2017). Silt often consists of quartz and other minerals, but because of its small size, it has a larger specific surface and can be chemically more active than sand. Soil rich in silt is called silty or loamy.

3. Clay: Size < 0.002 mm. These are colloidal particles, not visible to the eye. Clay feels “sticky” or “greasy”, very plastic; when dry it forms hard, dense clods (Foth, 1990). Clay particles have a huge specific surface area (up to 800 m²/g) and a high negative charge. They can swell when wet and shrink when dry. Soil rich in clay is called clayey.

Field Determination of Particle‑Size Distribution (the “Ribbon Test”)

In the laboratory, particle‑size distribution is precisely determined using hydrometers and sieves. But in the field, when you need a quick assessment, a simple and reliable “ribbon test” (or “ribbon method”) is used (Foth, 1990; Ganzhara et al., 2002; Mukha et al., 2003).

The essence of the method: moisten a small lump of soil to a dough‑like consistency (like clay for modelling) and try to roll it into a ribbon between the thumb and forefinger. The behaviour of this ribbon tells us about the clay content.

Step‑by‑step instructions:

1. Take a small lump of soil (about the size of a hazelnut) from the middle of the horizon.

2. Gradually add water and knead the soil until it becomes plastic, like modelling clay. It should not be too dry (crumbles) or too wet (sticks to fingers).

3. Roll this mass into a ribbon about the thickness of a pencil (3–5 mm) and 5–7 cm long.

4. Try to bend this ribbon into a ring around your finger. Based on how it behaves, you determine the soil type (Mukha et al., 2003):

• Sand: The ribbon does not form. The soil crumbles in the hand. Very high sand content.
• Loamy sand (sandy loam): The ribbon forms, but when you try to bend it into a ring, it immediately breaks into small pieces. Little clay, much sand.
• Light loam: The ribbon forms, but when bent into a ring it cracks and breaks into 2–3 pieces. Medium clay content.
• Medium loam: The ribbon forms; when bent, it forms a ring, but the ring has many cracks along the edges. Clay is already sufficient.
• Heavy loam: The ribbon forms; when bent, it forms a ring with small cracks.
• Clay: The ribbon forms; when bent, it forms a smooth, crack‑free ring. Very high plasticity.

Supplementary indicators:

• Sandiness: If, when rubbing a moist lump between your fingers, you distinctly feel roughness, a “gritty” sensation of sand grains, then the soil contains much sand. Even if the ribbon forms, but the sand is strongly felt, it is a “sandy loam”.
• Siltiness: If, when rubbing, the soil feels like flour, smooth, “silky”, without roughness, then there is much silt. This is a “silty loam” or “silty sandy loam”.

Influence of Organic Matter and Clay Type

Two nuances to remember:

1. Organic matter: High humus content (e.g., in Chernozems) “softens” tactile sensations. A clayey soil with high humus may feel lighter and less plastic than its low‑humus analogue. This can lead to underestimating the clay content (Foth, 1990).

2. Clay type: In tropical and subtropical regions, kaolinite predominates, which has much less plasticity and stickiness than smectites (montmorillonite) in temperate soils. Therefore, field determination “by feel” requires calibration according to the region (Weil & Brady, 2017).

Practical conclusion: Particle‑size distribution is a “passport” characteristic of the soil. It can be quickly assessed in the field by the ribbon test, but one must be attentive to soil condition (moisture) and organic matter content. In the field notebook, the record of particle‑size distribution is simple, e.g., “medium loam, sandy” or “clay, silty”.

In the next part of the lecture, we turn to the “traces” of soil processes—new formations. These are specific accumulations of substances that tell us about chemical reactions and element migration in the profile.

Why are new formations important?

1. Diagnosis of processes: They indicate specific chemical reactions occurring in the soil. For example, carbonate features indicate calcium accumulation, gypsum features indicate sulphate accumulation, and iron concretions indicate redox processes.

