Soil Geography

Subject and Tasks of Soil Geography

The subject of soil geography is the soil cover (or pedisphere) — the totality of all soils covering the Earth's surface. Soil geography does not study individual soils or their profiles but rather the patterns of their spatial distribution and the reasons determining this distribution.

The main tasks of soil geography are:

1. Identifying patterns in the distribution of various soil types across the Earth's surface.

2. Explaining the reasons for the spatial differentiation of the soil cover.

3. Establishing relationships between soils and soil-forming factors.

4. Zoning territories based on their soil cover.

5. Predicting the nature of soils in unexplored territories.

The Fundamental Formula of Soil Formation

The geographical approach to studying soils is based on the concept that soil is a natural-historical body formed through the interaction of several factors. This was first formulated by V.V. Dokuchaev in the late 19th century and later developed in the classic formula by the American scientist Hans Jenny (Jenny, 1941):

Soil = f(climate, organisms, relief, parent material, time)

Or in abbreviated notation (Jenny, 1941; White, 2006):

Where:

• cl (climate) — climate (primarily temperature and moisture)
• o (organisms) — organisms (vegetation, animals, microorganisms)
• r (relief) — relief (landscape position, slope steepness, and aspect)
• p (parent material) — soil-forming parent rock
• t (time) — duration of soil formation

This is the 'key formula' of soil geography. Each of these factors acts not in isolation but in close interaction with the others. The task of the soil geographer is to understand exactly how the combination of these factors in different parts of the Earth creates the diversity of soil types (Buol et al., 2011; Weil & Brady, 2017).

Soil Geography and Related Sciences

Soil geography occupies an intermediate position between soil science proper and physical geography. It is closely related to:

• Climatology — since climate determines the direction and intensity of soil-forming processes.
• Geobotany — since vegetation is the most important source of organic matter for soils.
• Geomorphology — since relief determines the redistribution of moisture and material on the surface.
• Hydrology — since the water regime is one of the leading factors of soil formation.
• Geology — since parent rocks determine the mineralogical and chemical composition of soils.

Research Methods

Soil geography uses two groups of methods:

Field methods:

• Route surveys
• Digging soil pits and auger holes
• Describing soil morphological features
• Soil mapping

Laboratory and analytical methods:

• Laboratory analyses of soil physicochemical properties
• Comparative geographical analysis
• Constructing soil maps and profiles
• Statistical data processing

The most important methodological principle is the comparative geographical method, which requires studying soil not in isolation but in connection with the entire set of conditions of its formation (Naumov, 2016). This method is what allows us to answer the main question of soil geography: "why did this particular soil arise here?"

Practical Significance

Knowledge of geographical patterns of soil distribution has enormous practical significance:

• For agriculture — rational placement of crops, selection of farming systems and reclamation measures.
• For forestry — selection of tree species and forest management regimes.
• For land management — rational use of the land fund.
• For environmental protection — assessing soil resistance to anthropogenic impacts.

As V.V. Dokuchaev aptly put it, soil is the "mirror of the landscape" (Naumov, 2016). Soil geography teaches us to read this mirror, understanding how natural conditions are reflected in soil properties.

Key Concepts We Will Use

Before moving on to a detailed examination of soil-forming factors and patterns of soil distribution, let's introduce a few basic terms:

Soil profile — the vertical section of soil from the surface to the parent material, consisting of genetic horizons (Weil & Brady, 2017; White, 2006).

Pedon — a three-dimensional "unit" of soil, the smallest volume that can be called soil (area approximately from 1 to 10 m²) (Weil & Brady, 2017).

Soil cover — the continuous shell of the Earth formed by the totality of all soils (Naumov, 2016).

Catena — a regular sequence of soils on a slope from the watershed to the valley (Wysocki et al., 2012; White, 2006).

Bioclimatic belt — a large regional unit of the soil cover, distinguished by the characteristics of climate and vegetation (Naumov, 2016).

Soil as a "Child" of the Environment

To understand why different soils arise in different places on Earth, we must envision soil formation not as an isolated process but as part of an integral natural system. Soil forms at the interface of four of Earth's "spheres": the lithosphere (rock), atmosphere (climate and air), hydrosphere (water), and biosphere (living organisms). This concept, first clearly formulated by V.V. Dokuchaev, remains the methodological foundation of soil geography (Naumov, 2016; Buol et al., 2011).

The interaction of these spheres in a limited area of the Earth's surface creates that unique environment we call the soil-forming environment. Its components — climate, vegetation, fauna, relief, parent rock, and groundwater — act not separately but in a complex interplay, forming a unified functional complex (Wysocki et al., 2012). A change in any element of this complex inevitably "echoes" in the soil. This is why the same rock yields completely different soils under different climatic conditions, and a change in vegetation on one slope leads to a transformation of the soil profile.

Let's consider each factor sequentially, and then how they interact.

2.1. Climate

Climate is the most important soil-forming factor, as it determines the energy (heat) and water regime (moisture) under which all processes occur (Weil & Brady, 2017; White, 2006). The two main climatic parameters — temperature and the amount and distribution of precipitation — control the rate of chemical reactions, biological activity, and the direction of substance migration.

2.2. Organisms (Vegetation, Animals, Microorganisms)

Plants are the main source of organic matter in the soil, while animals and microorganisms are its processors. Organisms not only supply carbon but also actively alter the soil's chemical environment by releasing acids, bases, and complexing agents (Weil & Brady, 2017; White, 2006).

