The World's Main Soil Types

1.1. Chernozems

Distribution. These are the classic soils of steppes and forest‑steppes. The largest areas are found in Russia (in the so‑called “chernozem belt” from Ukraine to Siberia), as well as in the northern USA (prairies) and Canada. The name comes from the Russian word for “black earth” (Buol et al., 2011).

Forming conditions. The decisive condition is herbaceous vegetation (steppes, prairies, meadow steppes). Every year, a huge mass of roots (up to 20–25 t/ha of dry matter) dies off, decomposes, and humifies under neutral or slightly alkaline conditions. The climate ranges from moderately continental to continental, with a non‑leaching or periodically leaching water regime. Annual precipitation is 350–600 mm. Parent materials are predominantly loess and loess‑like carbonate loams (Naumov, 2016).

Typical profile. A deep, dark‑coloured humus horizon (A) with a thickness of 40–50 cm to 120–150 cm. Transition to a carbonate horizon (Bca). A granular structure is typical, with abundant coprolites (earthworm casts). Down the profile, carbonate neoformations (“white‑eye”, pseudomycelium) appear. The profile is weakly differentiated: the content of clay particles and oxides is evenly distributed (Naumov, 2016; Buol et al., 2011).

Main processes. Humus‑accumulative process (chernozem formation), biogenic structuring, carbonatisation. Intensive bioturbation (earthworms, rodents) ensures mixing of the profile.

Key properties. Humus – 5–10% and more (up to 14%), humus of the humate type (C_ha:C_fa > 1.5–2.0). Cation exchange capacity – 40–60 cmol_c/kg, the complex is saturated with calcium. Soil reaction is neutral (pH 6.5–7.5) in the upper horizons. The granular structure is water‑stable. Humus reserves – up to 600–800 t/ha (Naumov, 2016).

Limitations. Under moisture deficiency (dry years), yields may be unstable. Wind and water erosion. With irrigation without proper norms, secondary salinisation may occur. On arable land, dehumification (loss of humus) takes place.

Agronomic value. The benchmark of fertility. The best soils for growing cereals (wheat, maize), sugar beet, sunflower. They are considered the “breadbasket of the world”. V. V. Dokuchaev called chernozem the “king of soils” (Naumov, 2016). With proper farming practices, they can give high stable yields for decades.

1.2. Kastanozems

Distribution. South of the chernozem zone – in dry steppes. In Russia – the Volga region, the Caspian lowland, the south of Western Siberia. In the USA – the drier parts of the Great Plains.

Forming conditions. The climate is more arid than for chernozems. Annual precipitation – 250–450 mm. Evapotranspiration significantly exceeds precipitation. Vegetation – dry steppes with predominance of fescue, feather grass, wormwood (Naumov, 2016). Water regime – non‑leaching.

Typical profile. The humus horizon is noticeably thinner (30–45 cm), colour dark grey with a brownish tint (chestnut‑coloured). A carbonate horizon (Bca) is pronounced, often with gypsum accumulations at 120–200 cm depth. Solonetzic features (a solonetzic horizon Bsn) are a zonal trait. Profile thickness – 80–120 cm (Naumov, 2016; Buol et al., 2011).

Main processes. Less intensive humus‑accumulative process. Solonetzic process – incorporation of sodium ions into the soil exchange complex. Carbonatisation. Accumulation of readily soluble salts in the lower part of the profile.

Key properties. Humus 2.5–4.5%. C_ha:C_fa ratio about 1. Soils may be solonetzic, containing exchangeable sodium (3–15% of CEC). Reaction slightly alkaline (pH > 7.5). The carbonate horizon occurs closer to the surface than in chernozems (Naumov, 2016).

Limitations. The main limiting factor is moisture deficit. Common salinisation and solonetzicity. Wind erosion (especially on light soils).

Agronomic value. Used for spring wheat, sunflower, millet. Livestock (pastures). Irrigation with proper management can give high yields, but it is critically important to control the groundwater level to avoid secondary salinisation (Naumov, 2016).

1.3. Phaeozems

Distribution. Moist meadow steppes and forest‑steppes, where precipitation is higher than in the Kastanozem zone. In Russia – forest‑steppe regions. Found in Central and Eastern Europe, in the Pampas of Argentina.

Forming conditions. Climate – more humid and warmer. Vegetation – meadow steppes with abundant forbs. Water regime – periodically leaching.

Typical profile. Similar to Chernozem, but with a less saturated colour (dark grey). Transitional horizons are more pronounced. Carbonates are often absent in the profile, in contrast to Chernozems and Kastanozems. May show signs of lessivage (clay formation and translocation) (IUSS Working Group WRB, 2014; Buol et al., 2011).

Main processes. Strong humus‑accumulative process (similar to Chernozem, but with deeper wetting and leaching of carbonates). Structuring.

Key properties. High humus content (4–8%), but often lower than Chernozems (due to the leaching regime). Reaction close to neutral or slightly acidic. High cation exchange capacity, base‑saturated. Good granular structure.

Limitations. Tendency to leaching and lessivage, which may lead to formation of compacted illuvial horizons. On arable land, structure may be lost.

Agronomic value. Very fertile. Used for cereals, fodder crops. Respond well to fertilisation. Can give high yields with sufficient moisture.

1.4. Luvisols

Distribution. Broad‑leaved forests of the temperate zone (Europe, eastern USA, East Asia). In Russia – central and southern regions (in forest‑steppe and deciduous‑forest zones).

Forming conditions. Climate – moderately humid and warm (with a leaching water regime). Vegetation – broad‑leaved and mixed coniferous‑broad‑leaved forests. Litter is rich in ash elements (especially calcium), which allows humus to accumulate, but not enough to form a thick Chernozem horizon.

Typical profile. Characteristic of the so‑called Parabraunerde in the German classification and grey forest soils in the Russian one. Profile: O horizon (litter) – A (humus) – E (eluvial, or AEL) – BT (textural, illuvial, with clay accumulation). The textural horizon (BT) is the main diagnostic feature of Luvisols (Naumov, 2016; Buol et al., 2011). It shows an increase in clay content and clay cutans on ped faces.

Main processes. Lessivage (eluvial‑illuvial redistribution of clay without destruction), humus‑accumulative process, clay formation, decarbonatisation (leaching of carbonates). Acidic organic acids formed in the forest litter enhance leaching.

Key properties. Humus content in the A horizon – 2–5%, in the eluvial – very low (0.2–0.5%). Humus is fulvate‑humate (C_ha/C_fa ~ 1.0). Profile differentiation in clay (1.5–2.5 times more in BT than in AEL). Reaction slightly acidic in the upper part (pH 4.5–5.5) and neutral in the lower. CEC – 20–40 cmol_c/kg. Carbonates are at a depth of 100–150 cm (Naumov, 2016; Buol et al., 2011).

Limitations. Under cultivation, they easily lose structure and are subject to erosion. Acidic reaction of upper horizons (liming required). Compaction of the illuvial horizon may hinder tillage and root development.

Agronomic value. Important arable soils. With proper agronomic management (liming, organic fertilisers), they can be very productive. Crops: cereals, sugar beet, vegetables.

1.5. Albeluvisols / Retisols

Nomenclature note: In the older WRB (2006), the group was called Albeluvisols. In the new version (2014) – Retisols. In the Russian classification, they correspond to Fahlerde.

Distribution. In the northern part of the temperate zone (boreal forest zone), where the climate is colder and wetter. In Russia – vast areas of taiga (European part, Western Siberia). Found in Scandinavia, Canada (Buol et al., 2011; Naumov, 2016).

Forming conditions. Climate – cold, humid. Water regime – leaching. Vegetation – coniferous and mixed forests. Litter (needles) is rich in acids, enhancing podzolisation. Parent materials – mostly non‑carbonate loams and clays (moraine deposits).