2. Indication of moisture regime: The nature and depth of new formations allow us to judge the type of water regime (percolative, non‑percolative, evaporative) and the groundwater level.

3. Assessment of fertility: Some new formations (e.g., carbonates) can affect nutrient availability, while others (e.g., gypsum) affect physical properties.

Forms of New Formations

In the field, new formations can have a wide variety of morphologies (Scheffer et al., 2018; Mukha et al., 2003):

• Efflorescences and coatings: Thin powdery accumulations on aggregate surfaces or in pores. Often seen with carbonates and salts.
• Veins and tubes: Infillings of cracks or root channels with new material.
• Concretions (nodules): Rounded, dense bodies of various sizes—from fractions of a millimetre to several centimetres. They form around a centre (sand grain, organic residue) and can be loose or very hard.
• Layers: Horizontal or inclined strata of new material.
• Crusts: Hard surface formations on pit walls or on the soil surface.
• Pseudomycelium: Thin, branching, white thread‑like accumulations of carbonates, resembling fungal mycelium.

Main Types of New Formations

1. Carbonate New Formations (CaCO₃)

Carbonates are among the most common new formations in soils of arid and semi‑arid regions, as well as in carbonate parent materials (Buol et al., 2011). In the field, they are easily diagnosed by reaction with 10% hydrochloric acid (HCl): upon acid application, carbonates “effervesce” (release CO₂ bubbles).

Forms:

• Pseudomycelium: Thin white threads penetrating the horizon. An indication of active CaCO₃ migration in capillaries (Scheffer et al., 2018).
• Powdery efflorescences and “white‑eye” (beloglazka): Loose, white accumulations on aggregate surfaces or in pores. Sometimes appear as round white spots (“white‑eye”). This is typical for Chernozems and Chestnut soils.
• Nodules (zhuravchiki): Dense, rounded bodies, often irregular, ranging from pea‑sized to chicken‑egg‑sized. They form by cementation of sandy‑clayey material by carbonates.
• Layers and horizons: Continuous carbonate horizons (e.g., Cca horizon, or calcic horizon), sometimes cemented into a hard pan—calcrete (petrocalcic horizon). Such horizons can be impenetrable to roots.

Diagnostic value: The depth of carbonate occurrence is an important indicator of the degree of leaching. The deeper they occur, the more humid the climate and the more intense the leaching. The presence of carbonates indicates a neutral or alkaline reaction.

2. Iron and Manganese New Formations

Iron (Fe) and manganese (Mn) are the main indicators of redox conditions in the soil (Weil & Brady, 2017; Scheffer et al., 2018).

Iron oxides and hydroxides (Fe³⁺): Form under oxidising conditions (well‑drained soils).

• Forms: Rusty‑brown, reddish, or yellow‑brown concretions, coatings, crusts on aggregates, tubes along root channels, films.
• Example: Ortstein—hard, often cemented concretions or layers, found in Podzolic and bog soils. They result from iron concentration.

Manganese oxides (Mn⁴⁺): Often occur together with iron, but can also form independent features.

• Forms: Black, dark‑brown, almost black coatings, dots, small (1–5 mm) concretions. They resemble soot or coal dust.
• Diagnostic value: Manganese concretions often indicate periodic waterlogging followed by drying.

Ferrous iron compounds (Fe²⁺): Form under reducing (anaerobic) conditions during prolonged waterlogging.

• Forms: The soil acquires bluish, grey‑blue, or greenish‑grey shades (gleyic tones). These are not separate new formations but the overall colour of the horizon (matrix).
• Diagnostic value: Gleyic colours indicate permanent or prolonged waterlogging and oxygen deficiency. This is an important feature of hydromorphic soils (Gleysols, bog soils).

Ortstein and pseudofibres: These are hard, cemented layers, often with a rusty colour, formed by the concentration of iron and manganese in the zone of groundwater fluctuation. They can act as a “soil screen” hindering root penetration.