2.3. Relief

Relief redistributes water, heat, and solid material, creating local conditions that can differ sharply from the regional climate (Wysocki et al., 2012; White, 2006). The main relief elements influencing soil formation are:

• Slope aspect (north–south, east–west). In the Northern Hemisphere, south-facing slopes receive more solar radiation; they are warmer and drier. This affects the composition of vegetation, the intensity of weathering, and the type of humus. South-facing slopes in the temperate zone more often feature steppes or steppe-like meadows, while north-facing ones feature forests (Buol et al., 2011; Foth, 1990).

• Slope steepness. Steep slopes increase surface runoff, decrease infiltration, and increase the risk of erosion. Soils here are often shallow, poorly developed, with a truncated profile. On gentle slopes and in depressions, conversely, moisture and fine earth accumulate, and soils can be thicker and richer (Wysocki et al., 2012).

• Slope shape (convex, concave, straight). Convex slopes lose moisture and material; concave slopes accumulate them. Therefore, concave landscape positions often feature hydromorphic or semi-hydromorphic soils.

• Altitude above sea level determines climate change (temperature decrease, precipitation change) and the succession of vegetation belts — altitudinal zonation of soils (Naumov, 2016).

Relief not only passively "transmits" climatic conditions but also actively forms catenas — regular sequences of soils from the watershed to the valley. We will discuss this separately in Section 5.

2.4. Parent Material

Parent material is the mineral basis of the soil. Its properties determine:

• Granulometric composition (sand, loam, clay). This affects water permeability, water-holding capacity, aeration, and thermal regime. Sandy rocks yield light, well-drained but nutrient-poor soils; clayey rocks yield heavy, colloid-rich, and often more fertile soils (Buol et al., 2011; Weil & Brady, 2017).

• Mineralogical composition. The composition of primary minerals determines the set of elements that can be released upon weathering. For example, granites, rich in quartz and feldspars, yield soils dominated by quartz sand and potassium-bearing clays. Basalts and gabbros (basic rocks) are rich in iron, magnesium, and calcium, yielding fertile, clayey, dark-colored soils (Buol et al., 2011; White, 2006).

• Chemical composition (presence of carbonates, gypsum, readily soluble salts). Carbonate rocks (limestones, dolomites) neutralize acids, promote the accumulation of humate-type humus, and form neutral or slightly alkaline soils (rendzinas, sod-carbonate soils). Saline rocks can produce solonchaks and solonetz even under relatively humid climates (Scheffer et al., 2018; Naumov, 2016).

• Age and degree of rock weathering. If the rock had already undergone weathering before becoming soil-forming material (e.g., reworked sediments), its initial composition may be depleted in primary minerals. This is typical for many ancient continental deposits in the tropics (Brazilian Plateau, Central Africa), where soils form on intensely weathered, depleted material (Buol et al., 2011).

Parent material has its strongest influence on young soils; over time, its influence gradually weakens, and climatic and biological factors gain importance. However, even in ancient soils, the parent rock continues to affect trace element composition and some physical properties.

2.5. Groundwater

Although groundwater is often included in the hydrological factor, its role in soil formation is so significant that it deserves separate mention. Groundwater affects the soil in two main ways (Wysocki et al., 2012; Foth, 1990):

1. Capillary rise — when groundwater is shallow, its dissolved salts can rise to the surface and accumulate in the soil (formation of solonchaks, solonetz). At the same time, capillary feeding can supply plants with moisture during the dry season.

2. Influence on aeration — a high groundwater table creates anaerobic conditions, leading to gleyzation, reduction of iron and manganese, and the formation of gleyic horizons.

Groundwater is closely linked to relief: in depressions, it lies closer to the surface, hence hydromorphic soils are characteristic of low landscape elements. Simultaneously, the chemical composition of groundwater is determined by parent rocks and climate (leaching intensity).

2.6. Time

Time is a factor that does not act on its own but determines the degree of manifestation of all other factors (Weil & Brady, 2017; White, 2006). The same combination of climate, vegetation, and rock, lasting for decades, yields only a primitive, poorly developed soil (Entisol). If it continues for thousands or millions of years, the soil can become deeply differentiated, highly weathered, and even polygenetic (i.e., reflecting several phases of climatic change).

In soil geography, the absolute age of the soil (in years) is distinguished from its relative age (degree of development). Soils on young geomorphic surfaces (river terraces, moraines, lava flows) are usually younger and less developed. On ancient surfaces (e.g., on the plateaus of Brazil or Africa), soils can be hundreds of thousands to millions of years old, and their profiles are often significantly altered.

Time interacts with other factors in complex ways. For instance, on a steep slope, erosion constantly removes the upper layers, so the soil there can remain "eternally young," despite the great geological age of the surface itself (Buol et al., 2011). This brings us to understand that soil formation is not a linear process but a dynamic equilibrium, where the soil can maintain its properties for a long time if the rate of soil formation equals the rate of destruction (White, 2006).

2.7. Interrelationship of Factors: The Soil-Forming Environment as a System

We have considered each factor separately, but in nature they never act in isolation (Wysocki et al., 2012; Weil & Brady, 2017). Moreover, there are feedback loops between them: soil, as it forms, begins to influence vegetation and the hydrological regime, which in turn alters further soil formation.

Let's look at a few examples of systemic interactions:

1. Climate → vegetation → soil → climate (local). A humid climate promotes forest growth; the forest creates a litter layer that shades the soil, reduces evaporation, and alters the water regime, potentially enhancing leaching and podzolization. In turn, the podzolic soil with an acidic reaction can hinder the development of broad-leaved species, reinforcing the coniferous forest.

2. Relief → redistribution of moisture → vegetation → soil. On a south-facing slope, there is more heat and less moisture, so more xerophytic vegetation (steppe) settles there, producing less litter but richer in roots. On the north-facing slope, there is forest with a thick litter layer. The soils on the two slopes will be completely different, despite identical climate and parent rock (Buol et al., 2011; Foth, 1990).

