World soil classification systems

Classification as a Fundamental Mental Operation

Imagine a world where every thing has no name. You walk through a forest and see trees, but you cannot say, "this is a pine," "this is a birch"—for you they are all just "green objects." You go to a store and see products, but you cannot distinguish bread, milk, and meat—you simply see "various items." Such a picture seems absurd, because in reality we constantly and unconsciously classify the world around us. We do it instantly: we divide clothes into everyday and festive, books into fiction and scientific, people into acquaintances and strangers.

Classification is a fundamental operation of our thinking, without which neither cognition of the world nor communication is possible (Buol et al., 2011). We classify in order to:

• organize knowledge—instead of chaos of individual facts, we obtain a coherent system;
• reduce cognitive load—instead of remembering the properties of each individual object, we remember the properties of the class to which it belongs;
• discover patterns—when we see that an object belongs to a certain class, we already know a lot about its properties, even without studying it in detail;
• transfer knowledge—classification creates a common language for specialists (Rosch & Lloyd, 1978).

Perhaps the most precise definition of classification was given by the English philosopher John Stuart Mill in the 19th century: classification is the arrangement of objects into groups according to their similarities, in such a way that the relationships of similarity and difference become obvious (Mill, 1925, cited in Cline, 1949). This definition is important because it emphasizes the main goal of classification—not just to "put things on shelves," but to reveal substantive connections between objects.

Classification and Categorization: What's the Difference?

In cognitive science and logic, there are two close but not identical concepts:

Categorization is the assignment of a specific object to a pre‑existing group (category) based on certain rules. For example, when we say, "this particular piece of land belongs to chernozems"—that is categorization. We know what a chernozem is (we have its description) and we check whether the given object matches that description (Rosch, 1978).

Classification (in the narrow sense) is the broader process of creating the system of groups itself, i.e., developing the principles by which we will divide objects and defining boundaries between classes. For example, developing Soil Taxonomy (USDA) is classification. Using that system to identify a particular soil is categorization.

In everyday soil science practice, we often mix these terms. That is acceptable—the main thing is to understand that underlying any work with soils are two processes: creating a system and applying that system in practice.

Why is Soil Classification a Difficult Task?

It would seem that classifying soils is easier than, say, plants or animals. Plants have clear characteristics: flower, leaf, stem. Animals have anatomy and genetics. But with soils, it is much more complicated. Here is why.

First difficulty: soil is a continuous body. Plants and animals are discrete objects: they are born, live, and die as separate individuals. Soils, however, form a continuous cover, where properties change gradually (White, 2006). Between chernozem and chestnut soil there is no sharp boundary—there is a transition zone tens of meters wide. How do you draw the line where chernozem "ends" and chestnut soil "begins"? This is always to some extent an arbitrary decision (Jenny, 1980).

Second difficulty: soil is a four‑dimensional object. It changes not only in space (horizontally and vertically) but also in time. Today's soil is not the same as the soil that was here a hundred years ago. It is the result of a continuous process, not a static object. Classifying a process is always more difficult than classifying a static object (Buol et al., 2011).

Third difficulty: soil has no "heredity." For plants and animals we can trace a pedigree. Soil has no "genes"—its properties are determined by the interaction of factors that may have changed throughout its history (White, 2006). Therefore, soil classification cannot use the same approach as biological systematics.

Fourth difficulty: diversity of purposes. Soil is used in many ways: for growing crops, for construction, for water filtration, as a habitat for millions of organisms. A classification that is good for one purpose may be useless for another.

That is why soil classifications have emerged and evolved differently in different countries and different eras. And that is why to this day there is no single "ideal" soil classification—there are several systems, each solving specific tasks.

Why Does an Agronomist Need Classification? Answering the Main Question of the Lecture

Soil classification is not an academic game. It is a practical tool that helps an agronomist (and any land user) make informed decisions. Here are four key functions of classification in agronomy.

1. Predicting soil behavior. If you know that your site belongs to a certain soil type, you already know a lot about its properties: how it responds to fertilizers, whether it needs liming, how prone to erosion, how it behaves under waterlogging (Buol et al., 2011). Instead of experimenting for years on each field, you can use the knowledge accumulated by scientists for that soil type.

2. Technology transfer. Research results obtained on one site can be transferred to other sites with similar soils. Without classification, you would have to start from scratch each time. Classification creates a "bridge" between different geographical points (Buol et al., 2011).

3. Information exchange. When agronomists from different regions or countries use a common classification language, they can discuss soil properties without seeing them in the field. This is especially important in the modern world, where information spreads instantly.

4. Land‑use planning. Soil maps are classifications applied to the landscape. They allow planning where to sow cereals, where to plant an orchard, and where to leave forest or pasture (Eash et al., 2016).

In the words of the founder of modern soil classification, V.V. Dokuchaev: "Soils must be studied and classified according to their profiles" (cited in Buol et al., 2011). But not only studied—classification is needed to use that knowledge in practice.

2.1. Classification by Origin (Genetic)

This is the oldest and perhaps the most intuitive approach: grouping soils by how they formed. Its founder is rightfully Vasily Vasilyevich Dokuchaev, who in 1883 published his famous work "Russian Chernozem" (Dokuchaev, 1883). Dokuchaev showed that soils are not just "ground rocks" but independent natural bodies that form under the influence of five factors: climate, organisms (plants and animals), relief, parent material, and time (Jenny, 1941). It is the combination of these factors that determines what soil will arise in a particular place.

In genetic classification, soils are grouped by similarity of conditions and processes of their formation. For example:

• Podzolic soils—form in coniferous forests of cold and temperate climates, where a leaching water regime removes from the upper horizons everything that can dissolve, leaving a light, depleted layer—"ash" (hence the name: pod—under, zola—ash).
• Chernozems—form in steppes, where dense herbaceous vegetation creates a powerful humus horizon, and the climate with periodic droughts does not allow this humus to be leached deep down.
• Chestnut soils—form in more arid steppes, where the humus horizon is thinner and carbonates appear in the profile.

Dokuchaev and his followers (Sibirtsev, Glinka) created a system in which the main taxa were genetic soil types—groups formed under "uniformly associated biological, climatic, and hydrological conditions" (Ganzhara et al., 2002). This system dominated in Russia and had a huge influence on soil science worldwide.

Advantages of the genetic approach:

• It gives a holistic understanding of soil as the result of environmental interactions.
• It explains the geographical distribution of soils—zoning.
• It is convenient for teaching because it links soil to landscape and climate, which are easy to observe (Buol et al., 2011).

Disadvantages of the genetic approach:

• The main drawback is that it is often subjective. The processes that formed the soil occurred in the past, and we cannot always reconstruct them precisely. One scientist may interpret soil genesis differently from another (Smith, 1983).
• Soil genesis is difficult to quantify. How do you measure "degree of podzolization"? It is a qualitative description, not a number.
• The same soil may form through different pathways under different conditions. Conversely, different soils may form under similar conditions due to differences in parent material or time.
• Genetic names are often ambiguous. For example, the term "podzolic" is interpreted differently in different countries (White, 2006).