Typical profile. Horizons: O (litter) – EL (eluvial, light grey, whitish) – BEL (transitional, light brown with whitish powder) – BT (textural, brown, compacted). Characteristic are tongues and fingers – light eluvial material penetrates as “tongues” into the textural horizon. This is called the retic property (hence the name Retisols). Structure – platy in EL, nutty‑prismatic in BT (Naumov, 2016; IUSS Working Group WRB, 2014).

Main processes. Lessivage, podzolisation (acid hydrolysis of minerals). Gleyisation (eluvial‑gley process) enhances the removal of iron and aluminium.

Key properties. Humus – up to 2–4% in upper horizons, in eluvial – less than 1%. Humus is fulvic (C_ha:C_fa < 0.5). Reaction strongly acidic (pH_KCl 3.0–4.5). Eluvial‑illuvial differentiation in clay and total sesquioxides is very pronounced. CEC is low. Soils are base‑unsaturated (Naumov, 2016).

Limitations. Acidity, low nutrient availability, poor structure. Tendency to waterlogging (gleyisation) in upper horizons.

Agronomic value. In natural state, fertility is low. With intensive cultivation (liming, organic matter, complete fertilisation), they can give high yields of cereals, potatoes, fodder crops. Mainly used for forestry, hayfields, pastures.

1.6. Podzols

Distribution. In cold and humid forest ecosystems – northern taiga, forest‑tundra. Widely represented in Scandinavia, Canada, Siberia, the north of European Russia.

Forming conditions. Climate – cold, humid (precipitation exceeds evapotranspiration). Vegetation – coniferous forests (spruce, pine, larch) with acidic litter poor in bases. Parent materials – sandy (quartz) or loamy sand, with very low CEC (Buol et al., 2011).

Typical profile. The most characteristic profile of all soils. Includes: O horizon (litter), then E horizon (eluvial, light, almost white, “podzolic”), and below – B horizon (illuvial, black, brown, ochre). The B horizon is subdivided into Bh (humus, black) and Bs (iron, brown or ochre). Transition sharp, wavy.

Main processes. Podzolisation (or podzol formation). The essence: organic acids formed during needle decomposition actively destroy minerals in the upper part of the profile. Soluble complex compounds with iron and aluminium (metal‑organic complexes) are formed, leached downward, and precipitated in the B horizon. The upper part remains without iron and aluminium – pure white quartz (E) (Buol et al., 2011). This is one of the strongest eluvial processes.

Key properties. E horizon composition – almost pure quartz. In the B horizon – accumulation of amorphous sesquioxides (Al and Fe) and humus. Soils strongly acidic (pH_KCl < 4), unsaturated. Humus is fulvic. CEC is low. Very high mobility of iron and aluminium compounds.

Limitations. Extreme nutrient poverty, strong acidity, low water‑holding capacity (sands), and the presence of ortstein (cemented horizon) in Bs – an almost insurmountable barrier for roots.

Agronomic value. Usually – forest land. On a limited scale – with intensive cultivation (liming, organic matter, fertilisers) – potatoes and winter rye can be grown (Buol et al., 2011). However, the environmental risk of cultivation is high (groundwater pollution).

Summary for the temperate zone.

Thus, in the temperate zone we see a regular sequence of soils from north to south. In the north (cold and moisture) – Podzols, acidic and infertile. Further (forest zone) – Luvisols and Retisols, with less pronounced acidity, but with textural differentiation and requiring cultivation. To the south (forest‑steppe) – Phaeozems, fertile chernozem‑like soils. Further south (steppes and dry steppes) – Chernozems and Kastanozems, the most fertile, but limited by moisture. This pattern is explained by the change in climatic conditions: from north to south, temperature increases and humidity decreases (Naumov, 2016; Buol et al., 2011).

End of Part 1. Soils of the Temperate Zone.

In the first part of the lecture, we examined the main soil types of the temperate zone. The following groups were established:

1. Chernozems – benchmark of fertility, characteristic of steppes, thick humus horizon, high humus content, neutral reaction, granular structure.

2. Kastanozems – south of Chernozems, in dry steppes. Smaller humus horizon thickness, carbonate and gypsum horizons, solonetzicity, moisture limitation.

3. Phaeozems – at the forest‑steppe/steppe boundary, rich in humus, but without carbonates.

4. Luvisols – forest soils with distinct textural differentiation (eluvial AEL and illuvial BT). Main process – lessivage.

5. Albeluvisols / Retisols (Fahlerde) – soils of the northern taiga with tongued textural differentiation, acidic, low fertility.

6. Podzols – soils of cold coniferous forests with a pronounced eluvial E horizon and illuvial B. Acidic, poor, but can be used under cultivation.

Each of these groups has its own distribution, genesis, properties, and agronomic value. Their distribution pattern clearly follows climatic zonality. Understanding these patterns is the basis for rational land use and crop selection.

In the next part, we will move on to the soils of humid tropics (Ferralsols, Acrisols, Alisols, Nitisols) – a zone with fundamentally different soil‑forming conditions: high temperatures and abundant rainfall, where soil formation proceeds along the path of deep weathering and desilication.

2.1. Ferralsols

Distribution. On ancient continental shields – in Brazil, Central Africa (Congo basin), northern Australia, India. These are the most widespread soils of the humid tropics (Buol et al., 2011; Naumov, 2016). They occupy about 20% of tropical land area. In the Russian classification, they correspond to ferrallitic soils, in the old American classification – Oxisols.

Forming conditions. The key condition is a hot and humid climate (prolonged weathering period). Precipitation exceeds evapotranspiration, water regime – leaching. Vegetation – humid tropical forest (hylaea), which produces huge biomass but is itself poor in nutrients (Sanchez and Buol, 1975). Parent materials – usually deeply weathered (ancient weathering crusts). Importantly, Ferralsols form on old, stable surfaces (age – millions of years) (Beinroth, 1982).

Typical profile. The profile is poorly differentiated in horizons. The upper layer – often ochric or umbric (depending on humus), may be intensely red or yellow. Below – a thick ferralic horizon (B) – loose, deep (up to 5–10 m or more), with bright red or yellow‑red colour. Boundaries between horizons are usually diffuse, gradual. Characteristic pseudo‑sandy structure: aggregates that look like sand grains are very strong, water‑stable, and easily absorb moisture (Buol et al., 2011; Scheffer et al., 2018).

Main processes. Ferrallitisation – deep chemical weathering under warm and humid conditions. The essence: almost complete destruction of all primary silicate minerals (except quartz). Removal of silicon (SiO₂) and bases (Ca, Mg, K, Na). Accumulation in the profile of iron and aluminium oxides, as well as 1:1 clay minerals (kaolinite) (Buol et al., 2011). Iron and aluminium accumulate as Fe₂O₃ (hematite, goethite) and Al₂O₃ (gibbsite). Desilication (removal of silicon) is the leading process.

Key properties.

• Acidity: pH (H₂O) 4.5–5.5, often lower in upper horizons (Naumov, 2016).
• Very low cation exchange capacity (CEC): usually < 10–15 cmol_c/kg, mainly due to organic matter. Cations are weakly retained and easily leached.
• High phosphorus fixation: because of the huge amount of active Fe and Al oxides (only 0.5–2% of phosphorus is mobile) (Sanchez, 1976; Buol et al., 2011).
• Good physical properties: stable microaggregate structure, high permeability, low bulk density (1.0–1.3 g/cm³). Easy to till even at high moisture (Buol et al., 2011).
• Nutrient poverty: natural vegetation (tropical forest) stores almost all biomass in trunks and leaves (living tissues), while soil nutrients are very low (Sanchez and Buol, 1975).

Limitations. Very low natural fertility. High acidity and aluminium toxicity (Al³⁺) for plants (Sanchez, 1976). Huge need for phosphorus fertilisers (due to fixation). Under low‑input agriculture (slash‑and‑burn), they are quickly depleted. Long‑term use without fertilisation and liming leads to degradation (Sanchez and Buol, 1975; Buol et al., 2011).