3. Gypsum New Formations (CaSO₄·2H₂O)

Gypsum is a characteristic new formation in soils of arid and semi‑arid regions, especially under evaporative water regimes (Buol et al., 2011).

• Forms: Colourless or white, needle‑like crystals, aggregates, druses, loose powdery accumulations, concretions (“gypsum roses”).
• Diagnostic value: They do not effervesce with HCl (unlike carbonates). Gypsic horizons indicate a very dry climate and shallow occurrence of gypsum‑bearing rocks or groundwater. Gypsum promotes soil structuring, but its excess can be toxic to some crops. In Russian soils, gypsum is often found in Solonchaks, Solonetzes, and Chestnut soils.

4. Readily Soluble Salts (NaCl, Na₂SO₄, etc.)

These are white, greyish, or yellowish efflorescences, crusts, coatings on the soil surface or in upper horizons. They appear under conditions of strong evaporation and rising saline groundwater. They are easily recognised by their salty taste (which, of course, is not recommended to taste in the field). This is a sign of Solonchaks and Solonetzes.

5. Silica (SiO₂)

In the form of opal concretions or layers (“siliceous pans”), it occurs less frequently, mainly in soils of dry subtropics. It forms by accumulation of silicic acid released during silicate weathering.

6. Humus New Formations

These are films, streaks, humus tongues penetrating from the humus horizon downwards along cracks and root channels. They indicate active migration of organic matter and its interaction with the mineral part (White, 2006).

Practical conclusion: New formations are “chemical records” in the soil profile. Their type, form, colour, and depth of occurrence provide keys to understanding soil‑forming processes, especially the migration and accumulation of elements. In the field notebook, always record all new formations, describing their colour, form, size, and distribution (e.g., “abundant carbonate pseudomycelia, white, from 80 cm depth”).

In the next part, we consider inclusions—objects that got into the soil from outside and are not related to soil‑forming processes. They are “foreign objects” in the soil profile.

Why are inclusions important for diagnosis?

1. They provide information about soil genesis and age: For example, the presence of pebbles and boulders indicates fluvioglacial or glacial origin of the parent material. Artefacts (pottery, charcoal) indicate anthropogenic impact and allow dating of the soil.

2. They affect agronomic properties: Stoniness determines the suitability of the soil for tillage, the depth of the root zone, and erosion resistance. Plant roots indicate biological activity and fertility.

3. They help interpret analytical results: Inclusions (e.g., large stones) can distort laboratory analysis results if not removed before analysis.

Main Types of Inclusions

1. Plant Roots

Roots are living or dead plant parts penetrating the soil profile. They are the most important indicator of biological activity and fertility (Weil & Brady, 2017). In field descriptions, roots are characterised by three parameters:

• Quantity: Assessed visually as a percentage of the wall area, or on a scale: few (<2%), common (2–20%), many (>20%). Some methods use scales like “very rare”, “rare”, “common”, “abundant”.
• Size (diameter): Distinguish very fine (<1 mm), fine (1–2 mm), medium (2–5 mm), coarse (5–10 mm), and very coarse (>10 mm) roots.
• Distribution in the profile: Note in which horizons roots are concentrated, whether they penetrate deeply, and whether vertical root channels exist.

Diagnostic value: Root penetration depth is an important indicator of the thickness of the root zone and the availability of moisture and nutrients. Abundance of roots in the upper horizon indicates high biological activity. Absence of roots in dense, compacted horizons indicates physical or chemical (e.g., salinity) degradation.

2. Stones and Rock Fragments

These are rock fragments of various sizes found in the profile. Size classification (according to USDA system):

• Gravel: 2–75 mm.
• Channers: flat fragments 2–150 mm.
• Cobbles: 75–250 mm (rounded).
• Stones: 250–600 mm.
• Boulders: > 600 mm.