3. Groundwater ↔ vegetation ↔ soil. When mineralized groundwater is shallow, vegetation becomes halophytic, increasing salt input into the upper horizons; this, in turn, enhances salinization and forms solonetz.

4. Time and climate change. Since climate has changed throughout geological history, many modern soils are polygenetic — they bear traces of processes that occurred under conditions different from today's. For example, in the deserts of the Southwestern United States, relict clayey horizons formed during the moist Pleistocene period are found (Buol et al., 2011; Weil & Brady, 2017).

This very interconnectedness and historical variability makes soil geography both a complex and fascinating science. By understanding the system, we can predict which soil will arise on a new site given a known combination of factors.

3.1. Concept of a Soil-Bioclimatic Belt

We have already seen that climate and vegetation are the leading factors determining the appearance of soils over large areas. This was first observed by V.V. Dokuchaev during his famous travels across Russia: he noticed that the change in natural zones from north to south is accompanied by a regular change in soils. Thus, the doctrine of soil zonality was born (Naumov, 2016; Weil & Brady, 2017).

In modern soil geography, soil-bioclimatic belts are distinguished — the largest regional subdivisions of the soil cover, characterized by similar heat and moisture regimes and, consequently, by similar types of soil-forming processes and vegetation (Naumov, 2016). These belts generally have a latitudinal extent but are interrupted by mountain systems and oceanic influences.

The main criterion for distinguishing belts is the sum of active temperatures (>10 °C) and the moisture coefficient (ratio of precipitation to evapotranspiration). However, for soil geography, it is equally important which processes dominate in each belt: humus accumulation, podzolization, lessivage, gleyzation, salinization, ferrallitization, etc. (Buol et al., 2011; Scheffer et al., 2018).

3.2. Main Soil-Bioclimatic Belts of the Earth

It is customary to distinguish several large belts that successively replace each other from the poles to the equator. Let's examine them in general terms, emphasizing the connection between climate, vegetation, and soil-forming processes.

3.3. Soil-Bioclimatic Zones of Russia

The territory of Russia encompasses three main soil-bioclimatic belts (Naumov, 2016):

1. Polar belt — Arctic and tundra soils (Eurasian polar region).

2. Boreal belt — taiga-forest soils (European-West Siberian, East Siberian, and Far Eastern taiga-forest regions).

3. Subboreal belt — forest-steppe, steppe, dry-steppe soils (deciduous forest and forest-steppe region; steppe and dry-steppe region; semi-desert and desert region).

Within each belt, regions and zones are distinguished, corresponding to latitudinal zonality (Naumov, 2016). For example:

• In the boreal belt, northern, middle, and southern taiga zones are distinguished.
• In the subboreal belt — zones of gray forest soils, chernozems (various subtypes), chestnut soils, brown semi-desert, and gray-brown desert soils.

In addition, mountainous regions are distinguished on the territory of Russia (Caucasus, Urals, Altai, Sayans, Kamchatka, etc.), where soil zonality has a vertical character — belts change with elevation (Naumov, 2016).

3.4. Limitations of the Zonal Concept

Although the zonal approach provides a good first approximation, the real soil cover is always more complex (Weil & Brady, 2017; White, 2006). There are:

• Intrazonal soils — formed under the influence of local factors that disrupt zonality (e.g., solonetz, solonchaks, bog soils, alluvial soils).
• Azonal soils — poorly developed soils on young surfaces (river alluvium, volcanic ash, bedrock outcrops) where zonal processes have not yet had time to manifest.
• Relict (polygenetic) soils — bearing features of past climatic epochs and not fully corresponding to modern conditions.

Furthermore, oceanic influences, monsoon regimes, and mountain barriers create significant distortions of latitudinal zonality. For example, in the Russian Far East, due to the monsoon climate, specific volcanic and brown-taiga soils form, which differ from their West Siberian counterparts (Naumov, 2016).

Thus, zonality is a general tendency, not a rigid law. It sets the general background upon which local factors create a rich mosaic of soils. In the following sections, we will examine how relief and hydrology form local patterns (catenas) and how time and changing conditions create chrono-, litho-, and climosequences.

Question for Reflection

Why do chernozems not form in the taiga zone, despite the presence of herbaceous vegetation in clearings? Which factor (or combination of factors) is decisive? The answer lies in the balance of heat and moisture, as well as the nature of the biological cycle — we will return to this when discussing soil sequences.

4.1. Forest Ecosystems

Forest is the most widespread soil-forming environment in temperate and tropical latitudes. A characteristic feature of forest soils is the supply of the bulk of organic matter to the surface (leaf or needle litter) and, consequently, the formation of an organic horizon (litter) above the mineral mass (Weil & Brady, 2017; White, 2006).

Forest ecosystems are subdivided into several major types, each yielding its own soils:

4.2. Herbaceous (Steppe and Savanna) Ecosystems

In these ecosystems, the bulk of organic matter enters the soil via roots, which die off annually and decompose directly within the mineral horizon. This contributes to the formation of a thick, dark-colored, well-structured humus horizon (Weil & Brady, 2017; White, 2006).

4.3. Tundra Ecosystems

• Vegetation: mosses, lichens, dwarf shrubs, low-growing grasses; treeless.
• Climate: cold, short summer, permafrost (cryolithozone). Little precipitation, but evapotranspiration is extremely low, hence excessively moist regime.
• Main processes: sharply slowed decomposition of organic matter (accumulation of coarse humus and peat); gleyzation due to permafrost water barrier; cryoturbation (frost mixing); weak chemical weathering.
• Main soils: Tundra Gleyic (Gleysols), Cryosols, Peat-Gleyic, Bog (Naumov, 2016; Weil & Brady, 2017).