It is precisely these shortcomings that led to the search for other classification principles.

2.2. Classification by Properties (Morphological, Diagnostic)

If genesis is difficult to measure, then soil properties—color, texture, structure, humus content, pH, cation exchange capacity—can be measured objectively. So we can build a classification on what is now rather than on what once happened.

This principle underpinned the American Soil Taxonomy, whose development began in the 1950s under the leadership of Guy Smith and was published in 1975 (Soil Survey Staff, 1975). The second edition came out in 1999, and the keys are regularly updated (Soil Survey Staff, 1999, 2010).

The main idea of Soil Taxonomy: classify soils by their diagnostic properties—traits that can be observed or measured by standard methods. Genesis is used only as a "thread" to help select which properties are important, but the classification criteria themselves are properties, not hypotheses about origin (Smith, 1983; Buol et al., 2011).

Key concepts became diagnostic horizons—soil layers with clear, quantitatively described characteristics. If a soil has, for example, a mollic epipedon (dark, soft, humus‑rich, with high base saturation)—it is a serious candidate for Mollisols. If it has an argillic horizon (accumulation of clay translocated from above)—it is a sign of Alfisols or Ultisols (Weil & Brady, 2017).

Advantages of the diagnostic approach:

• Objectivity: two different scientists measuring the same properties will get the same result (provided methods are standardized).
• Possibility of automation: a computer can classify soil from numerical data.
• Clear boundaries: classes are defined by specific numbers (e.g., "clay content >35%"), not vague descriptions.
• The system is resilient to changing views on genesis: if scientists revise theories of soil formation, the classification itself does not collapse.

Disadvantages of the diagnostic approach:

• Complexity and cost: accurate diagnosis requires laboratory analyses. In the field, not all diagnostic traits can always be determined (Eash et al., 2016).
• Loss of connection with landscape: the name "Typic Hapludalf" tells the agronomist nothing about where this soil occurs, what its vegetation and climate are.
• Artificiality: numerical thresholds are often arbitrary. Why, for example, is the boundary between "active" and "semi‑active" clays set at CEC/clay = 0.24, not 0.25?

Nevertheless, the diagnostic approach dominates in international systems (Soil Taxonomy and WRB) because it ensures reproducibility and universality (Huang et al., 2012).

2.3. Classification by Use (Technical, Special)

This principle does not aim to reflect the nature of the soil or its genesis. Its task is to answer a specific practical question: how suitable is this soil for a particular use?

Such classifications are called technical or special. They can be created for:

• Assessing soils for irrigation (e.g., division into non‑saline, slightly saline, solonetzic, etc. based on EC and ESP) (White, 2006).
• Classifying soils for construction (by bearing capacity, compressibility, frost heave).
• Assessing contamination (by heavy metals, petroleum products).
• Agro‑production grouping—combining soils similar in crop cultivation conditions and requiring uniform agronomic practices (Ganzhara et al., 2002).

In technical classification, one or several relevant indicators are used, and boundaries between classes are drawn according to values that matter for practice, not natural patterns (White, 2006). For example, a classification for agriculture may single out "soils requiring liming" based on pH, regardless of genetic type.

Advantages: direct practical benefit, simplicity, link to specific technologies.

Disadvantages: such a classification becomes obsolete when technologies or goals change. It does not provide a general understanding of soil as a natural body. And it does not help transfer knowledge from one soil to another if their uses differ (Buol et al., 2011).

2.4. Natural vs. Artificial Classification

In the methodology of science, two approaches are distinguished:

Artificial (pragmatic) classification—grouping by one or several traits chosen solely for user convenience. Technical classifications are examples of artificial ones. They may place completely different objects side by side if they share the required trait.

Natural classification—grouping that reflects all essential properties of objects and their interrelations. Ideally, such a classification reveals the "natural structure" of the world (Mill, 1925). A class should unite objects with many common properties, and these properties should correlate with each other (Rosch et al., 1976).

Soil Taxonomy claims the status of a natural classification: it aims to use properties that have the greatest number of associated (correlating) characteristics. For example, the presence of an argillic horizon correlates with increased water‑holding capacity, nutrient availability, etc. (Buol et al., 2011).

2.5. Principles of Cognitive Science and Soil Classification

Interestingly, modern views on classification rely not only on logic but also on the psychology of cognition (cognitive science). Studies have shown that people naturally group objects guided by two principles (Rosch & Lloyd, 1978; Buol et al., 2011):

1. Principle of cognitive economy: classification should provide maximum information with minimum mental effort. We do not remember all properties of each object, but the properties of the class.

2. Principle of perceived world structure: there are natural "clusters" of similarity in the world—objects that are more similar to each other than to others. People tend to single out such groups—they seem "natural" to us.

These principles explain why soil classifications based on diagnostic traits have proven convenient: they group soils by properties that indeed correlate with each other, creating "natural" groups.

2.6. Brief Summary: Evolution of Principles

In the history of soil classification, we see a movement:

• from genetic classifications, which answered the question "how did the soil originate?";
• to diagnostic classifications, which answer "what are its observable properties?";
• while preserving technical classifications for applied tasks.

The genetic approach gave us an understanding of soil as a landscape element. The diagnostic approach gave us precision and objectivity. Technical classifications give practical recommendations. Modern systems (Soil Taxonomy, WRB) try to combine the best of both: they use diagnostic traits, but select them so that they reflect genesis and are meaningful for use (Smith, 1983; IUSS Working Group WRB, 2014).

In the next section, we will look at how soil classifications actually developed in historical perspective—from Dokuchaev's early attempts to modern international systems.

3.1. Prehistory: from Antiquity to the 19th Century

People classified soils long before scientific soil science appeared. The farmer who said, "this land is good for wheat, and this for grapes," was already classifying, albeit primitively.

In ancient China, as early as the Zhou dynasty (about 3000 years ago), there was a classification of soils by color, texture, and hydrological features. The famous book "Yu Gong" divided the soils of China into three categories and nine classes (Gong et al., 2003; Huang et al., 2012). This is perhaps the oldest soil classification that has come down to us.

In ancient Greece and Rome, thinkers—Aristotle, Theophrastus, Cato, Varro, Virgil, Columella—described soil properties in relation to plant suitability. But these descriptions were more practical notes than systematic classifications (Buol et al., 2011).

In the 19th century, the first more or less systematic classifications began to appear in Europe. One of the most famous is the classification of Albrecht Daniel Thaer (Thaer, 1853), published in "Principles of Rational Agriculture." Thaer divided soils by texture (clay, loam, sand, etc.) and within these groups by agricultural suitability. For example, among clay soils he distinguished "strong wheat land," "weak wheat land," "thin wheat land," etc. (Buol et al., 2011). This was a technical classification—for practical farming.