If the content of particles larger than 2 mm exceeds 15% by volume, the texture name is modified accordingly (e.g., “gravelly loam” or “cobbly sandy loam”) (Buol et al., 2011).

Diagnostic value: Stoniness is not only a limitation for tillage but also a source of information about rock origin. For example, rounded gravel indicates water sedimentation, while angular rubble indicates eluvium of bedrock or weathering products.

3. Anthropogenic Inclusions (Artefacts)

These are objects created or modified by humans that have found their way into the soil: pottery, glass, bricks, charcoal, slag, construction debris, etc. Their presence indicates technogenic impact, often associated with construction, ancient settlements, or landfills (Scheffer et al., 2018).

Diagnostic value: Artefacts can serve as markers of soil age (e.g., cultural layer in cities), indicate ameliorative measures (e.g., sand or peat addition), or pollution.

4. Other Inclusions

• Mollusc shells: Indicate marine or freshwater origin of parent material.
• Animal bones: Can be either natural (remains of animals living in the soil) or anthropogenic (burials).
• Buried horizons: These are parts of an older soil profile covered by younger deposits (e.g., loess, sand). They are designated as buried horizons (b) and are of great importance for reconstructing landscape history (White, 2006).

How to record inclusions in the field notebook?

In the horizon description, always indicate:

• Type of inclusions (roots, stones, artefacts).
• Quantity (few, common, many, or %).
• Size (for stones, in mm).
• Distribution pattern (uniform, patchy, along root channels, in layers, etc.).

Example: “Horizon A1: roots abundant, fine, penetrating the whole mass; occasional boulders 20–30 cm in size; charcoal and pottery fragments occur at 40–50 cm depth”.

Practical conclusion: Inclusions are an important element of the morphological portrait of the soil. They help reconstruct the history of profile formation, assess its agronomic potential, and diagnose anthropogenic influence. In the field notebook, they are recorded alongside other features.

In the next, concluding part of the lecture, we will examine how horizons interact with each other—how to describe the boundaries between them. Boundaries, like all previous features, carry important diagnostic information about the rates and intensity of soil formation.

Why are boundaries so important?

1. They reflect the intensity of processes: Sharp boundaries often indicate a sharp change in conditions (e.g., tectonic disturbance, abrupt change in parent material, or illuviation processes where substances are deposited at a clear geochemical barrier). Gradual boundaries, on the contrary, indicate a smooth, continuous flow of processes, a gradual change in the intensity of weathering or humus accumulation with depth.

2. They help diagnose processes: For example, tongued, “tongue‑shaped” boundaries are characteristic of podzolisation, when organic acids “eat” their way into underlying horizons. Wavy boundaries often arise from root and soil faunal activity (bioturbation).

3. They are necessary for accurate sampling: In laboratory analysis, we must know exactly where one horizon ends and another begins to obtain representative data.

Parameters for Describing Boundaries

Boundaries are described by two main parameters: distinctness (degree of expression) and shape (topography) (Buol et al., 2011; Weil & Brady, 2017).

1. Boundary Distinctness (Transition width)

This is the thickness of the transitional layer in which properties are mixed. The thicker this layer, the less distinct the boundary.

• Very abrupt: Transition occurs over less than 0.5 cm. It is like a “cut” boundary. Often seen at the contact of a plough layer with the subsoil, at the boundary of peat with mineral soil, or at the boundary with a buried horizon.
• Abrupt: Transition occurs over 0.5–2 cm. Typical of many illuvial horizons, where the change in colour and structure is clearly visible.
• Clear: Transition occupies 2–5 cm. This is the most common type of boundary in well‑developed soils. For example, the transition from the humus horizon A to the transitional AB, or from eluvial A2 to illuvial B.
• Gradual: Transition occurs over 5–15 cm. Properties change slowly and gradually. Such boundaries are typical for soils with a smooth change in humus content with depth, e.g., Chernozems.
• Diffuse: Transitional layer more than 15 cm. Properties change so imperceptibly that boundaries between horizons are almost indistinguishable. Often found in homogeneous parent materials or in soils with very slow processes.