4.4. Desert and Semi-Desert Ecosystems

• Vegetation: extremely sparse, xerophytic shrubs, wormwood, saltworts, ephemerals.
• Climate: very dry, precipitation < 250 mm/year, evapotranspiration greatly exceeds precipitation (effusive regime).
• Main processes: salt accumulation and carbonatization. Chemical weathering is weak, physical (thermal, wind) dominates. Organic matter hardly accumulates. Characterized by the presence of carbonate, gypsum, and salt horizons, often cemented (calcretes, gypsicretes, salcretes) (Weil & Brady, 2017; Scheffer et al., 2018).
• Main soils: Gray-Brown Desert, Brown Semi-Desert, Solonchaks, Takyr-like soils; on sands — Arenosols (Naumov, 2016).

4.5. Bog (Hydromorphic) Ecosystems

Bogs are ecosystems with constant or prolonged excessive moisture, leading to anaerobiosis and peat accumulation (Foth, 1990; Weil & Brady, 2017).

• Raised bogs (oligotrophic): fed only by atmospheric precipitation, vegetation — sphagnum mosses, cotton grass, cranberry. Peat is poor in bases, acidic, poorly decomposed.
• Fen bogs (eutrophic): fed by groundwater rich in mineral salts, vegetation — sedges, reeds, cattail, alder. Peat is more decomposed, ash-rich, near-neutral.
• Transitional bogs (mesotrophic): intermediate type.

Main soils: Peat (oligotrophic, eutrophic, transitional) and Peat-Gleyic (Naumov, 2016; Scheffer et al., 2018).

4.6. Floodplain (Alluvial) Ecosystems

River floodplains are dynamic landscapes where soil formation is combined with periodic alluvium deposition (river sediments) (Naumov, 2016; Foth, 1990).

• Vegetation: floodplain meadows, willow thickets, alder forests, floodplain forests (oak, elm, poplar).
• Hydrology: periodic floods (high water), bringing suspended material (silt, sand, organic matter) and creating conditions for the sod process and alluvial soil formation.
• Main processes: annual renewal of the surface by a layer of alluvium, active biological cycle, often — proximity of groundwater (gleyzation in the lower profile part).
• Main soils: Alluvial Sod (on riverine levees, sandy), Alluvial Meadow (in the central floodplain, rich in humus), Alluvial Meadow-Bog and Bog (in the terraced floodplain, with excessive moisture) (Naumov, 2016).

4.7. Mountain Ecosystems

In mountains, soil formation follows the law of altitudinal zonation — the change of natural zones with altitude (Naumov, 2016; White, 2006). Characteristic features of mountain soils:

• Shallow profile due to erosion and limited weathering time (slopes are constantly "rejuvenated").
• High stoniness (large content of rock fragments).
• Strong dependence on slope aspect (north–south, windward–leeward).
• Frequent manifestation of cryogenic processes in high belts.

In mountain systems, specific mountain soils are distinguished, having no analogues on plains: mountain-meadow, mountain-meadow-steppe, mountain-tundra, mountain-forest (brown and podzolic) (Naumov, 2016). For example, in the Caucasus and Altai, belts are distinguished:

• Nival (soils almost absent, bare rocks, glaciers);
• Mountain-tundra (primitive soils, cryosols);
• Mountain-meadow (alpine and subalpine meadow soils);
• Mountain-forest (brown forest, podzolic, sod-podzolic);
• Mountain-steppe (chernozems, chestnut), etc.

4.8. Volcanic Territories (Ash Soils)

Volcanic eruptions create a special type of soil-forming parent rock — volcanic ashes and tuffs (Weil & Brady, 2017; Scheffer et al., 2018).

• Parent rock: volcanic glass, pyroclastic material rich in amorphous phases.
• Rapid weathering: ashes are chemically active; upon weathering, allophane and imogolite form — minerals with a large specific surface area and anion exchange capacity.
• Soils: Andosols (in the US — Andisols). They are characterized by: low density, high water-holding capacity, intense humus accumulation (aluminum-humus complexes form), high phosphate-fixing capacity.
• Additional feature: periodic input of fresh ash on the surface creates stratified (layered) soils, where ash layers and humus horizons alternate. This is especially characteristic of Kamchatka, the Kuril Islands, Japan, and Indonesia (Naumov, 2016; Buol et al., 2011).

4.9. Soils of Anthropogenically Modified Environments

Although this is a separate large topic, it should be mentioned that in recent centuries, significant areas have been occupied by agrocenoses and urbanized territories. Here, soil formation proceeds under strong human influence: tillage, fertilization, reclamation, compaction, pollution, soil movement. Such soils are classified as anthropogenic or technogenic (in international classifications — Anthrosols and Technosols) (Scheffer et al., 2018). Their properties often differ significantly from zonal analogues.

4.10. Summary Table of Soil-Forming Environments

For convenience, let's summarize the main characteristics in a table (based on Naumov, 2016; Buol et al., 2011; Weil & Brady, 2017).

Summary for Section 4

Each soil-forming environment is a stable combination of climatic, biotic, and hydrological conditions that sets the dominant type of soil-forming process. However, within one environment, there can be significant variations due to differences in parent rock, relief, and age. These variations, and their regular combinations in space, are the subject of the next section — on catenas and soil sequences.

5.1. Definition and Historical Background

Catena (from Latin catena — "chain," "link") is a sequence of soils that regularly change from the top of a hill (watershed) to the foot of the slope and further to the valley bottom, given homogeneous parent material and similar climatic conditions (White, 2006; Wysocki et al., 2012).