Agronomic value. Huge potential with high‑input technologies. With regular application of lime, phosphorus and potassium fertilisers, and micronutrients (Zn, Cu, B), they can give very high yields of agricultural crops: soybean, maize, sugarcane, coffee, rubber, oil palm (Lopes, 1996; Buol et al., 2011). Ferralsols in Brazil (Cerrado) are the basis of the modern agricultural boom (Lopes, 1996). With proper agrotechniques, they are among the most productive soils in the world.

2.2. Acrisols

Distribution. Extensive areas of humid and subhumid tropics (West Africa, Amazon basin, Southeast Asia). In the Russian classification – red earths and yellow earths. In the old American classification – Ultisols (Buol et al., 2011).

Forming conditions. Climate – humid tropical, but often with a pronounced dry season (3–6 months). It is this break in moisture that allows the process of lessivage (clay translocation) to proceed. Parent materials – usually acidic (granites, gneisses, sandstones), less often basic.

Typical profile. Horizons: A – eluvial E (often lighter) – transitional BEL – argic or kandic horizon Bt (with clay accumulation). Colour often red, yellow, or reddish‑yellow. The most characteristic feature is distinct textural differentiation: in the upper part (E) – sand, loamy sand; in the lower (Bt) – clay. The boundary between E and Bt is often sharp, leading to the formation of a plow pan (dense horizon) at the boundary (Buol et al., 2011).

Main processes. Lessivage (clay translocation), weathering (including desilication, but less intensive than in Ferralsols). Accumulation in the Bt horizon – kaolinite, Fe and Al oxides. In the upper part (A and E) – removal of clay and sesquioxides.

Key properties.

• Low CEC (usually 5–15 cmol_c/kg).
• Acidic reaction (pH 4–5.5).
• Low base saturation (<50%).
• Pronounced textural differentiation (from sand in E to clay in Bt).
• Elevated aluminium toxicity (high exchangeable Al³⁺ content).
• Often contain plinthite (a secondary material that hardens into an iron crust upon exposure to air) – a serious limitation (Buol et al., 2011).

Limitations. Low fertility, acidity, toxic Al, poor physical properties under erosion (exposure of clayey Bt), and plinthite crust.

Agronomic value. Under cultivation, they require liming, high doses of phosphorus and potassium fertilisers, often organic matter. With good management, they can be productive for rice, maize, soybean, cassava, tea. In Asia and Africa, many Acrisols are used for rice (paddy fields) (Buol et al., 2011). However, many Acrisols remain under natural vegetation (forests) or are used for extensive livestock.

2.3. Alisols

Distribution. Alisols are often found in the same regions as Acrisols, but under slightly wetter conditions and on more base‑rich parent materials. They usually occupy a transitional position between Ferralsols and Acrisols (IUSS Working Group WRB, 2014). Found in Southeast Asia, Central Africa, some parts of South America. In the Russian classification there is no direct analogue (close to red earths). They differ from Acrisols by having a higher content of 2:1 clay minerals (hydromicas, vermiculite) and a higher cation exchange capacity while maintaining low base saturation (Buol et al., 2011).

Forming conditions. Similar to Acrisols, but with slightly better conditions for the preservation of 2:1 layer silicates (less intense weathering, often on basic rocks). Climate – humid tropical, with seasonal waterlogging, but less intense than in Ferralsols (IUSS Working Group WRB, 2014).

Typical profile. Similar to Acrisols: A – E – Bt (argic or kandic horizon). However, colour – often reddish‑brown. Characteristic is clear textural differentiation and presence of clay cutans (films) in Bt. Important difference: the clay horizon often has a high cation exchange capacity (due to 2:1 layer silicates) (IUSS Working Group WRB, 2014; Buol et al., 2011).

Main processes. Lessivage (clay translocation). Weathering (but less intense than in Ferralsols). Preservation of 2:1 layer silicates (unlike Ferralsols). Often show andic properties (if volcanic ash is present).

Key properties.

• Acidic reaction, but often pH 4.5–5.5 (higher than in Acrisols).
• High CEC (usually > 20 cmol_c/kg) due to 2:1 clays.
• Low base saturation (<50%).
• Good structure in Bt.
• Contain Al‑interlayered clay minerals.
• Lower phosphorus fixation than Ferralsols, but acidity and Al toxicity remain a problem (IUSS Working Group WRB, 2014).

Limitations. Acidity, aluminium toxicity. With high CEC – good retention of bases, but they are in deficit. Under cultivation (especially after forest clearance) – rapid depletion.

Agronomic value. With cultivation (liming, fertilisation) they can give very good yields. Due to high CEC, they retain potassium and magnesium more effectively than Ferralsols. In Asia, they are used for rice, maize, vegetables. With a good cropping system (rotations, organic matter) – they can be very productive (Buol et al., 2011).

2.4. Nitisols

Distribution. These are soils that stand out among other tropical soils for their exceptional structural development. Found in East Africa (highlands), in some parts of India, Indonesia, Central and South America. In the Russian classification there is no direct analogue (close to red clay‑metamorphic soils), in the old American classification – Nitosols were included in Ultisols or Alfisols (IUSS Working Group WRB, 2014; Buol et al., 2011).

Forming conditions. Climate – humid tropical or subtropical, often with a distinct dry season (which favours clay translocation). Parent materials – often basic (basalts, basic clay shales). It is the richness in ferromagnesian minerals that creates ideal conditions for the development of bright‑red, structured soils (Buol et al., 2011; Scheffer et al., 2018).

Typical profile. The distinguishing feature is a deep, bright‑red, very well‑structured B‑horizon (nitic horizon). Structure – nutty, very strong, with shiny slickensides on aggregates. Profile is usually deep (up to 2–3 m), with gradual transitions. Clay content – high (often > 40–50%), but due to good structure – excellent permeability (Buol et al., 2011; Scheffer et al., 2018).

Main processes. Nitisol formation (development of characteristic nutty structure). Weathering (formation of kaolinite, goethite, hematite). Lessivage. The dominant process is metamorphic structuring, not just clay translocation (clay cutans are often weakly expressed, as structure forms in situ) (IUSS Working Group WRB, 2014; Scheffer et al., 2018).

Key properties.

• Bright‑red colour (hematite).
• High clay content, but excellent aggregate structure.
• CEC – often medium or high (20–30 cmol_c/kg), depends on parent material.
• Reaction – usually neutral or slightly acidic (pH 5.5–6.5). Due to good buffering (high Ca and Mg content in the rock).
• Good phosphorus availability (less fixation than in Ferralsols, due to fewer amorphous oxides).
• High available potassium and magnesium (due to rich parent rock).

Limitations. Located in zones with seasonal droughts – need irrigation. May be subject to erosion if structure is lost (due to tillage). With shallow bedrock – limited depth.

Agronomic value. Among the best tropical soils. Due to good structure, high CEC, and phosphorus availability – very productive. Used for tea, coffee, tobacco, cotton, cereals, sugarcane (especially in East Africa). With proper management – consistently high yields without large inputs (Buol et al., 2011). Nitisols often form the basis for high‑intensity commercial agriculture in the tropics.

Summary for tropical soils.

Thus, in the humid tropics we see a sequence of soils from ancient, strongly weathered (Ferralsols) to younger or on richer parent materials (Acrisols, Alisols, Nitisols). The key difference is the degree of weathering and preservation of primary minerals. Ferralsols – most weathered, poor (but highly productive with inputs). Acrisols and Alisols – with clay accumulation in the subsoil, have better CEC and are often more fertile. Nitisols – the elite of the tropics, with unique structure and relatively high fertility.

Understanding these differences is critically important for choosing land‑use systems. Slash‑and‑burn agriculture quickly depletes Ferralsols and Acrisols, while on Nitisols and with proper management on Ferralsols, stable yields can be obtained for decades.