2. Boundary Shape (Topography)

This is the configuration of the boundary in vertical section, reflecting non‑uniformity of processes and the influence of bioturbation.

• Smooth: The boundary is an almost straight horizontal line. Characteristic of uniformly proceeding processes without local anomalies.
• Wavy: The boundary has gentle, undulating bends, where vertical deviations are smaller than horizontal ones. Arises from uneven moisture distribution, root system influence, or microrelief.
• Irregular: The boundary is strongly indented, vertical deviations are greater than horizontal ones. It has “bays” and “tongues” of one horizon into another. This is a diagnostic feature of podzolisation (tongues of whitish eluvial horizon cutting into the illuvial) or lessivage (tongues of bleached material into the clayey horizon) (Scheffer et al., 2018).
• Broken: The boundary is discontinuous, i.e., the horizon is not continuous and occurs as separate “lenses” or “pockets” in the underlying horizon. Characteristic of complex, polycyclic profiles, soils subjected to erosion or cryoturbation.

Relationship to processes:

• Smooth, gradual boundaries are more common in soils with predominance of in situ transformation processes (e.g., weathering, humus accumulation) without substantial substance migration.
• Abrupt, irregular or wavy boundaries indicate active migration processes (eluviation, illuviation) or a sharp change in parent material (lithological contact). For example, the sharp contact between a bleached eluvial horizon (A2) and a dark illuvial horizon (Bh or Bt) is one of the key diagnostic features of the podzolic process (White, 2006).

How to record a boundary in the field notebook?

In the horizon description, the lower boundary is recorded as a combination of distinctness and shape. For example:

• “Boundary abrupt, smooth” — typical for a Podzolic horizon.
• “Boundary clear, wavy” — for well‑developed Sod‑Podzolic soils.
• “Boundary gradual, smooth” — for Chernozems.
• “Boundary irregular, with tongues of bleached material” — for Podzols or Fahlerds.

Practical conclusion: Horizon boundaries are the “dividing lines” between different pages of soil history. Correct description of their distinctness and shape allows us to reconstruct the sequence of events, diagnose migration processes, and correctly interpret laboratory analysis data.

In the next, concluding part of our lecture, we will bring all this knowledge together into a single whole—compile a complete, comprehensive description of a soil profile using a concrete example. This will be a practical demonstration of how theory transforms into professional mastery.

Real Example of Profile Description

For illustration, I provide an example of a typical profile of Sod‑Podzolic soil (typical of the taiga‑forest zone of the European part of Russia). This description is synthesised from real field materials and well illustrates all the features we have considered (Ganzhara et al., 2002; Mukha et al., 2003).

General information:

• Pit number: 1‑2023
• Date: 15 June 2023
• Location: Moscow region, Sergiev Posad district, block 17, 200 m northeast of the village Lugovaya.
• Relief: Northern exposure slope, gradient 3‑5°, middle part.
• Vegetation: Mixed forest (spruce, birch, aspen) with dense moss‑herbaceous cover.
• Parent material: Mantle loam, loess‑like.
• Pit depth: 150 cm.
• Groundwater level: not detected (deeper than 2 m).

Horizon‑by‑horizon description:

A0 (0–5 cm) — Forest litter

• Colour: Brownish‑brown (moist: 7.5YR 3/4; dry: 7.5YR 5/6).
• Composition: Slightly decomposed litter of needles, leaves, twigs. Abundant fungal mycelium.
• Consistence: Loose, very porous (air‑filled).
• Inclusions: Many fine roots.
• Lower boundary: Abrupt, smooth.