Around the same time, the German scientist Fallou (Fallou, 1862) proposed a classification based on the geological origin of the material: soils on residual rocks, alluvial, etc. And Ferdinand von Richthofen (von Richthofen, 1886) developed this idea, creating a more complex system with categories "residual," "accumulated," etc. (Buol et al., 2011).

All these early classifications were either technical (for agriculture) or geological (soil as a product of rock weathering). Soil was not yet conceived as an independent natural body with its own history and developmental patterns.

3.2. The Birth of Pedology: V.V. Dokuchaev and the Russian School

The turning point came in 1883, when Vasily Vasilyevich Dokuchaev published his famous work "Russian Chernozem". This was the first scientific work in the world where soil was presented not as "weathered rock," but as an independent natural body forming under the influence of a complex of factors (Dokuchaev, 1883; Buol et al., 2011).

Dokuchaev showed that soils have a regular geographical distribution: over vast areas of the Russian Plain, where parent material is relatively uniform (loess‑like loams), soils change from north to south depending on climate and vegetation. Under coniferous forests—podzols, under broad‑leaved forests—grey forest soils, in steppes—chernozems, in semi‑deserts—chestnut soils. This discovery of soil zones was revolutionary (Sibirtsev, 1901; Buol et al., 2011).

In 1886, Dokuchaev proposed a definition of soil that still remains relevant (in paraphrased form): soil is "the horizons of rocks that daily or almost daily change their relationships under the combined influence of water, air, and various organisms, living and dead" (cited in Buol et al., 2011).

Dokuchaev's main idea: soil is the result of the interaction of five soil‑forming factors. Later, in the 20th century, Hans Jenny formalized this idea in his famous equation:

Soil = f (climate, organisms, relief, parent material, time) (Jenny, 1941, 1980).

Dokuchaev's students—N.M. Sibirtsev and K.D. Glinka—developed his ideas. Sibirtsev introduced the concept of soil zones and wrote the first soil science textbook (Sibirtsev, 1901). Glinka published works that introduced the Western world to the Russian school of soil science (Glinka, 1914, 1927; Buol et al., 2011).

It was this school that created the genetic classification of soils, which dominated in Russia and the USSR throughout the 20th century and had a huge influence on soil science worldwide. At the heart of this classification is the concept of genetic soil type—a group of soils formed under similar bioclimatic conditions and having a similar profile (Ganzhara et al., 2002).

Soil types according to Dokuchaev‑Sibirtsev: podzols, chernozems, chestnut soils, serozems, solonetzes, solonchaks, etc.

This classification was brilliant for its time. It explained the connection between soils and landscape and climate, was visual and convenient for geographical maps. But it also had shortcomings that became apparent as new knowledge accumulated.

3.3. The American Stage: from Marbut to Soil Taxonomy

3.4. International Attempts: the FAO‑UNESCO Legend and WRB

Parallel to the development of Soil Taxonomy, there was a growing demand for an international system. Soils do not recognize national borders, and for global projects—world maps, resource assessments, climate change predictions—a common language was needed.

In 1961, the Soil Map of the World project at scale 1:5,000,000 was launched under the auspices of FAO and UNESCO. In 1974, the Legend to this map was published, which contained not just a map but a system for classifying the world's soils (FAO‑UNESCO, 1974; Huang et al., 2012).

The FAO‑UNESCO system included 26 major soil groups and 106 soil units. It was a mono‑categorical system—each soil belonged to one soil unit, without hierarchy. Its task was primarily mapping, not exhaustive classification.

In 1988, a revised legend was published (FAO‑UNESCO, 1988). The number of major groups increased to 28, and units to 153. A two‑level structure appeared: major groups and soil units within them. This system became the predecessor of WRB.

In the 1990s, work began on the World Reference Base for Soil Resources (WRB). This was a joint project of ISSS (International Society of Soil Science, now IUSS), FAO, and ISRIC (International Soil Reference and Information Centre) (Huang et al., 2012).

The first official version of WRB was adopted in 1998, and the second version came out in 2006 (IUSS Working Group WRB, 2006, 2014).

WRB is a two‑level system:

• 32 Reference Soil Groups—highest level, analogous to "orders" in Soil Taxonomy.
• Qualifiers—prefixes and suffixes added to the group name to refine properties.

It is important to note that WRB deliberately does not use climatic criteria (soil temperature and soil moisture regimes) as Soil Taxonomy does (IUSS Working Group WRB, 2014; Buol et al., 2006). This is so that the classification does not depend on climatic data, which are not always available in developing countries.

3.5. National Systems—Why Haven't They Disappeared?

Many national soil classifications exist in the world. For example:

• Russia—genetic classification (types, subtypes, genera, species) which is continuously evolving, and its latest version (2004) differs significantly from the classical Dokuchaev system (Shishov et al., 2004; Ganzhara et al., 2002).
• Canada—Canadian System of Soil Classification, close to Soil Taxonomy but adapted to Canadian conditions.
• Australia—Australian Soil Classification, which uses 14 orders, many distinct from the American ones (Isbell, 1996; White, 2006).
• China—Chinese Soil Taxonomic Classification, combining principles of Soil Taxonomy and national traditions (Gong & Zhang, 2007; Huang et al., 2012).
• France—Référentiel Pédologique—a kind of "reference base" for soils, which is not a rigid classification but rather a system for describing soil cover (AFES‑INRA, 1992; Huang et al., 2012).

National systems exist because each country has its own unique soils, research traditions, and practical tasks. A soil scientist in Russia often finds it more convenient to speak of "sod‑podzolic" and "chernozem" than "Alfisols" and "Mollisols"—for him and his colleagues, these terms carry more meaning tied to real landscapes.

Moreover, national systems can respond faster to local scientific discoveries and practical needs. International systems like Soil Taxonomy and WRB change more slowly because they must consider the opinions of scientists from many countries.

3.6. Summary: from Dokuchaev to the Present

Let us briefly summarize the historical development:

In the next section, we will examine in more detail the logic behind hierarchical taxonomies—both in Soil Taxonomy and in WRB.

4.1. Russia: Genetic Tradition in a New Guise

The Russian school of soil science, founded by V.V. Dokuchaev, developed a genetic classification for over a hundred years, based on the concept of soil type—a group of soils formed under similar bioclimatic conditions and having a characteristic profile (Ganzhara et al., 2002). For a long time, the main taxonomic unit was the genetic type, followed by subtypes, genera, species, and varieties.

However, by the end of the 20th century, the classical genetic classification began to face the same problems as the American 1938 system: subjectivity, fuzzy boundaries, and difficulty in applying to human‑modified soils. Therefore, in 1997–2004, a new classification of Russian soils was developed and published (Shishov et al., 2004; see also Huang et al., 2012; Scheffer et al., 2018).