In the next part, we will move on to soils of arid regions – zones where moisture deficit determines everything: Calcisols, Gypsisols, Solonchaks, Solonetz.

3.1. Calcisols

Distribution. In deserts and semi‑deserts of Africa (Sahara, Namib), Asia (Arabian Peninsula, Central Asia, Gobi), Australia, North and South America. In Russia – in the Caspian lowland, the south of Western Siberia. These are typical soils of arid areas with carbonate‑type accumulation (Buol et al., 2011; Naumov, 2016).

Forming conditions. Climate – arid or semi‑arid (precipitation 100–350 mm/year). Evapotranspiration is 5–10 times higher than precipitation. Water regime – non‑leaching. Vegetation – sparse (wormwood, saltworts, ephemerals). Parent materials – usually carbonate (loess, marls, limestones) or non‑carbonate, but with intensive input of calcium from dust and atmospheric precipitation. These are soils of “non‑leaching type”, where carbonates are not leached but redistributed within the profile (Gile et al., 1966; Buol et al., 2011).

Typical profile. Upper horizon (A) – light grey or pale, thin, with low humus content. Below – a calcic horizon (Bk or Bca), where secondary carbonates accumulate. Forms of carbonates vary: from thin threads (pseudomycelium) and spots to dense concretions (“white‑eye”) and continuous petrocalcic horizons (Bkm) – calcite crusts (calcrete) up to several metres thick (Gile et al., 1966; Naumov, 2016). Below – parent material (C), often also carbonate.

Main processes. Calcification – dissolution of carbonates in upper horizons (under the action of carbonic acid produced by roots and microorganisms) and their re‑precipitation in the lower part of the profile. Carbonate migration is seasonal: in the wet season they dissolve and move downward, in the dry season they precipitate. This process is especially intensive when there is a source of calcium (dust, rain, rock) and limited removal (non‑leaching regime) (Gile et al., 1966; Buol et al., 2011).

Key properties.

• Alkaline reaction (pH 7.5–8.5), especially in the carbonate horizon.
• Low humus content (0.5–1.5%) in the upper horizon.
• High CaCO₃ content (up to 30–80% in Bk) (Naumov, 2016).
• Often gypsum and readily soluble salts are present in the lower part of the profile.
• Water permeability can be greatly reduced in the presence of a dense petrocalcic horizon.

Limitations. The main limitation is drought. With a petrocalcic horizon – roots cannot penetrate deep, further limiting moisture availability. Irrigation is required for agricultural use, but with irrigation there is a high risk of secondary salinisation (Naumov, 2016; Buol et al., 2011).

Agronomic value. Under rainfed conditions – only extensive pastoralism (sheep, camels). With irrigation – can be productive for cereals (wheat, barley), cotton, vegetables (provided salinity is controlled). However, the presence of a carbonate horizon often requires special tillage (deep ripping) to break the crust and improve permeability (Naumov, 2016).

3.2. Gypsisols

Distribution. In deserts and semi‑deserts with more arid climates than Calcisols. Widely represented in the Sahara, Arabian Peninsula, Central Asia, Australia. In Russia – small areas in the Caspian lowland and some intermountain depressions of Southern Siberia (Naumov, 2016; Buol et al., 2011).

Forming conditions. Climate – very arid (precipitation < 150–200 mm/year). Evapotranspiration is 10–20 times higher than precipitation. Vegetation – extremely sparse. Parent materials – often gypsum‑bearing (marine deposits, evaporites), or with intensive input of gypsum from dust or groundwater. Gypsum (CaSO₄·2H₂O) accumulates in the profile because its solubility (2.4 g/l) is much lower than that of chlorides and sodium sulphates, but higher than that of carbonates (Buol et al., 2011; Scheffer et al., 2018).

Typical profile. Upper A – very weakly developed, light. Below – a gypsic horizon (By or Bca) with accumulation of gypsum in the form of crystals, druses, pseudomycelium. At high concentration, a petrogypsic horizon (Bym) is formed – a gypsum crust (gypcrete). The profile is often weakly differentiated, as gypsum fills pores and cements particles (Naumov, 2016; Scheffer et al., 2018).

Main processes. Gypsum accumulation – accumulation of gypsum in the zone of maximum evaporation. Gypsum is easily transported by upward water flows and precipitates upon drying. Unlike carbonates, gypsum is not always confined to a specific depth; it can accumulate in upper horizons as well (Buol et al., 2011; IUSS Working Group WRB, 2014).

Key properties.

• Alkaline or neutral reaction (pH 7.0–8.5).
• High gypsum content (up to 30–60% or more).
• Low humus content (< 0.5%).
• Often contain readily soluble salts (NaCl, Na₂SO₄) and carbonates.
• Gypsum strongly affects physical properties: upon wetting – softens; upon drying – cements (Scheffer et al., 2018).

Limitations. The main – drought. Petrogypsic horizon – an insurmountable barrier for roots. Under irrigation, gypsum can dissolve, leading to subsidence and damage to structures (concrete corrosion). Gypsum can also reduce permeability when saturated (Scheffer et al., 2018).

Agronomic value. In natural state – only pastures (extremely low productivity). With water and absence of continuous gypsum horizon – possible cultivation of some crops (e.g., barley, alfalfa), but with low economic efficiency. Gypsum, with proper management, can be useful for improving Solonetz (gypsum application) (Naumov, 2016).

3.3. Solonchaks

Distribution. In endorheic depressions of arid and semi‑arid areas – takyrs, salt deserts, ancient river deltas. In Russia – the Caspian lowland, the south of Western Siberia, some depressions of Southern Siberia. These are soils where accumulation of readily soluble salts (NaCl, Na₂SO₄, MgSO₄, CaCl₂) has reached toxic levels (Naumov, 2016; Buol et al., 2011).

Forming conditions. Climate – arid, with very high evapotranspiration. Water regime – evaporative (efflorescence): groundwater with dissolved salts rises to the surface and evaporates, leaving salts in the upper horizons. Groundwater usually lies at a depth of 1–3 m. Vegetation – halophytes (salt‑loving plants: glasswort, sea blite, saxaul) or absent (Naumov, 2016).

Typical profile. The distinguishing feature is intensive surface salinisation. Upper horizon – salt crust, efflorescences, or loose saline layer. May contain up to 10–20% readily soluble salts. Profile weakly differentiated, often with signs of gleyisation below (due to groundwater). Both automorphic (on saline parent material) and hydromorphic (with groundwater salinisation) variants occur (Naumov, 2016; Scheffer et al., 2018).

Main processes. Solonchak process – accumulation of readily soluble salts (chlorides, sulphates, sometimes soda) in the upper part of the profile. This is the result of salt input from groundwater, high evapotranspiration, and lack of leaching. Salt sources: ancient marine deposits, rock weathering, atmospheric precipitation (impulverisation) (Naumov, 2016).

Key properties.

• Very high content of readily soluble salts (more than 1–2% in upper horizons).
• Strongly alkaline reaction under soda salinisation, neutral under chloride salinisation.
• Low humus content (less than 1–2%).
• Severely degraded physical properties (colloid dispersion, lack of structure) in the presence of soda, or conversely, coagulation and crusting under chloride salinisation.
• High osmotic pressure of the soil solution, making water physiologically unavailable to most plants (Naumov, 2016; Buol et al., 2011).

Limitations. Salt toxicity for agricultural crops. Osmotic shock, ion imbalance (Na, Cl), often high alkalinity. Non‑halophytes cannot grow. Complex reclamation measures (leaching, drainage) are required to remove salts (Naumov, 2016).

Agronomic value. Without reclamation – unsuitable for arable farming, used as pastures for camels and sheep (halophytes). After leaching (flooding, drainage) and liming (under soda salinisation), they can be reclaimed. However, the process is very expensive and often unprofitable. There are areas where, after leaching, rice is grown (paddy fields). Overall, these are indicator soils of extreme aridity, for which humanity has not yet learned to apply cheap agriculture (Naumov, 2016; Buol et al., 2011).