The term was introduced by the English soil scientist George Milne in 1935 while working in East Africa (present-day Tanzania). Milne noticed that moving from the hilltop to the foot, soils change with a certain regularity, forming like "links of one chain." He distinguished two types of catenas:

1. Catena on homogeneous parent rock — soil differences are related only to the redistribution of moisture, erosion, and accumulation.

2. Catena on several parent rocks — the factor of changing parent rock along the slope is added (characteristic of dissected relief) (Milne, 1935; cited in White, 2006; Wysocki et al., 2012).

Subsequently, the concept of the catena was refined and expanded. In the American school of soil science, a catena is more often understood as a toposequence — a series of soils where relief (slope position) is the leading differentiating factor, while all other factors (climate, parent rock, vegetation, time) are maximally equalized (Buol et al., 2011). However, in the modern understanding, a catena is not just a series of soils but a geochemically coupled system, where substances and water move from upper to lower elements, creating exchange links (Wysocki et al., 2012).

5.2. Elements of a Catena

A typical catena under temperate hilly relief includes the following positions (Buol et al., 2011; Wysocki et al., 2012; White, 2006):

• Watershed (flat summit, interfluve) — the most level, maximally drained part. Here, precipitation only infiltrates (no lateral inflow). Soils are usually the most developed, with a deep profile, reflecting the regional climate and vegetation. It is on the summits that "zonal" soils (e.g., Chernozem, Podzol, Ferralsol) typically form.

• Upper slope (shoulder) — a transitional zone where runoff begins and erosion somewhat intensifies. The soil profile may be truncated, often with signs of degradation of the upper horizons.

• Middle slope (backslope) — the steepest section. Here, erosion dominates, with the removal of fine earth and organic matter; soils are shallow, often skeletal, poorly developed, with exposure of lower horizons or even bedrock.

• Lower slope (footslope) — an accumulation zone. Here, particles eroded from above (colluvium) accumulate; moisture increases due to lateral water inflow. Soils are thicker, with higher humus content, often waterlogged.

• Valley bottom or depression (toeslope) — the lowest position, where water from the entire slope accumulates. Here, groundwater moisture (if there is an aquiclude) and even water stagnation can form. Hydromorphic soils (Gleyic, Meadow, Bog) with characteristic signs of gleyzation and anaerobiosis form (Wysocki et al., 2012; White, 2006).

The nomenclature of catena elements varies across classifications, but the general idea remains: from top to bottom, moisture and material accumulation increase, and the thickness and degree of profile development change non-monotonically (maximum on the summit and in the lower part, minimum on the middle slope).

5.3. Factors Determining Soil Differentiation in a Catena

A catena results from several interrelated processes:

1. Redistribution of moisture. On a slope, part of the precipitation does not have time to infiltrate and flows downhill. Therefore, the lower elements of the catena receive additional moisture from runoff, while the upper ones lose it. This affects leaching, the degree of weathering, and the type of vegetation. In arid regions, it is at the bottom of the slope that meadow or even forest patches may persist among the dry steppe.

2. Redistribution of solid material (erosion and accumulation). Erosion from steep slopes exposes less weathered horizons, "rejuvenating" the soil. At the foot of the slope, colluvium accumulates — a mixture of humus material and mineral particles, yielding thicker, often more fertile soils.

3. Geochemical migration. Water moving along the slope, both on the surface and within the soil, transports dissolved substances. For example, iron and manganese compounds can be leached from the upper part of the slope and deposited in the lower part, forming iron concretions or even ortsteins (Foth, 1990; White, 2006). Carbonates and readily soluble salts behave similarly in arid conditions.

4. Vegetation often changes along the catena in response to changes in moisture and nutrient availability. This creates an additional feedback loop: wetter lower areas may overgrow with forest or dense grass, enhancing humus accumulation.

5.4. Examples of Catenas

5.5. The Catena as a Geochemical System

It is important to understand that a catena is an open system (Wysocki et al., 2012). Material removed from the upper part can accumulate in the lower part or even leave the catena's boundaries, entering the groundwater flow or river network. In this sense, a distinction is made (White, 2006; Wysocki et al., 2012):

• Open catenas — runoff and erosion remove part of the material beyond the slope in question (e.g., into a gully or river valley). Soils in the upper part are impoverished; the lower part is only partially enriched.
• Closed catenas — material washed from the summit remains entirely within the depression (hollow, lake basin, endorheic basin). In this case, soils in the lower position accumulate all the material and become particularly thick and rich in organic matter and nutrients.

This distinction explains why soils in closed depressions have very high humus content, whereas on open slopes such accumulation does not occur.

5.6. Significance of Catenas for Soil Geography and Practice

Studying catenas has several important applications:

1. Prediction of soil cover. Knowing catenary patterns, a soil scientist can predict which soils will be encountered on certain relief elements, even without making numerous pits.

2. Soil mapping. Soil boundaries on a map largely follow relief elements. Catenas serve as the basis for identifying soil combinations — areas where different soils regularly repeat in space.

3. Rational land use. Different parts of a catena require different agricultural approaches: upper slopes are better left under forests or perennial grasses to avoid erosion; middle slopes — for arable land with anti-erosion measures; lower slopes — for hayfields or pastures (considering waterlogging).

4. Ecological modeling. Catenas underlie many ecological and landscape models, as they reflect fundamental hydrological, geochemical, and biogeocenotic relationships.

Summary for Section 5

A catena is an elementary "cell" of the landscape in which relief (slope) creates a regular redistribution of water and solid material, forming a sequential series of soils from well-drained at the summit to hydromorphic in the depression. The catenary principle is key to understanding the local organization of the soil cover and is an important tool for soil mapping and land-use planning.