The new Russian classification retained a genetic orientation but introduced important changes:

• The concept of diagnostic horizons was introduced (by analogy with Soil Taxonomy), though they are not as rigidly quantitative.
• Instead of a single "type" taxon, a more complex hierarchy appeared: trunk → section → type → subtype → genus → species → variety → category.
• Three trunks were distinguished: post‑lithogenic (soils developing on already formed material), syn‑lithogenic (soils with simultaneous accumulation of new material on the surface), and organogenic (peat soils).
• Types reflect not only natural processes but also anthropogenic changes. For example, agrozems—soils strongly altered by agricultural tillage, and agroabrazems—eroded arable soils (Scheffer et al., 2018).

Features of the Russian classification:

• Consideration of hydromorphism (gleying)—very important for the vast wetlands of Russia. Gleyic horizons (G) are diagnostic.
• Consideration of solonetzicity and salinity—for southern arid regions.
• Consideration of cryogenic processes—for Siberia and the Far East, where permafrost soils (cryozems) are widespread.
• Retention of traditional names: podzolic, sod‑podzolic, grey forest, chernozems, chestnut, solonetzes, solonchaks, brown semi‑desert, etc.

For a Russian agronomist, these names carry much more meaning than, say, "Alfisols" or "Mollisols," because they are tied to the specific landscapes and climatic zones of Russia that he knows well. Moreover, the Russian classification is actively used in land cadastre and reclamation design.

Important note: The Russian classification is constantly evolving. Efforts are underway to bring it closer to WRB to facilitate international communication, but a complete abandonment of the national system is not planned (Ganzhara et al., 2002; Shishov et al., 2004).

4.2. Germany: Systematics Based on Water Regime

The German soil classification (Bodensystematik) is one of the most detailed in Europe. Its modern version was developed by a working group of the German Soil Science Society and published in the "Soil Mapping Manual" (Ad‑hoc‑AG Boden, 2005; see also Scheffer et al., 2018).

The main feature of the German system is division of soils by water regime into four major departments (Abteilungen):

1. Terrestrial soils (Landböden)—soils not dependent on groundwater. They develop under downward moisture flow (precipitation → soil → subsoil). This is the majority of soils: from initial (Syroseme) to mature (Parabraunerden, Podsole, Braunerden, etc.).

2. Semiterrestrial soils (Grundwasserböden)—soils influenced by groundwater. These include gleys (Gleye), floodplain soils (Auenböden), and marsh soils (Marschen) of coastal areas.

3. Subhydric soils (Unterwasserböden)—soils permanently covered by water (bottom deposits of lakes and seas).

4. Peat soils (Moore)—a separate department for organic soils with thick peat horizons.

Within each department, classes are distinguished, then soil types (Bodentypen). Types are defined by a combination of genetic horizons denoted by letter symbols. For example, a typical profile of Braunerde (brown earth) has Ah / Bv / C. Parabraunerde—Ah / Al / Bt / C. Podsol—O / Aeh / Ae / Bh / Bs / C (Scheffer et al., 2018; Ad‑hoc‑AG Boden, 2005).

The German system is very detailed and includes more than 50 soil types, not counting subtypes and variations. It is widely used for large‑scale soil mapping in Germany, Austria, and Switzerland.

For an agronomist, the German classification is valuable because the soil type immediately indicates its water regime and drainage status—key factors for crop selection, timing of field operations, and reclamation measures.

4.3. France: Référentiel Pédologique—a "Reference Base"

The French soil classification system is not a rigid hierarchy of taxa but a Référentiel Pédologique ("Pedological Reference Base"), developed by the French Association for Soil Study (AFES‑INRA, 1992; Huang et al., 2012).

This system is fundamentally different from others. Instead of forcibly fitting each soil into predetermined classes, it offers a flexible system for describing soil covers. The basic unit is the soil cover (couverture pédologique), considered as a three‑dimensional spatial body with a certain internal organization (horizons, structures, inclusions).

The system identifies about 90 references—a kind of "standard samples" of soils, each described by a specific combination of diagnostic horizons. But a soil does not have to strictly correspond to one reference—it can be described as a combination of features from several references (AFES‑INRA, 1992).

What does this provide?

• Flexibility—the system does not break down when encountering a soil that does not fit standard frameworks.
• Consideration of spatial heterogeneity—soils often change over short distances, and the Référentiel allows this to be reflected.
• Connection with landscape and processes—emphasis is placed not on formal taxa but on real soil systems.

The Référentiel Pédologique is used in France and in many French‑speaking African countries. It harmonizes well with WRB but retains its uniqueness.

4.4. China: Synthesis of Traditions and Modernity

The Chinese Soil Taxonomic Classification (CSTC) is a relatively new system actively developed since the 1990s under the leadership of Professor Gong Zitong (Gong & Zhang, 2007; Huang et al., 2012).

Historically, China used first American (in the first half of the 20th century), then Soviet genetic classification. But by the end of the 20th century, it became clear that a own system was needed, one that would account for the unique diversity of China's soils (from permafrost to tropical laterites) and millennia of experience in rice cultivation and terraced farming.

Structure of the Chinese taxonomy:

• 14 orders, analogous to Soil Taxonomy.
• 39 suborders, 78 groups, 301 subgroups.
• Diagnostic horizons (33 defined) and diagnostic properties (25) are used, many borrowed from Soil Taxonomy, but with specific Chinese ones.
• One important feature is the identification of anthropic epipedons related to centuries of rice cultivation: for example, the hydragric horizon (accumulation of iron and organic matter under flooding) and the anthraquic horizon (mixed layer with plough pan) (Huang et al., 2012; Gong & Zhang, 2007).

Orders of the Chinese classification include both familiar ones (Histosols, Spodosols, Andosols, Vertisols, Aridisols, Gleyosols) and specific ones: Ferrosols (soils with a ferralic horizon), Isohumosols (with isohumic profile, characteristic of chernozem regions of China), Halosols (saline soils), etc. (Huang et al., 2012).

The Chinese system is actively used for soil mapping and land cadastre, especially at detailed levels. It closely correlates with WRB, and publications often give equivalents in both systems, facilitating international communication.

4.5. Other National Systems (Briefly)

Apart from those listed, there are many other national classifications:

• Canadian System—very close to Soil Taxonomy but adapted to cold Canadian soils. Has 10 orders (including specific Cryosolic and Vertisolic) (Soil Classification Working Group, 1998; Weil & Brady, 2017).
• Australian Soil Classification—uses 14 orders, quite different from the American. For example, Kurosols—acid soils with a textural B horizon, Sodosols—soils with high sodium content, Calcarosols—carbonate soils (Isbell, 1996; White, 2006).
• Brazilian System (Sistema Brasileiro de Classificação de Solos)—focused on tropical soils, uses many Soil Taxonomy criteria but adds local indicators (e.g., weathering indices Ki and Kr) (EMBRAPA, 2006; Huang et al., 2012).