A1 (5–15 cm) — Humus‑eluvial horizon

• Colour: Grey (moist: 10YR 5/2; dry: 10YR 6/3).
• Particle‑size distribution: Medium loam, silty (feels like flour when rubbed, but with slight sandiness).
• Structure: Weak, fine crumb‑granular. Aggregates are fragile, easily crumble.
• Consistence: Loose, fine‑porous.
• Moisture: Fresh.
• New formations: Occasional small (1–2 mm) ferruginous‑manganese concretions.
• Inclusions: Roots numerous, fine (1–2 mm), penetrate the whole horizon.
• Lower boundary: Clear, wavy.

A2 (15–28 cm) — Podzolic (eluvial) horizon

• Colour: Whitish, pale (moist: 10YR 6/3; dry: 10YR 7/2).
• Particle‑size distribution: Sandy loam (strong sandiness felt; ribbon does not form).
• Structure: Weak, thin platy (laminar). Soil splits into thin plates.
• Consistence: Slightly firm, fine‑porous.
• Moisture: Fresh.
• New formations: Abundant whitish siliceous powder, characteristic of the Podzolic horizon.
• Inclusions: Roots occasional, very fine. Rare charcoal fragments (traces of ancient fire).
• Lower boundary: Abrupt, irregular. The boundary has tongues and pockets penetrating into the underlying horizon (evidence of the podzolic process).

A2B (28–45 cm) — Transitional eluvial‑illuvial

• Colour: Mottled, brownish‑whitish (moist: 10YR 5/4 with whitish spots 10YR 7/2).
• Particle‑size distribution: Medium loam.
• Structure: Moderate, medium subangular blocky.
• Consistence: Firm, medium‑porous.
• Moisture: Fresh.
• New formations: Occasional iron films on aggregate faces.
• Inclusions: Roots rare, fine.
• Lower boundary: Clear, wavy.

B (45–100 cm) — Illuvial horizon

• Colour: Brownish‑brown (moist: 7.5YR 4/4; dry: 7.5YR 6/4).
• Particle‑size distribution: Heavy loam (ribbon forms; when bent, it cracks).
• Structure: Moderate, medium angular blocky. Aggregates strong, with sharp edges.
• Consistence: Firm, fine‑porous, with pore channels along root traces (biopores).
• Moisture: Slightly moist.
• New formations: Glossy films (cutans) on aggregate surfaces—evidence of clay illuviation.
• Inclusions: Roots occasional, fine, only along channels.
• Lower boundary: Gradual (transition to parent material), smooth.

C (100–150 cm) — Parent material

• Colour: Pale‑brown (moist: 10YR 6/4; dry: 10YR 7/4).
• Particle‑size distribution: Medium loam, homogeneous.
• Structure: Structureless (massive). Soil when extracted breaks into shapeless clods.
• Consistence: Firm, fine‑porous.
• Moisture: Fresh.
• New formations: Occasional carbonate concretions (“zhuravchiki”) at 120 cm depth.
• Inclusions: No roots.
• Lower boundary: Not reached (exposed to 150 cm).

General conclusion on the pit

• Soil name (according to Russian classification): Sod‑Podzolic, moderately podzolic, medium‑loamy soil on mantle loam.
• Genesis: The soil developed under mixed forest. The profile clearly shows two processes: sod process (humus accumulation in horizon A1) and podzolic process (removal of iron and clay from horizon A2 into horizon B). The illuvial horizon B is enriched with clay and iron oxides, as confirmed by colour, structure, and the presence of cutans.
• Agronomic assessment: The soil is poor in nutrients, acidic, requires liming and application of organic and mineral fertilisers. The upper horizons (A1, A2) have low coherence, which may promote wind erosion upon ploughing. The illuvial horizon is dense, which may limit deep root penetration. In general, with cultivation, it can be used for agricultural crops, but requires special agronomic practices.

Practical conclusion: A complete pit description is the culmination of all our field work. It allows not only to classify the soil but also to provide scientifically based recommendations for its use and improvement. This is an art that requires attention, systematic approach, and theoretical knowledge. I hope that today’s lecture, from pit preparation to full description, has laid a solid foundation for you to master this art.