3.4. Solonetz

Distribution. In steppe, dry‑steppe, and semi‑desert zones of Eurasia (southern Russia, Ukraine, Kazakhstan, Hungary). Also found in the USA, Canada, Australia. In Russia – large areas in the Caspian lowland, southern Western Siberia, Transbaikalia. These are soils that form during desalinisation of Solonchaks, but with preservation of sodium in the exchange complex (Naumov, 2016; Buol et al., 2011).

Forming conditions. Climate – arid or semi‑arid (precipitation 200–400 mm/year). Water regime – non‑leaching or periodically leaching. Vegetation – wormwood‑grass communities (fescue, slender grass, wormwood). Parent materials – often saline (marine clays, loess‑like loams). Solonetz process – incorporation of sodium ions (Na⁺) into the soil exchange complex, leading to dispersion of colloids, deterioration of physical properties, and a sharp increase in alkalinity (Naumov, 2016; Buol et al., 2011).

Typical profile. Profile sharply differentiated (eluvial‑illuvial type):

• Upper horizon (A) – humus, often lightening (solodized), with crumb or platy structure.
• Solonetzic horizon (Bsn) – the most characteristic: dense, columnar or prismatic (large columns with shiny surfaces). This horizon is strongly compacted, fissured, and contains much exchangeable sodium.
• Below – subsolonetzic horizon (Bca, Bcs) with carbonates, gypsum, often with readily soluble salts (Naumov, 2016; Scheffer et al., 2018).

Main processes. Solonetz process – exchangeable adsorption of Na⁺ ions by colloids when present in the soil solution (often a result of Solonchak desalinisation). Na⁺ causes strong peptisation (dispersion) of clay and organic colloids. As a result:

• Colloids become mobile and can move downward (formation of illuvial horizon).
• Physical condition deteriorates sharply: when wet, soil becomes viscous, sticky, almost impermeable; when dry, extremely hard, cracking into large columns. This phenomenon is called Solonetz complex (Naumov, 2016; Buol et al., 2011).

Key properties.

• Strongly alkaline reaction (pH 8.5–10), especially in the Bsn horizon.
• High content of exchangeable Na⁺ (> 15–20% of CEC in Bsn).
• Eluvial‑illuvial differentiation (depletion of upper part, enrichment of middle).
• Extremely unfavourable physical properties: low permeability, high hardness when dry, stickiness and viscosity when wet.
• Carbonates and gypsum are often present in lower horizons (Naumov, 2016; Buol et al., 2011).

Limitations. Practically cannot be tilled with ordinary farm implements. In dry state – stony; in wet – impassable. Strongly restrict root growth. Low biological activity. With shallow groundwater – additionally salinised (Naumov, 2016).

Agronomic value. Without reclamation – of low suitability. The main reclamation method is gypsuming (application of gypsum CaSO₄·2H₂O), which replaces Na⁺ with Ca²⁺ in the exchange complex, improving physical properties. Applied over large areas in Kazakhstan, Russia (Naumov, 2016). After gypsuming and deep ripping, they become suitable for wheat, barley, alfalfa. Also used as pastures (wormwood, wheatgrass) with moderate stocking.

Summary for arid soils.

Thus, in arid regions we see a regular sequence of soils according to degree of salinisation and alkalinity:

• Calcisols – accumulation of carbonates (CaCO₃) in the soil, usually as secondary deposits, typical of semi‑arid and arid conditions.
• Gypsisols – accumulation of gypsum (CaSO₄·2H₂O), forming under more arid conditions than Calcisols.
• Solonchaks – accumulation of readily soluble salts (NaCl, Na₂SO₄, etc.) in the upper part of the profile, making them extremely toxic.
• Solonetz – soils with high exchangeable sodium (Na⁺), severely degraded physical properties, and high alkalinity.

All these soils have common features: low humus content, alkaline or neutral reaction, frequent presence of carbonates, gypsum, and salts, weak profile differentiation (except for Solonetz). They are widespread in arid regions of the world. Agricultural use is possible only after special reclamation: irrigation (with drainage), gypsuming, leaching, and selection of salt‑tolerant crops.

Understanding these soils is critically important for rational water resource use, prevention of secondary salinisation, and desertification.

In the next part, we will move on to young and weakly developed soils – initial stages of soil formation, where differentiation processes have not yet had time to manifest: Cambisols, Regosols, Leptosols, Fluvisols.

4.1. Cambisols

Distribution. Cambisols are among the most widespread soils of the temperate zone. They occur everywhere: in Europe (especially in mountains), North America, southern Australia, New Zealand, and also over vast areas of Russia (in taiga and forest‑steppe zones). These are “typical” soils that show signs of soil formation but have not yet reached the stage of mature soils (Buol et al., 2011; Naumov, 2016). In the Russian classification – brown forest soils and sod‑podzolic in initial stages, in the old American – Inceptisols.

Forming conditions. Climate – from moderately humid to subhumid. Water regime – leaching or periodically leaching. Vegetation – forests (broad‑leaved, coniferous) or meadows. Parent materials – various: from loose sands to dense clays (moraine, alluvium, colluvium). Importantly, Cambisols form on relatively young surfaces or on rocks that weather slowly (Buol et al., 2011; Scheffer et al., 2018).

Typical profile. The profile usually consists of: O (litter) or A (humus) – Bw (weakly developed, cambic horizon) – C (parent material). Bw – an altered horizon where weathering signs are noticeable: colour change (browning), structure (crumbly or nutty), but no pronounced accumulation of clay, humus, or salts. Transitions between horizons are gradual, profile often shallow (30–80 cm) (Buol et al., 2011; Scheffer et al., 2018). There are also Cambisols with gleyisation (Bg) or with carbonates (Bk).

Main processes. The main process is brown soil formation (or cambisolization): weathering (mainly physical and initial chemical), colour change due to release of iron from minerals, structuring. Weak redistribution of clay (lessivage) may occur, but insufficient to form an argic horizon. Processes of podzolisation (under acidic conditions) or calcification (on carbonate rocks) may also appear (Buol et al., 2011; Naumov, 2016).

Key properties.

• Neutral or slightly acidic reaction (pH 5.5–7.0), often depends on parent material.
• Average humus content (1–4%) in A horizon.
• CEC – medium (10–25 cmol_c/kg), depends on clay minerals.
• Good structure, often crumbly, permeability good.
• Weak differentiation in clay or oxide content (no sharp increase in Bw).
• No pronounced accumulative horizons (salts, gypsum, carbonates), though they may occur (Naumov, 2016; Buol et al., 2011).

Limitations. Shallow depth on steep slopes. Stoniness. Possible gleyisation under waterlogging. Low CEC may limit cation retention. Often located on slopes, limiting mechanised tillage.

Agronomic value. Cambisols are widely used in agriculture, especially on flat areas and in valleys. With proper management (fertilisation, liming) they can give good yields of cereals, potatoes, fodder crops. They also support good forest growth (used in forestry). Their value lies in good drainage and aeration, important for root systems (Buol et al., 2011).

4.2. Regosols

Distribution. Regosols are weakly developed soils on loose sandy or loamy sand deposits. They occur in deserts, on marine coasts (dunes), river valleys, young alluvial terraces. In Russia – on sandy river terraces, dunes, areas with light soils. In world practice, they are often associated with Psamments in the American classification (Buol et al., 2011; Weil, 2017). They can occur in both arid and humid regions, but more often in semi‑arid.

Forming conditions. Soil formation is strongly limited due to:

• Sandy texture (low specific surface, weak chemical weathering).
• Active erosion or accumulation (soil is constantly rejuvenated).
• Often – moisture deficit (in arid regions).
• getation – poor, often pioneer (mosses, lichens, rare grasses) (Weil, 2017; Naumov, 2016).