The concept of the catena logically leads to more general concepts of soil sequences (topo-, climato-, chrono-, lithosequences), which we will examine in the next section.

6.1. Concept of a Soil Sequence

Recall Jenny's fundamental formula (Jenny, 1941):

Soil = f(climate, organisms, relief, parent rock, time)

If we want to determine how exactly one of these factors influences soil properties, we need to find or create situations where this factor varies while the other four are constant or close to constant. Then, the changes in soil properties along the series will be primarily related to the change in that specific factor (Jenny, 1941; Buol et al., 2011).

This approach is called the method of sequences (or "controlled comparisons" method). Depending on which factor is studied, the following are distinguished:

• Toposequence — relief (this is essentially a catena, but in a stricter definition)
• Climosequence — climate
• Chronosequence — time
• Lithosequence — parent material
• Biosequence — organisms (primarily vegetation)

Each of these sequences serves as a natural experiment, allowing for the quantitative assessment of the contribution of a particular factor to the formation of soil properties. Let's consider them in turn.

6.2. Toposequence

A toposequence is a series of soils forming on different relief elements (summit, slope, foot, valley) under the same climate, vegetation, parent rock, and age (White, 2006; Buol et al., 2011).

In this definition, the toposequence practically coincides with the catena in its narrow sense (Milne's catena on homogeneous parent rock). However, in modern literature, a toposequence is often considered a more general concept, including both slope sequences and sequences on terraces of different levels (if the age is the same). Nevertheless, in most cases, these terms are used as synonyms (Weil & Brady, 2017).

Classic examples of toposequences (catenas) we have already discussed in the previous section: the Clarion–Nicollet–Webster sequence in the USA (Buol et al., 2011), Chernozem–Gray Forest–Meadow-Chernozem in the Russian forest-steppe (Naumov, 2016), and various hydromorphic sequences in floodplains and on terraces (Wysocki et al., 2012).

The main conclusion of toposequences: with a change in relief, the water regime, erosion load, and biological productivity change, which is directly reflected in profile depth, humus content, degree of gleyzation, and other properties.

6.3. Climosequence

A climosequence is a series of soils forming under different climatic conditions (different temperature and/or moisture) with similar relief, parent rock, vegetation (as much as possible), and age (Jenny, 1941; Weil & Brady, 2017).

This is perhaps the most well-known type of sequence because it is climatic zonality that sets the main contours of the world's soil map. Jenny (1941) and his followers used climosequences to establish quantitative relationships between climatic parameters and soil properties.

Examples of climosequences:

1. Transect from prairies to deserts in the USA. Moving from east to west (from Nebraska to Colorado), annual precipitation decreases from 700 to 300 mm/year at approximately the same temperature. Soils on homogeneous loess deposits change regularly: Chernozems (Mollisols) → Chestnut → Brown Semi-Desert → Gray-Brown Desert. The thickness of the humus horizon and humus content decrease, while the depth of carbonate effervescence increases (Weil & Brady, 2017; Foth, 1990).

2. Climosequence on loess in Europe. Jenny and Leonard (1934) showed that with an increase in annual precipitation from 370 to 900 mm on loess deposits at approximately the same temperature (about 11 °C), humus content increases from 1 to 5%, and cation exchange capacity increases (Buol et al., 2011; cited in Scheffer et al., 2018).

3. Altitudinal climosequence. In mountains, for every 100 m ascent, temperature drops by approximately 0.6 °C, and precipitation often increases up to a certain altitude. Therefore, on one slope, one can observe a change in soils analogous to latitudinal zonality — this is a vertical (altitudinal) climosequence. For example, in the Caucasus: at the foot — Chernozems; higher — Gray Forest; higher — Brown Forest; then Mountain-Meadow; and in the highlands — primitive soils (Naumov, 2016).

Climosequences show that climate sets the main direction of soil formation, determining which processes will dominate — humus accumulation, podzolization, ferrallitization, or salt accumulation.

6.4. Chronosequence

A chronosequence is a series of soils differing in age (time of development) under identical or similar conditions of climate, relief, parent rock, and vegetation (Weil & Brady, 2017; White, 2006). Chronosequences allow assessing the rate of soil formation and the sequence of appearance of various horizons and properties.

Ideal chronosequences are found on geomorphic surfaces of different ages:

• River terraces. The river gradually cuts down, leaving a series of terraces, each older than the previous one (the higher the terrace above the floodplain, the older it is). Soils on such terraces have the same parent rock (alluvium), climate, and vegetation but different ages. Studying them allows tracing soil evolution over time.

• Moraine ridges (glacial deposits). In areas where the glacier retreated in stages, leaving moraines of different ages, chronosequences can be found. For example, in the northern US (Michigan), Franzmeier and Whiteside (1963) studied soils on lake beach terraces aged 2250, 3000, and 8000 years (cited in Foth, 1990; Weil & Brady, 2017). Over 2250 years, an A-C profile (initial sod-podzolic) formed; over 3000 years, a Bs horizon appeared (accumulation of Fe, Al oxides); over 8000 years, a Bhs horizon (with humus). This is a classic example of podzolic soil development over time.

• Volcanic ash. In areas of active volcanism, ash layers of different ages create natural chronosequences. For example, in Kamchatka and Japan, soils on fresh ashes (age tens to hundreds of years) are primitive, while on ancient ones (thousands of years) — fully formed Andosols with a thick humus horizon and allophane minerals (Naumov, 2016; Scheffer et al., 2018).