4.6. Why Don't National Systems Disappear?

It may seem that with the advent of WRB and Soil Taxonomy, national classifications should become obsolete. But this does not happen, and for good reasons:

1. Language and tradition. For a soil scientist in Russia, "chernozem" is not just a word—it is a whole world of understanding about soil, landscape, climate, and agricultural history. An agronomist raised in the steppe zone intuitively understands what "ordinary chernozem" means. Switching to "Mollisol" requires retraining and loss of this intuitive connection.

2. Accounting for local features. National systems can include diagnostic traits important specifically for that country. For example, the Chinese system accounts for rice soils, the German system elaborates water regimes, the Russian system focuses on permafrost and saline soils.

3. Responsiveness. National systems can change faster than international ones. If a new soil type is discovered in a country or a new reclamation technology developed, the national classification can respond within a few years, whereas changes in WRB or Soil Taxonomy go through lengthy international negotiations.

4. Land management practice. In most countries, cadastral and land‑management documents rely on the national classification. Switching to an international system would require enormous costs for retraining specialists and re‑issuing documents.

5. Hierarchy of purposes. National systems often solve specific tasks: e.g., agricultural land evaluation, reclamation design, forestry. International systems are more universal but precisely for that reason may be less convenient for specific practical applications.

Therefore, in actual work, soil scientists and agronomists usually master several classification languages. For international publications, they use WRB or Soil Taxonomy. For internal reports, cadastres, projects—the national system. The ability to "translate" from one system to another is an important professional skill.

In the next section, we will examine in detail the logic of taxonomy construction using Soil Taxonomy as an example—and see how this knowledge helps the agronomist in practice.

5.1. What is Taxonomy?

The term "taxonomy" (from Greek @ln‑greek[taxis]—arrangement, nomos—law) in the broadest sense means the science of classification. But in the context of soil science, we more often use this term to denote a specific formal classification system—one built according to strict rules, using quantitative criteria, and having a hierarchical structure (Buol et al., 2011; Soil Survey Staff, 1999).

Soil Taxonomy is precisely a taxonomy. It was created not just as a "list of names" but as a tool for identification and communication that can be applied uniformly in any region of the world (Smith, 1983; Huang et al., 2012).

5.2. Hierarchical Structure: from General to Specific

Soil Taxonomy has six categories (levels of generalization). From highest (most general) to lowest (most specific):

1. Order—12 classes (e.g., Alfisols, Mollisols, Oxisols…)

2. Suborder—about 65 classes

3. Great Group—more than 400 classes

4. Subgroup—more than 2600 classes

5. Family—about 8000 classes

6. Series—more than 20,000 classes in the USA and tens of thousands worldwide (Weil & Brady, 2017; Eash et al., 2016).

Each lower category is included in the higher one. That is, all soils belonging to a certain series automatically belong to a certain family, subgroup, great group, suborder, and order. This property is called hierarchy or nesting (Buol et al., 2011).

Example (from Buol et al., 2011; Weil & Brady, 2017):

• Order: Alfisols (soils with an argillic horizon and medium‑high base saturation)
• Suborder: Udalfs (Alfisols with udic moisture regime—moist most of the year)
• Great Group: Hapludalfs (Udalfs with minimal development of additional horizons)
• Subgroup: Typic Hapludalfs (typical representative of the great group)
• Family: Fine‑silty, mixed, active, mesic Typic Hapludalfs (with added particle‑size, mineralogy, exchange activity, and temperature characteristics)
• Series: Fayette (specific local name)

Why such a complex hierarchy? Because the same soil may be needed for different purposes. An engineer designing a road may only be interested in the order or great group to understand general properties (clay content, drainage). An agronomist choosing a fertilizer system may go down to the series to account for nuances of a particular field (Eash et al., 2016).

5.3. Eight Principles of Soil Taxonomy Construction

The creators of Soil Taxonomy (primarily Guy Smith) built the system on eight key principles (Soil Survey Staff, 1975; Buol et al., 2011; Huang et al., 2012). They determine why the system is structured the way it is:

1. Same meaning for all users. Each taxon is defined by quantitative characteristics that can be reproduced by any trained specialist. There should be no room for subjective interpretations.

2. Multi‑categorical. Hierarchy from general to specific—so that one can choose the appropriate level of detail for a given task.

3. Reality of taxa. Classes should reflect soils that actually exist in nature, not be artificial combinations of properties that never occur together.

4. Use of measurable properties. Classification criteria are properties that can be observed in the field or measured in the laboratory using standard methods. No "assumed" processes.

5. Flexibility and evolution. The system should allow changes as new knowledge accumulates, but with minimal disruption to the existing structure.

6. Stability under anthropogenic change. Plowed or human‑altered soils should, as far as possible, remain in the same taxa as their virgin analogs. This is important for predicting soil behavior.

7. Completeness of coverage. Every soil existing in the world should find a place in the system at all categorical levels (the principle of "integrity of taxonomic categories").

8. Incorporation of new knowledge. The system must be open to the inclusion of new diagnostic traits and new taxa as science advances.

5.4. Diagnostic Horizons and Properties—"Building Blocks"

The central innovation of soil taxonomy is the use of diagnostic horizons and diagnostic properties as the main criteria for division. These are not just "descriptions" but strict definitions with quantitative thresholds (Weil & Brady, 2017; Soil Survey Staff, 1999).

Diagnostic horizon is a soil layer that has a certain set of properties (color, thickness, clay content, organic carbon, base saturation, etc.) meeting specified numerical criteria. Diagnostic horizons can be surface (epipedons) and subsurface.

Epipedons—diagnostic horizons that form at the surface (or include the surface) (Weil & Brady, 2017):

• Mollic—dark, humus‑rich, with high base saturation (> 50%), characteristic of chernozems and grassland soils.
• Umbric—like mollic, but with low base saturation (< 50%), characteristic of acid humus horizons.
• Ochric—light, or too thin, or with low humus content. The most common, essentially a "residual" epipedon for soils that do not fit others.
• Histic—organic horizon (peat) 20–60 cm thick over mineral soil.
• Melanic—very dark, with high organic carbon (> 6%), characteristic of volcanic soils.
• Anthropic and Plaggen—anthropogenic, altered by long‑term cultivation (e.g., plow horizons with organic amendments).

Subsurface diagnostic horizons (Weil & Brady, 2017; Eash et al., 2016):

• Argillic (Bt)—accumulation of clay (silicate) translocated from overlying horizons (illuvial).
• Kandic—accumulation of low‑activity clay (kaolinite, oxides), without obligatory clay coatings.
• Natric—like argillic, but with high exchangeable sodium (> 15%) and columnar structure.
• Spodic (Bh, Bs)—accumulation of organic matter and aluminum oxides (sometimes iron), characteristic of podzols.
• Oxic (Bo)—highly weathered, low cation exchange capacity, dominated by iron and aluminum oxides (ferralitic soils).
• Calcic (Bk)—accumulation of secondary calcium carbonates.
• Gypsic (By)—accumulation of gypsum.
• Salic (Bz)—accumulation of soluble salts.
• Cambic (Bw)—horizon with signs of alteration (color, structure) without clear illuvial accumulation.
• Albic (E)—light eluvial horizon, depleted in clay and iron oxides.