Typical profile. The simplest profile: A – C. Humus horizon (A) – thin (1–10 cm), weakly expressed, usually ochric. Parent material (C) – sand or loamy sand, often uniform. Sometimes a weak Bw appears (but rarely). Profile is practically undifferentiated (Buol et al., 2011; Weil, 2017).

Main processes. Minimal. May include settling (weak humus accumulation in the top layer), weak weathering (browning due to iron). In arid conditions – salt accumulation, but Regosols are usually poor in salts due to rapid leaching of sandy layers during rare rains (Buol et al., 2011).

Key properties.

• Very low CEC (< 5–10 cmol_c/kg) due to sandy texture.
• Low humus content (< 0.5–1%).
• Neutral or slightly alkaline reaction (depends on parent material).
• Excellent permeability, but low water‑holding capacity.
• Easily subject to wind erosion and deflation.

Limitations. Very low fertility. Strong dependence on irrigation. Easily dry out. Practically do not retain nutrients. When vegetation cover is disturbed – rapid desertification.

Agronomic value. Under rainfed conditions – used as pastures with very low productivity. With irrigation and intensive fertilisation (especially organic), they can be productive for melons (watermelons, melons), grapes (on loose sands), and some industrial crops. In oases – an important resource, but requires constant care and protection from erosion (Weil, 2017; Naumov, 2016).

4.3. Leptosols

Distribution. Leptosols are shallow, skeletal soils that often occur on steep slopes, mountainous areas, rock outcrops. They occupy significant areas in the Alps, Himalayas, Andes, Balkans, Scandinavia. In Russia – in the Urals, Caucasus Mountains, Siberia, and areas with shallow bedrock (Buol et al., 2011; Naumov, 2016). In the American classification – Lithic or Entisols.

Forming conditions. The main factor – shallow bedrock (within 25–50 cm of the surface). Soil formation is limited by:

• Shallow loose layer.
• Often high erosion (slopes).
• Stoniness, rubble.
• lief – steep or rocky. Vegetation – sparse (lichens, mosses, low shrubs) (Buol et al., 2011; Scheffer et al., 2018).

Typical profile. Profile: A – R (or C – R). Very thin humus horizon (often 5–15 cm), often ochric or umbric (if humus is abundant). Below – bedrock (R) or dense rubble layer (C). A weakly developed Bw may be present (if chemical weathering occurs). Profile very stony, often with rock outcropping (Buol et al., 2011; Weil, 2017).

Main processes. Minimal. Physical weathering (frost, thermal) dominates over chemical. Weak humus accumulation (due to low vegetation productivity). Carbonate removal may occur (on carbonate rocks). Profile often fully reflects rock composition.

Key properties.

• High stoniness, skeletal (>40% stones).
• Shallow depth (< 50 cm).
• Low water‑holding capacity.
• Reaction strongly depends on rock (acidic on granites, neutral on limestones).
• Low CEC, though may be high on basic rocks.
• Often have high humus content in the surface layer (in mountain conditions) (Naumov, 2016; Weil, 2017).

Limitations. Shallow, stony, poor moisture retention, erosion hazard. Unsuitable for mechanised tillage. On slopes – high landslide risk.

Agronomic value. Only for forestry and recreation. On some Leptosols (e.g., Rendzinas – Leptosols on limestones with high Ca content) there may be productive pastures (especially in the Alps and Caucasus). With terracing and fertilisation, they may be used for grapevines, olives (in the Mediterranean). But mainly – lands with low agricultural potential (Weil, 2017; Naumov, 2016).

4.4. Fluvisols

Distribution. Fluvisols are alluvial soils forming on river floodplains, deltas, floodplain meadows. They occur along major rivers of the world: Nile, Ganges, Amazon, Mississippi, Volga, Danube. These are among the most fertile soils, used by humans since ancient times (Buol et al., 2011; Weil, 2017). In the Russian classification – alluvial, floodplain soils.

Forming conditions. Climate – any (from arid to humid). The decisive factor – periodic flooding and deposition of sediment. The river brings fresh mineral material, which mixes with organic matter. Soil formation is constantly interrupted by new deposits (alluvium). Water regime – periodically leaching (during floods) and evaporative (in low‑water periods). Vegetation – floodplain meadows, forests (willows, poplars) (Naumov, 2016; Weil, 2017).

Typical profile. Profile weakly differentiated and stratified. Stratification is visible, due to different deposition regimes: coarser fractions (sand) are deposited in the channel part, finer (loam, clay) on the outer side of the floodplain. Often buried A‑horizons occur (with accumulation of new layers). Morphologically distinguished: A (often thick, with high humus content) – C (alluvium) – sometimes B (with signs of gleyisation). Saline and carbonate variants also occur (Naumov, 2016; Buol et al., 2011).

Main processes. Accumulation of organic matter (due to high productivity of meadow vegetation). Gleyisation (in lower horizons due to high groundwater table). Salinisation (in arid areas). Importantly, Fluvisols constantly receive “nutrient replenishment” from sediments, so they do not deplete as quickly as many other soils (Weil, 2017).

Key properties.

• High humus content (3–8%) in the upper horizon (A).
• Excellent permeability (due to loose structure).
• Neutral or slightly alkaline reaction (often with carbonates).
• High content of nutrients (N, P, K, Ca) from alluvium.
• Good supply of micronutrients.
• Often high productivity of natural plant communities (meadows) (Naumov, 2016).

Limitations. Flooding (risk of plant death during prolonged floods). Salinisation (in arid regions). Complex stratification may hinder tillage. Acidic variants occur (in taiga zone). With dam construction – loss of floodplain regime and fertility decline.

Agronomic value. Very high. Fluvisols are among the most fertile soils in the world. Civilisations arose on them since ancient times (Nile, Ganges, Yellow River). Used for growing cereals (wheat, rice), cotton, vegetables, fodder crops. In natural state – the best hayfields and pastures. With irrigation (where required) and proper agrotechniques, they give consistently high yields (Weil, 2017; Naumov, 2016). Their main enemy is over‑intensification and loss of floodplain regime due to hydro‑construction.

Summary for young and weakly developed soils.

Thus, in this group we see a variety of soils united by short soil‑formation time or its constant interruption:

• Cambisols – initial stages of soil formation (weakly developed Bw), widespread, have good potential with cultivation.
• Regosols – sandy, weakly developed, very poor, but can be used with irrigation.
• Leptosols – shallow, stony, on slopes, with low agricultural potential.
• Fluvisols – alluvial (floodplain) soils, very fertile, with high productivity, but with flood risk.

All these soils play an important role in agriculture and forestry. Knowledge of their properties allows choosing the right strategy for use and preventing degradation.

In the next part, we will move on to hydromorphic soils (Gleysols, Histosols) and specific soils (Vertisols, Andosols).

5.1. Gleysols – waterlogged mineral soils

Distribution. Gleysols occur everywhere – in river floodplains, along lake shores, in depressions, in areas with high groundwater table or under stagnant surface waterlogging. In Russia, they are widespread in forest, forest‑steppe, tundra, and taiga zones (Naumov, 2016). In world practice, they are often associated with Aquepts and Aquents in the American classification, as well as with gley and bog soils in the Russian one (Buol et al., 2011).

Forming conditions. The main condition – excessive moisture, which may be caused by:

• High groundwater table (groundwater gleyisation).
• Stagnation of surface water (atmospheric gleyisation, perched water).
• Proximity to water bodies or rivers (floodplain and lake‑side waterlogging).

Under such conditions, anaerobiosis (oxygen deficiency) develops in the soil. Microorganisms switch to anaerobic respiration, using not oxygen but Fe(III) and Mn(IV) compounds as electron acceptors. This leads to their reduction to Fe(II) and Mn(II), which become mobile and can be leached, causing characteristic grey‑blue and greenish‑grey colours (gleyisation) (Buol et al., 2011; Naumov, 2016). Vegetation – moisture‑loving (sedge, reed, willows, black alder).