General patterns of chronosequences (Jenny, 1980; White, 2006):

• Humus accumulation is rapid initially, then slows down, approaching an equilibrium state (exponential curve).
• Weathering depth and clay mineral content increase over time, but the rate decreases (law of diminishing returns).
• Profile differentiation (appearance of distinct horizons) typically requires several thousand years for temperate zones and tens to hundreds of thousands of years for tropical deeply weathered soils.
• The distinctness of eluvial and illuvial horizons increases with age but may reach a plateau (stationary state) — the so-called mature soil.

It is important to note that the absolute age of a soil can vary greatly depending on conditions. For example, a podzolic horizon can form in 1000 years on sand in a humid climate, whereas on dense rock in a dry climate, it might take 10,000 years (Buol et al., 2011). Therefore, chronosequences provide relative estimates of process rates, not absolute datings.

6.5. Lithosequence

A lithosequence is a series of soils formed on different parent rocks (of different mineralogical and granulometric composition) under the same climate, relief, vegetation, and age (Buol et al., 2011; White, 2006). Lithosequences allow assessing how the initial chemical and mineralogical composition of the rock affects the properties of the forming soil.

Classic examples:

1. Granite and gabbro (acid and basic rocks) under the same climatic conditions. On granite (acid rock, much quartz, little Ca, Mg, Fe), sandy, acidic, base-poor soils form — e.g., sandy Podzols or Sod-Podzolic. On gabbro or basalt (basic rock, little quartz, much Ca, Mg, Fe) — clayey, base-rich, often dark-colored soils (Chernozem-like, Cambisols, Ferrallitic) (Weil & Brady, 2017; Buol et al., 2011).

2. Limestone and sandstone in the same climatic zone. On limestone — Rendzinas (Sod-Carbonate) with high humus content and neutral reaction; on sandstone — acidic Podzolic or Sod-Podzolic soils with low base content (Naumov, 2016; Scheffer et al., 2018).

3. Loess and moraine loam in the forest-steppe. On loess (rich in carbonates, homogeneous) — Typical Chernozems with a thick humus horizon and deep carbonate profile. On moraine loam (coarser, often with admixture of boulders) — Chernozems less thick, sometimes with signs of gleyzation (Naumov, 2016).

Lithosequences show that parent material can modify or even redefine the zonal soil type. In particular, on carbonate rocks, even in the taiga zone, Sod-Carbonate soils can form; on sands — Podzols, while on loams — Podzolic soils. Therefore, the lithological factor is always considered in soil mapping.

6.6. Biosequence

A biosequence is a series of soils forming under different vegetation (or different sets of organisms) under the same climate, relief, parent rock, and age (Weil & Brady, 2017; White, 2006). Biosequences allow assessing the influence of the biotic factor — how exactly different types of vegetation alter soil formation.

Classic examples of biosequences:

1. Forest vs. steppe on the same parent rock. In the forest-steppe zone on loess deposits, patches of forest (oak grove) and patches of steppe can coexist on the same slope. Soils under forest — Gray Forest, with a less thick humus horizon, slightly acidic reaction, presence of an eluvial horizon. Soils under steppe — Chernozems (or Chernozem-like), with a thick humus profile, neutral reaction, without an eluvial horizon (Naumov, 2016; Foth, 1990). The reason is the different nature of organic matter input (surface litter for forest, root input for steppe) and the different litter composition (forest litter is poorer in bases).

2. Coniferous forest vs. broad-leaved forest under the same climatic conditions. In the middle taiga on the same parent rock (e.g., moraine loam), under spruce forest, a Podzolic soil with moss litter and acidic coarse humus may form; while under linden-oak forest, a Sod-Podzolic soil with a thicker humus horizon and less acidic reaction may form. The difference is explained by the chemical composition of the litter and the activity of soil fauna (earthworms are more active in deciduous forests) (Weil & Brady, 2017; Buol et al., 2011).

3. Monoculture experiment on the same parent rock (San Dimas, California). In the famous lysimeter experiment in San Dimas (USA), different species of shrubs and pines were planted on the same homogeneous parent rock. After 40–50 years, the soils under different species acquired noticeable differences in A-horizon thickness, humus content, acidity, and even clay mineral composition (Buol et al., 2011; Weil & Brady, 2017). This is a striking example of a biosequence under controlled experimental conditions.

Biosequences show that vegetation can significantly modify the course of soil formation even within the same climate, creating soils with varying degrees of podzolization, humus accumulation, and acidity.

6.7. Integration of Sequences and Their Practical Significance

In real landscapes, all factors act simultaneously, and separating their effects can be difficult. However, the sequence method allows us to "untangle" this knot. For example, if we find a site with the same parent rock and relief but different vegetation — it's a biosequence; if different climate — a climosequence; if different age — a chronosequence.

Sometimes sequences overlap. For instance, along a slope, moisture (toposequence), vegetation (biosequence), and the age of the surface (chronosequence, if the slope formed at different times) may all change. To isolate the pure effect of a single factor, researchers use statistical methods or look for sites where other factors are maximally equalized (Jenny, 1941; Buol et al., 2011).

Practical significance of studying sequences:

• Predicting soils. Knowing how properties change depending on the factor, we can predict soil properties in unexplored territories.
• Assessing resilience. Understanding process rates (chronosequences) helps assess how long it will take for a soil to recover after disturbance.
• Reconstructing paleoclimates. Ancient soils (paleosols) are often analyzed as elements of climosequences and chronosequences to reconstruct past climates.
• Rational land use. Knowing how vegetation affects the soil (biosequence), we can plan afforestation or crop rotation.