There are also diagnostic properties not tied to a specific horizon: for example, soil moisture regimes (udic, ustic, xeric, aridic, aquic) and soil temperature regimes (cryic, frigid, mesic, thermic, hyperthermic, and their iso‑variants) (Weil & Brady, 2017; Eash et al., 2016).

Why is this so important?

Diagnostic horizons and properties are the building blocks of the system. It is their combination that determines which class the soil belongs to. For example, if a soil has a mollic epipedon and high base saturation—it is likely a Mollisol. If it has an argillic horizon and low base saturation (< 35%)—it is an Ultisol. If it has a spodic horizon—Spodosol. And so on (Weil & Brady, 2017).

5.5. How the "Key" Works—Identification Logic

Soil Taxonomy is structured as a dichotomous key (like a plant identification guide). This means that to identify a soil, we sequentially answer questions, starting from the highest level (order) and moving downward (Weil & Brady, 2017; Buol et al., 2011; Eash et al., 2016).

A simplified key to the 12 orders looks like this (from Weil & Brady, 2017; Buol et al., 2011):

1. Is there permafrost within 100 cm? → Gelisols.

2. Is there more than 30% organic matter to a depth of 40 cm? → Histosols.

3. Is there a spodic horizon? → Spodosols.

4. Are there andic properties (volcanic minerals)? → Andisols.

5. Is there an oxic horizon (or kandic with very low weatherable minerals)? → Oxisols.

6. Does it contain >30% clay in all horizons and have periodic cracks? → Vertisols.

7. Are there any diagnostic subsurface horizons and an arid moisture regime? → Aridisols.

8. Is there an argillic or kandic horizon with base saturation <35%? → Ultisols.

9. Is there a mollic epipedon with base saturation >50% to 180 cm? → Mollisols.

10. Is there an argillic, kandic, or natric horizon with base saturation ≥35%? → Alfisols.

11. Is there an umbric, mollic, or plaggen epipedon, or a cambic horizon? → Inceptisols.

12. If none of the above traits apply → Entisols.

Note that order matters. If a soil has both a mollic epipedon and an argillic horizon but is in an arid climate (step 7), it will be classified as an Aridisol, not Mollisol or Alfisol. That is why you cannot "guess" the order from one trait—you must follow the key sequentially (Soil Survey Staff, 1999; Weil & Brady, 2017).

5.6. Categories Below Order: Names That Say a Lot

One of the strong points of Soil Taxonomy is connotative nomenclature. The name of a taxon at any level (except series) carries information about its position in the hierarchy and key properties (Heller, 1963; Buol et al., 2011; Huang et al., 2012).

Name‑building principle:

• The formative element (root) of the order gives the ending for all lower taxa.
• For example, Mollisols → root oll. All taxa in this order end with oll: Udolls, Argiudolls, Typic Argiudolls, etc.
• Suborder adds a prefix to this root: Ud (humid) → Udolls.
• Great Group adds another prefix: Argi (clay) → Argiudolls.
• Subgroup adds one or more adjectives: Typic → Typic Argiudolls.
• Family adds descriptive terms (texture, mineralogy, exchange activity, temperature): fine‑loamy, mixed, active, mesic → Typic Argiudolls.
• Series—local geographical name.

What does this give the practitioner?

Knowing the taxon name (e.g., Aquic Hapludalf), you can already understand:

• alf → this is Alfisol (order), so there is an argillic horizon, moderately leached, base saturation >35%.
• ud → Udalf, udic moisture regime (humid most of the year).
• Hapl → Hapludalf, simple group, minimal development of additional horizons.
• Aquic → signs of excessive wetness (redoximorphic).

In other words, the taxon name is a brief "annotation" of the soil (Buol et al., 2011; Eash et al., 2016).

5.7. Advantages of Soil Taxonomy

• Objectivity. Based on measurable criteria, eliminating subjectivity and allowing different specialists to reach the same results.
• Universality. Applicable to all soils worldwide, regardless of region.
• Informativeness. Taxon names carry specific information about soil properties.
• Flexibility. The system can be changed and supplemented without disruption (there have already been several significant revisions, e.g., the introduction of Gelisols in 1999) (Soil Survey Staff, 1999; Huang et al., 2012).
• Practical orientation. Used for soil mapping, land cadastre, predicting soil behavior under different uses (Eash et al., 2016).

5.8. Disadvantages and Criticism

• Complexity. Accurate classification requires laboratory data, not always available in the field. Especially in developing countries where soil survey services are poor.
• Cost. Determining many diagnostic traits requires expensive analyses (e.g., CEC of clay, oxalate‑extractable aluminum and iron, etc.).
• Artificiality of numerical thresholds. Why exactly 35% base saturation separates Ultisols from Alfisols? It is a convention, albeit supported by empirical correlations.
• Detachment from landscape. Unlike genetic classification, Soil Taxonomy does not give direct information about genesis, climate, vegetation. For an agronomist accustomed to the zonal principle, this can be unfamiliar.
• Nomenclature is difficult to memorize. Names like "Oxyaquic Fragiudalfs" intimidate beginners and require time to learn.
• Climatic criteria. Use of soil moisture and temperature regimes makes the system dependent on climatic data, which are not always reliable (especially under changing climate) (Buol et al., 2011; White, 2006).

5.9. Comparison with WRB: Two Systems, Two Approaches

We have already mentioned that there are two international systems: Soil Taxonomy and WRB. What is their fundamental difference?

Soil Taxonomy is a rigid hierarchical taxonomy, with fixed categories and rules. Its goal is to unambiguously identify any soil in the world using a standard procedure. It is a system for classification in the strict sense.

WRB is a more flexible reference system that does not impose a rigid hierarchy. It has 32 Reference Soil Groups (highest level) and two types of qualifiers (prefixes and suffixes) that allow detailed description. WRB deliberately avoids climatic criteria and does not have as deep a hierarchy as Soil Taxonomy. It is a system for correlation and communication (IUSS Working Group WRB, 2014; Huang et al., 2012; Buol et al., 2011).

Why are both needed?

• Soil Taxonomy—for countries with developed soil survey and laboratory capacity, where complex diagnostic criteria can be applied. Primarily for the USA and many other countries.
• WRB—as a universal language for international projects, especially where complex laboratory analyses are not possible. WRB is often used in Europe, Asia, Africa for global maps and databases (e.g., Harmonized World Soil Database).

Many countries use both systems in parallel: the national one for internal use, and WRB for international publications (Scheffer et al., 2018; Ganzhara et al., 2002).

5.10. Brief Summary

Soil taxonomy (in particular Soil Taxonomy) is a tool that allows:

• Identify soil by objective traits.
• Predict its properties and behavior.
• Communicate among specialists from different countries.
• Make decisions in land use.