Typical profile. The profile of Gleysols varies greatly, but in most cases the following are distinguished:

• Upper horizon (A) – may be humus, often with high organic matter content (up to 10–15% in meadow Gleysols).
• Gleyic horizon (G) – the main diagnostic feature. This is a horizon where Fe and Mn are reduced, colour grey‑blue, dirty grey, with rusty spots (Fe concretions) or without (in case of complete gleyisation). Structure often structureless or crumb‑curdy.
• Below – water‑bearing horizon (Cg) or parent material (Buol et al., 2011; Weil, 2017).

Main processes. Gleyisation – a complex of biochemical reactions leading to reduction of iron and manganese, destruction of clay minerals, accumulation of mobile compounds, and change in profile colour. Gleyisation can be surface (with stagnant atmospheric water) or groundwater (with shallow groundwater). Another important process – accumulation of organic matter (due to slowed decomposition). In some Gleysols, an illuvial horizon with accumulation of Fe, Mn, and organic matter forms (e.g., ortsteins – concretions of Fe/Mn oxides) (Naumov, 2016; Buol et al., 2011).

Key properties.

• Gleyic features: grey‑blue, steel, or greenish tones, rusty spots.
• High humus content (5–15%) in surface horizons (meadow Gleysols).
• Reaction may range from acidic (in taiga zone) to neutral and alkaline (in steppe zone, on carbonate rocks).
• High moisture and low aeration.
• CEC – medium or high (20–40 cmol_c/kg) due to high organic matter.
• Often high concentration of mobile forms of Fe and Mn.
• Low structural strength (when dry – very hard) (Naumov, 2016; Weil, 2017).

Limitations. The main limitation for agriculture – excess moisture (waterlogging, flooding) in natural state. Low aeration suppresses the growth of most crops. Upon drainage – compounds toxic to plants (Fe²⁺, Mn²⁺, H₂S) may appear. Also, in some Gleysols, salinisation and solonetzicity are possible (Naumov, 2016). Upon drainage of acidic Gleysols – formation of sulphuric acid (from sulphides) may occur.

Agronomic value. Gleysols are a very important resource provided drainage is installed. After creating drainage and regulating water regime, they become highly productive lands. Used for fodder crops (alfalfa, clover), cereals (rice, barley), vegetables. In river floodplains – the best haymaking lands (in meadow Gleysols). In tropics – for rice. Their value is high, but requires large investments in drainage and maintenance of optimal water level (Weil, 2017; Naumov, 2016).

5.2. Histosols – peat and muck soils

Distribution. Histosols are soils consisting predominantly of organic material (peat, sapropel). They occur in bogs, peatlands, river floodplains, lake terraces. The largest areas are concentrated in the boreal zone of the Northern Hemisphere (Canada, Russia, Scandinavia, northern USA), as well as in the tropics (Indonesia, Malaysia, Amazonia) (Buol et al., 2011; Weil, 2017). In the Russian classification – peat soils. In WRB – Histosols (including Fibric, Hemic, Sapric depending on degree of decomposition). Importantly, Histosols are soils without permafrost. In presence of permafrost, they are classified as Histels in the Gelisols group (Buol et al., 2011).

Forming conditions. The key condition – excess of organic matter accumulation over its decomposition. This is achieved under:

• Stagnant waterlogging (anaerobiosis).
• Low temperatures (slowed microbial activity).
• Acidic environment (suppression of microorganisms).
• High content of toxic compounds for microorganisms (phenols, tannins).

Vegetation – hydrophytes: sphagnum mosses, sedge, reed, cotton grass, willow, alder, pine (on raised bogs). The process of organic accumulation is called peat formation (Buol et al., 2011; Scheffer et al., 2018).

Typical profile. The profile of Histosols consists of layers of organic material of varying decomposition:

O (or T in Russian classification) – peat horizon, may be thick (up to several metres). By degree of decomposition, they distinguish:

• Fibric (weakly decomposed, fibrous) – plant structure preserved.
• Hemic (moderately decomposed, intermediate).
• Sapric (strongly decomposed, amorphous, mucky).

In the lower part of the profile – may be a gleyic horizon (G) or mineral (underlying). Also Limnic – lake deposits (sapropel) are distinguished (Buol et al., 2011; Weil, 2017). There are both low‑moor (eutrophic, rich) and high‑moor (oligotrophic, poor) bogs. Importantly, the profile of Histosols is often not differentiated in mineralogical composition, but varies greatly in peat decomposition degree.

Main processes. Peat accumulation – preservation of organic matter under anaerobic conditions. The process includes:

• Accumulation of plant residues.
• Weak decomposition (mainly anaerobic) with formation of humic substances (humic and fulvic acids).
• Formation of specific organic compounds (bitumens, waxes, resins).
• Acidification (during sphagnum decomposition).

In some cases (under drainage), the process of mineralisation is activated – intensive decomposition of peat with release of CO₂ (Buol et al., 2011; Weil, 2017).

Key properties.

• Very high porosity (70–90% of volume).
• Very low density (0.1–0.4 g/cm³).
• Acidic reaction (pH 3.5–5.5) in raised bogs, neutral or slightly alkaline in low‑moor bogs.
• High water‑holding capacity (up to 300–400% of dry matter mass).
• High CEC (100–200 cmol_c/kg) due to organic matter.
• Low ash content (2–10%) in raised peats, high (up to 20–50%) in low‑moor.
• N content – 1–3%, C/N – wide (20–60), indicating slow mineralisation (Buol et al., 2011; Weil, 2017).

Limitations. Upon drainage (for agriculture) – rapid mineralisation (loss of organic matter) with release of CO₂, leading to soil degradation. Subsidence and settling of the surface (up to 2–5 cm per year) (Buol et al., 2011; Naumov, 2016). High fire hazard. Poor bearing capacity (low density) – construction difficult. Toxic compounds (phenols, acids) may be released upon drainage.

Agronomic value. Histosols of low‑moor bogs (eutrophic) – highly productive soils when drained. Used for vegetables (cabbage, carrots), potatoes, fodder grasses. In tropics – for rice, sugarcane. Raised bogs (oligotrophic) – low productivity, used mainly as peat sources (for fuel, bedding, fertiliser). Histosols are important carbon stores (they contain huge amounts of organic carbon). Their role in global climate – significant (when drained, become sources of CO₂) (Buol et al., 2011; Weil, 2017).

Summary for hydromorphic soils.

Hydromorphic soils (Gleysols, Histosols) are soils whose essence is determined by excessive moisture. Gleysols – mineral soils with gleyisation, often highly productive when drained, but with risks of toxicity (Fe, Mn) and salinisation. Histosols – organic soils (peatlands), with low density, high moisture capacity, but very sensitive to drainage (mineralisation, subsidence). Their use requires a fine balance between the need for drainage (for agriculture) and preservation of unique properties (to prevent degradation and carbon emissions).

The study of hydromorphic soils is extremely important for designing drainage systems, protecting wetlands, and understanding the role of soils in the global carbon cycle.

In the next, final part of the lecture, we will move on to specific soils that are distinguished not so much by climatic or hydrological factors, but by special properties – Vertisols (clayey, cracking) and Andosols (volcanic, with amorphous minerals).

6.1. Andosols – volcanic soils

Distribution. Andosols are confined to areas of active or recent volcanism. Their ranges are the “Ring of Fire” of the Pacific Ocean (Japan, Indonesia, Philippines, New Zealand, western USA, Kamchatka, Kuril Islands), as well as rift volcanic zones (Iceland, East Africa, Canary Islands). In Russia – on Kamchatka, Kurils, some areas of the Far East (Naumov, 2016; Buol et al., 2011). They occupy no more than 1% of the land surface, but due to their exceptional fertility, they sustain the lives of almost 10% of the world’s population (Ping, 2000; McDaniel et al., 2011).