Summary for Section 6

• A soil sequence is a series of soils in which one factor changes systematically while others remain constant.
• Toposequence (catena) — relief.
• Climosequence — climate (temperature, moisture).
• Chronosequence — time (soil age).
• Lithosequence — parent material.
• Biosequence — vegetation (organisms).

These sequences are the primary tool of soil geography for establishing cause-and-effect relationships between soil-forming factors and soil properties. They show that each factor contributes, and that this contribution varies under different conditions. Modern soil classifications are built upon these empirical patterns.

7.1. Latitudinal Zonality: A Global Picture

Latitudinal zonality is the general pattern of soil distribution on plains, associated with changes in climate and vegetation from the poles to the equator. As a first approximation, the soil zones of the Earth follow the climatic and biome zones (Weil & Brady, 2017; Scheffer et al., 2018).

A generalized scheme from north to south for Eurasia and North America looks as follows (Buol et al., 2011; Naumov, 2016):

This scheme is an idealization. In reality, zones can be shifted, interrupted, or have complex configurations. Nevertheless, it reflects the main tendency: a change in climatic belts leads to a change in dominant soil-forming processes.

7.2. Altitudinal Zonation (Vertical Zonality)

In mountains, latitudinal zonality is "compressed" vertically: for every 100 m ascent, temperature drops by about 0.6 °C, and precipitation often increases up to a certain altitude (Weil & Brady, 2017; Naumov, 2016). Therefore, on a mountain slope, one can observe a change in natural zones analogous to moving from the equator to the pole, but over a distance of just a few kilometers.

This phenomenon is called altitudinal zonation (or vertical zonality) of soils. It is particularly pronounced in high mountain systems: the Andes, Himalayas, Caucasus, Altai, Tien Shan, Rocky Mountains.

Typical altitudinal sequence (example from the Caucasus):

As one moves into the continental interior, into more arid areas, altitudinal zonation can change: the forest belt may drop out, and mountain steppes may be replaced by semi-deserts and deserts. In very dry mountains (e.g., in Central Asia), an inversion can be observed — where drier and more xerophytic communities are found in the lower belts, and wetter ones in the upper belts (Naumov, 2016).

Important nuance: altitudinal zonation is not an exact analogue of latitudinal zonality because not only temperature changes in mountains but also other factors: ultraviolet radiation, wind regime, slope steepness, aspect, rockiness. Therefore, mountain soils often have specific features not found on plains (shallowness, stoniness, high erodibility) (White, 2006; Wysocki et al., 2012).

7.3. The Role of Local Factors: Relief, Hydrology, Lithology

The zonal picture is only a background. In each location, this background is modified by local conditions. This is why we see zones on small-scale maps but a mosaic of soils on large-scale maps.

7.4. Why is "Ideal" Zonality Disrupted?

Despite the apparent regularity, the real soil cover often does not conform to a simple latitudinal scheme. The main reasons (Weil & Brady, 2017; White, 2006; Buol et al., 2011):

1. Historical (relict) soils. Many modern soils formed under conditions different from today's. For example, in the modern deserts of Africa and Australia, soils with a thick clayey horizon (argillic) formed during the humid Pleistocene occur. These are relict (polygenetic) soils, not in equilibrium with the current climate.

2. Oceanic influences. Sea currents and monsoons create deviations from the latitudinal norm. For example, in the Russian Far East, due to the monsoon climate, specific brown-taiga and volcanic soils form, which are not found in Siberia. Western coasts of continents (Mediterranean climate) yield Brown soils, while eastern coasts (monsoon) yield Red Earth and Yellow Earth.

3. Mountain barriers. Mountains create rain shadows (dry on the leeward side) and trap moisture (humid on the windward side). Therefore, different soils can exist on different sides of the same ridge (e.g., on the western slopes of the Andes — humid tropical forests, on the eastern — dry savannas and deserts).

4. Human activity. Agriculture, deforestation, irrigation, drainage, and urbanization radically change soils, creating anthropogenic modifications that no longer fit the zonal classification (Scheffer et al., 2018).

7.5. An Overall View: The Soil Cover as a System

In conclusion, the Earth's soil cover is a complex, multi-level system in which the following operate:

• Global patterns (latitudinal zonality, altitudinal zonation) — set the general background.
• Regional and local factors (relief, hydrology, lithology, age, biota) — modify the background, creating diversity.
• Historical legacy — past climates and geomorphic processes leave traces that can persist for thousands and millions of years.
• Anthropogenic impact — a modern factor rapidly changing the face of soils.

It is precisely this multi-causality and multi-level nature that makes soil geography both a complex and fascinating science. The answer to the question "why is this particular soil here?" is rarely simple — it requires considering all factors and their interactions over time and space.

Lecture Conclusion

We have concluded our introduction to soil geography. During the lecture, we:

• Understood that soil geography studies the patterns of soil distribution and explains them through soil-forming factors.
• Analyzed the five main factors and showed how they interact to create the soil-forming environment.
• Examined bioclimatic zones and showed how climate and vegetation set the main trend.
• Studied in detail the main soil-forming environments — forest, steppe, tundra, desert, bog, floodplain, mountains, volcanoes.
• Introduced the concept of the catena — a local sequence of soils on a slope where relief redistributes water and matter.
• Familiarized ourselves with soil sequences — a tool for isolating the influence of each individual factor.
• And finally, synthesized everything into a picture of global patterns, emphasizing that zonality is a tendency, and reality is always richer.

Soil geography is not just a set of facts, but a way of seeing the landscape. By understanding how the factors work, we can predict soil properties, plan land use, and assess environmental risks. This knowledge is the foundation for rational natural resource management, as soil is a non-renewable resource on the scale of a human lifetime, and we are obliged to understand how it is structured.