Despite criticism, it is precisely this approach—diagnostic, quantitative, hierarchical—that has become the standard of modern soil science. Understanding the logic of taxonomy is not just academic knowledge but a practical skill for an agronomist, allowing him to "read" the soil and its characteristics from its name.

In the next section, we will examine WRB in more detail—how it differs from Soil Taxonomy, how its Reference Soil Groups and qualifiers are structured, and why it does not replace national systems.

6.1. Prehistory: from the Soil Map of the World to WRB

The history of WRB begins with the grand Soil Map of the World project at scale 1:5,000,000, launched by FAO and UNESCO in 1961. The map was published in 1974 together with a Legend containing 26 major soil groups and 106 soil units (FAO‑UNESCO, 1974; Huang et al., 2012). This was a mono‑categorical system designed for mapping, not for detailed classification. In 1988, a revised legend was issued, with the number of major groups increased to 28 and units to 153, and a two‑level structure appeared (FAO‑UNESCO, 1988). This system became the direct predecessor of WRB.

In the 1990s, the International Society of Soil Science (ISSS, now IUSS), FAO, and ISRIC (International Soil Reference and Information Centre) began work on creating a world reference base that would unify and harmonize existing national systems. The first official version of WRB was adopted in 1998 at the 16th World Congress of Soil Science in Montpellier, and a second, substantially revised version appeared in 2006 (IUSS Working Group WRB, 2006, 2014; see also Huang et al., 2012; Buol et al., 2011).

6.2. Structure of WRB: Two Levels of Detail

Unlike the multi‑level hierarchy of Soil Taxonomy (6 categories), WRB has only two levels (IUSS Working Group WRB, 2014; Huang et al., 2012):

1. Reference Soil Groups (RSG)—the highest level (32 groups). This is analogous to "orders" in Soil Taxonomy, but the groups are broader and more "genetically" oriented.

2. Qualifiers—refining traits added to the group name. They are divided into prefix qualifiers (placed before the group name) and suffix qualifiers (placed in parentheses after the group name).

Thus, a full WRB soil name might look like:

Calcic Mollic Vertisol (Pellic) (example from IUSS Working Group WRB, 2014; Scheffer et al., 2018)

This means: main group Vertisol (cracking clay soil with high‑shrinkage clay), prefix qualifiers Calcic (with carbonates) and Mollic (with mollic horizon), suffix qualifier Pellic (very dark, with low value and chroma).

6.3. Reference Soil Groups (RSG)—32 Groups

WRB has 32 Reference Soil Groups (in the 2014 version, 32 groups—Technosols and Stagnosols were added; in 1998 there were 30). They are listed in strict key order (as in Soil Taxonomy). The key starts with the most "special" soils and ends with the simplest (IUSS Working Group WRB, 2014; Huang et al., 2012):

• Histosols—organic soils (peat, muck) more than 40 cm thick.
• Anthrosols—strongly modified by humans (rice paddies, irrigated terraces, deep organic amendment).
• Technosols—soils with high content of technogenic materials (concrete, glass, slag, construction debris)—a new addition reflecting the Anthropocene.
• Cryosols—soils with permafrost (close to Gelisols but with a cryoturbation criterion).
• Leptosols—shallow (<25 cm to rock) or very skeletal soils.
• Vertisols—cracking clay soils (similar to Vertisols in Soil Taxonomy).
• Fluvisols—young alluvial soils.
• Solonetz—with a sodium‑rich horizon.
• Solonchaks—saline (with a salt horizon).
• Gleysols—groundwater‑gleyed, waterlogged.
• Andosols—volcanic (analogous to Andisols).
• Podzols—podzols (analogous to Spodosols).
• Plinthosols—with plinthite (ferruginous material that hardens on exposure).
• Nitisols—nitisols (clayey, with shiny aggregate surfaces, typical of tropics).
• Ferralsols—ferralitic, deeply weathered (analogous to Oxisols).
• Planosols—with abrupt textural change and signs of wetness.
• Stagnosols—with signs of stagnant wetness (analogous to pseudogleys).
• Chernozems, Kastanozems, Phaeozems—three groups with dark humus horizons (analogues of various chernozem subtypes and meadow‑steppe soils).
• Gypsisols, Durisols, Calcisols—with accumulation of gypsum, silica, or carbonates.
• Retisols—with an argic horizon and tonguing boundary (close to albeluvisols or ferralsols?).
• Acrisols, Alisols, Luvisols, Lixisols—four groups with an argic horizon, differing in clay activity and base saturation (these are groups that in Soil Taxonomy would be distributed between Alfisols and Ultisols, but with account of clay activity).
• Umbrisols—with umbric horizon (dark, acid).
• Arenosols—sandy (deep sands, analogous to Psamments).
• Cambisols—with cambic horizon (weak alteration, analogous to many Inceptisols).
• Regosols—weakly developed, without pronounced horizons (analogous to Entisols).

Important: The order of groups in the key is strict. If a soil meets the criteria for Histosols, it is classified as Histosol, even if it has other traits (e.g., technogenic materials). So one must always follow the key from top to bottom (IUSS Working Group WRB, 2014).

6.4. Qualifiers—Second Level of Detail

Each Reference Soil Group can be refined using qualifiers—traits that are not mandatory for the group but add important information. Qualifiers are divided into two categories (IUSS Working Group WRB, 2014; Huang et al., 2012):

Prefix qualifiers (before the group name)—they denote features that modify the main group characteristic and often indicate a transition to another group. For example, in the Vertisol group, prefixes can be:

• Mollic—presence of a mollic epipedon,
• Calcic—presence of a calcic horizon,
• Gypsic—presence of a gypsic horizon,
• Gleyic—signs of gleying,
• Sodic—high sodium content.

Prefixes are placed in order of importance and usually reflect the most significant properties for soil use.

Suffix qualifiers (in parentheses after the name)—they describe additional properties common to many groups (texture, depth, pH, concretions, etc.). Suffixes are listed alphabetically (inside parentheses). Examples of suffix qualifiers:

• Arenic, Loamic, Siltic, Clayic—particle‑size class.
• Dystric, Eutric—low or high base saturation.
• Calcaric—presence of primary carbonates.
• Humic, Ochric—type of epipedon.
• Episalic, Hypersalic—degree of salinity.
• Endoleptic—presence of bedrock within 1 m.
• Novic—young soils with a sedimentary layer 5–50 cm.

Thus, a full name may contain several prefixes and several suffixes, but it is always a limited set, and names do not become overly long.

Example classification by WRB (from Scheffer et al., 2018): the German soil Parabraunerde‑Pseudogley (old genetic term) would be classified as Eutric Luvic Stagnosol (Siltic). Decoding: Stagnosol—main group (soil with stagnant wetness), Luvic—there is an illuvial horizon (clayey), Eutric—high base saturation, Siltic—silty loam texture.