Forming conditions. Andosols form from volcanic ashes, pumice, tuffs, and other pyroclastic materials. The key feature – high reactivity of volcanic glass. Under the action of heat and moisture (often in humid climates), the glass weathers rapidly, but not to crystalline clay minerals, but to amorphous and poorly crystalline products – allophane, imogolite, ferrihydrite, and metal‑organic complexes. This process is called andosolisation (or andic soil formation) (Shoji et al., 1993; McDaniel et al., 2011). Importantly, the age of Andosols is usually small (Holocene, late Pleistocene), because over time amorphous minerals recrystallise into kaolinite and halloysite, and the soil loses its andic properties.

Typical profile. The profile of Andosols is often multilayered (due to repeated eruptions). Characteristic are dark (often black) melanic epipedons – thick (up to 30–60 cm) humus horizons with organic carbon content > 6% (Soil Survey Staff, 2010). Below – light ash layers or brown ochric horizons (Bw). Both layered and continuous profiles occur. Important features – very low density (0.5–0.9 g/cm³) and loose structure (Shoji et al., 1993; McDaniel et al., 2011). The profile is often composed of several buried soils.

Main processes. Andosolisation – a set of processes including:

• Rapid weathering of volcanic glass.
• Formation of allophane (amorphous aluminosilicate) and imogolite (fibrous).
• Accumulation of metal‑organic complexes – especially under acidic conditions (pH < 5), when aluminium binds with humic acids rather than with silicon (forming non‑allophanic Andosols).
• Accumulation of humus – very high, since aluminium and iron suppress microbial activity, slowing decomposition. Humus is stabilised in complexes with Al and Fe (Shoji et al., 1993; Dahlgren et al., 2004).
• Illuvial processes (translocation) are weakly expressed in Andosols – all products accumulate in situ.

Key properties.

• Low density, high porosity (up to 70–80% of volume).
• Very high water‑holding capacity (retain 2–3 times more water than their own mass).
• High organic carbon content (5–15% and more).
• Acidic reaction (pH 4.5–6.0) in most cases.
• High phosphorus fixation (Phosphate Retention > 85%) – the main chemical limitation.
• High CEC, but pH‑dependent (most charge is variable).
• Thixotropy – upon shaking (or loading) they can liquefy and flow, then re‑solidify (causing engineering problems).
• Tendency to crust formation upon drying (due to colloidal nature) (Shoji et al., 1993; Dahlgren et al., 2004; McDaniel et al., 2011).

Limitations. The main agronomic limitation – high phosphorus fixation, which makes phosphorus fertilisers inefficient. Special application methods are required (localised, in granules, combined with organic matter). Also – high acidity (requires liming) and low available silicon (for rice). Thixotropy and low bearing capacity create problems for construction. In dry weather – easily subject to wind erosion (Ping, 2000; McDaniel et al., 2011).

Agronomic value. Despite limitations, Andosols are among the most fertile soils in the world. They give high yields of rice (in Asia), tea, coffee, vegetables, fruits, and are the basis for intensive horticulture and plantation agriculture (Ping, 2000; Shoji et al., 1993). Their unique physical properties (good aeration, high moisture capacity) make them ideal for root crops. With proper management (organic matter, pH regulation, special P application methods) – they ensure stable food production in densely populated volcanic regions of the world.

6.2. Vertisols – clayey, cracking soils

Distribution. Vertisols occur in all warm regions of the world, but the largest areas are concentrated in India (about 80 million ha), Australia (about 70 million ha), Sudan (about 50 million ha), as well as in the USA, Ethiopia, China, northern South America. In Russia, small areas are found in the Krasnodar region, the Caspian lowland, and the south of Western Siberia (Naumov, 2016; Buol et al., 2011). Vertisols occupy about 2.4% of the land surface, but due to their specificity, they play an important role in agriculture.

Forming conditions. For the formation of Vertisols, a combination of three factors is necessary:

1. High content of 2:1 clay minerals (especially smectites and montmorillonite) – capable of great swelling when wet and shrinking when dry.

2. Seasonal moisture contrast – a pronounced dry and wet season (or periods). This ensures cycles of “swelling – shrinking”.

3. Neutral or slightly alkaline reaction, which promotes the preservation of smectites (acidic environment destroys them) (Buol et al., 2011; Coulombe et al., 1996).

Parent materials – often basic (basalts, diabases), as well as calcium‑ and magnesium‑rich clayey sediments (alluvium, marine clays). Relief – usually flat or gently undulating (depressions, floodplains) to allow water to pond and ensure deep wetting.

Typical profile. The profile of Vertisols is deep (often > 1 m), uniform in colour – dark grey, black, or brown. Characteristic:

• Deep network of cracks (in dry season) – they can reach 5–10 cm width and 1–2 m depth.
• Slickensides (polished, shiny, inclined slip surfaces) in the B‑horizon – result of soil mass displacement upon swelling.
• Wedge‑shaped structure (especially in the middle part of the profile) – aggregates are wedge‑shaped, inclined at 10–60°.
• Gilgai – micro‑relief of alternating mounds and depressions (not obligatory, but very characteristic) (Buol et al., 2011; Wilding and Tessier, 1988).

Main processes. Vertisol process (or argillipedoturbation) – cyclic self‑mixing of soil due to swelling and shrinking. In dry season, soil cracks, and surface material falls into cracks. Upon wetting, clay swells, volume increases, and the soil is “squeezed” upward and sideways. This process:

• Destroys horizons (makes profile uniform).
• Forms polished surfaces (slickensides).
• Leads to formation of micro‑relief (gilgai) (Wilding and Tessier, 1988; Coulombe et al., 1996).

Key properties.

• High clay content (> 30–40%, often up to 60–80%).
• Dominance of smectites (montmorillonite group) – they provide a high coefficient of linear extensibility (COLE > 0.06).
• Neutral or slightly alkaline reaction (pH 7.0–8.5).
• High CEC (30–60 cmol_c/kg) due to smectites.
• High humus content (1–3% in upper horizon, but often less than 1% due to intensive mixing).
• Extremely unstable physical properties: in dry state – hard, cracked; in wet – plastic, sticky, almost impassable.
• Very low permeability in wet state (due to swelling and crack closure) (Buol et al., 2011; Coulombe et al., 2000).

Limitations. The main limitation for agriculture – very narrow tillage window. Soil can be tilled only at certain moisture content (usually immediately after drying after rain, but before it dries into cracks). Otherwise:

• Too wet – sticks to implements, clods.
• Too dry – requires enormous effort for loosening.
• Cracks also create problems for roots (tearing) and for construction (foundations destroyed). Low infiltration leads to surface runoff and erosion, especially on slopes. Salinisation** may be aggravated by irrigation if drainage is poor (Coulombe et al., 2000; Naumov, 2016).

Agronomic value. Despite difficulties, Vertisols are potentially very fertile soils. They are rich in nutrients (Ca, Mg, K, P, N), retain moisture well in the off‑season (due to deep cracks through which water quickly penetrates deep). In India and Sudan, they are widely used for cotton, sorghum, wheat, rice (with irrigation). In Australia – for wheat and pastures. With proper management (minimum tillage, sowing into cracks, mulching, gypsum application to improve structure), Vertisols can give high and stable yields (Coulombe et al., 2000; Blokhuis, 2006). The main thing is to use their specific properties, not fight them.

Summary for specific soils.

Specific soils – Andosols and Vertisols – illustrate how strongly substrate properties and moisture dynamics can influence soil formation. Andosols – soils of volcanic origin with unique mineralogical composition (allophane, imogolite, metal‑organic complexes), which provide high fertility provided phosphorus fixation is managed. Vertisols – clayey soils with active self‑mixing, requiring special agrotechniques, but with the right approach they give good yields. Both soil types are of great importance for food security in their regions of distribution.