6.5. Diagnostic Horizons, Properties, and Materials in WRB

Like Soil Taxonomy, WRB relies on diagnostic horizons, properties, and materials. However, in WRB they are less rigidly quantified than in Soil Taxonomy, and often rely on morphological features observable in the field (IUSS Working Group WRB, 2014; Scheffer et al., 2018; Huang et al., 2012).

Main diagnostic horizons of WRB (list from IUSS Working Group WRB, 2014; see also Scheffer et al., 2018; Huang et al., 2012):

Surface and near‑surface:

• Mollic—dark, base‑saturated, structured.
• Umbric—like Mollic but with low base saturation.
• Chernic—very dark, with very high base saturation, characteristic of chernozems.
• Melanic—black, andic (volcanic), very rich in organic carbon.
• Histic—organic, waterlogged.
• Folic—organic, well aerated (dry).
• Hortic, Plaggic, Terric—anthropogenic (related to long‑term cultivation, organic amendments, irrigation).

Subsurface (illuvial, etc.):

• Argic—clay accumulation (close to Argillic and Kandic of Soil Taxonomy, but combines both).
• Natric—like Argic but with high sodium.
• Spodic—accumulation of organic matter and aluminum (with or without iron).
• Ferralic—highly weathered, low exchange capacity (analogous to Oxic, but with additional criteria on silt/clay ratio).
• Cambic—horizon with signs of alteration (color, structure).
• Calcic—accumulation of secondary carbonates.
• Gypsic—accumulation of gypsum.
• Salic—accumulation of soluble salts.
• Plinthic—iron‑clay material that hardens upon drying.
• Petroplinthic, Pisoplinthic—hardened forms of plinthite.
• Duric—silica concretions (durinodes).
• Petrocalcic, Petrogypsic, Petroduric—cemented (calcite, gypsum, silica).

Horizons related to reducing conditions:

• Anthraquic—rice paddy: mixed layer and plough pan.
• Hydragric—horizon with iron accumulation under flooding.
• Gleyic—signs of gleying (reduction).
• Stagnic—stagnant wetness with mottling.

Diagnostic properties of WRB (IUSS Working Group WRB, 2014; Scheffer et al., 2018):

• Andic—properties of volcanic soils (low density, high allophane content, etc.).
• Vitric—presence of volcanic glass.
• Abrupt textural change—sharp increase in clay.
• Albeluvic tonguing—tonguing boundary between albic and argic horizons.
• Gleyic colour pattern—reduction colours.
• Stagnic colour pattern—oxidation and reduction mottles under stagnant wetness.
• Aridic, Yermic, Takyric—properties of arid soils.

Diagnostic materials:

• Organic, Mineral—material dominated by organic or mineral component.
• Calcaric, Gypsiric, Dolomitic—carbonatic or gypsiferous materials.
• Sulfidic, Hypersulfidic—sulphidic materials, hazardous upon drainage (acid sulphate soils).
• Artefacts—artificial materials (of human origin).

WRB uses these diagnostic concepts to define Reference Soil Groups and to select qualifiers.

6.6. Why Doesn't WRB Replace National Systems?

Many students ask: "If there is WRB—an international system—why do we need national classifications?" The answer is multifaceted.

1. The purpose of WRB is correlation, not replacement. WRB was originally conceived as a Reference Base to allow national systems to be "translated" into each other. It is a "common denominator," not a system that should displace national ones (FAO‑ISRIC‑ISSS, 1998; IUSS Working Group WRB, 2014; Huang et al., 2012). As stated in WRB documents, it is not intended to replace national systems but to serve as a bridge between them.

2. WRB does not use climatic criteria. Unlike Soil Taxonomy, where soil moisture and temperature regimes are crucial differentiating traits, WRB deliberately avoids them. This is done so that the classification does not depend on climatic data, which are not always available in developing countries or may be inaccurate. However, for many agronomic and environmental tasks, climatic information is extremely important, so national systems often retain it (IUSS Working Group WRB, 2014; Buol et al., 2011).

3. WRB is more general (less detailed). WRB has only two levels (group + qualifiers), whereas Soil Taxonomy has six. This makes WRB convenient for global generalizations (world maps, databases), but insufficiently detailed for local tasks (e.g., fertilizer decisions, irrigation, reclamation at the field level) (White, 2006; Weil & Brady, 2017).

4. National systems account for local features. Classifications of Russia, China, Brazil, Germany include horizons and properties important specifically for those countries (e.g., Chinese rice soils, Russian cryogenic processes, German water regimes). WRB cannot cover all these nuances with sufficient detail (Ganzhara et al., 2002; Huang et al., 2012; Scheffer et al., 2018).

5. National systems are tied to land cadastre, legislation, and traditions. In each country, there are established soil maps, cadastral evaluations, reclamation projects based on the national classification. Switching to WRB would require huge costs for retraining personnel, reissuing maps, revising laws. This is economically and organizationally impractical.

Therefore, in practice, bilingualism is used: for internal purposes—national system, for international publications—WRB (or Soil Taxonomy). The ability to "translate" from one system to another is an important professional skill.

6.7. Comparison of WRB and Soil Taxonomy: Brief Table

6.8. Brief Summary

WRB is not a competitor to Soil Taxonomy but its complement. It solves the task of international communication, correlation, and creation of global databases. It is simpler, more flexible, and less demanding on laboratory data, making it accessible to countries with limited resources. But precisely because of its simplicity, it cannot replace detailed national systems that take into account local climatic, lithological, and anthropogenic features.

For an agronomist, knowledge of WRB is useful because it allows reading international literature, participating in global projects, and understanding how soil properties are described in an international context. However, in practice, when making decisions on his field, he will most often rely on the national classification, which provides more accurate and locally relevant information.

In the next section, we will answer the question that often troubles students: "Why are there two international systems? Doesn't that create confusion?"

7.1. Different Purpose: Identification vs. Communication

This is the main difference from which all others derive.

Soil Taxonomy was created as a tool for identification and classification (Huang et al., 2012; Buol et al., 2011; Soil Survey Staff, 1999). Its task is to give the soil scientist an unambiguous way to determine which taxon a particular soil belongs to, using objective, measurable criteria. It is a "bottom‑up" system: from detailed laboratory analyses to generalized categories. It is rigid because it must ensure reproducibility of the result regardless of who performs the classification.

WRB was created as a tool for correlation and communication (IUSS Working Group WRB, 2014; FAO‑ISRIC‑ISSS, 1998; Huang et al., 2012). Its task is to provide a common language for soil scientists from different countries to discuss their soils without going into the intricacies of national classifications. It is a "top‑down" system: from general groups to refining qualifiers. It is flexible because it must be applicable in countries with different levels of laboratory capacity and different scientific traditions.

As stated in WRB documents: it is a "reference base," not a taxonomy that should replace national systems (IUSS Working Group WRB, 2014). Soil Taxonomy, on the other hand, is directly called a "comprehensive system of classification" (Soil Survey Staff, 1999).

7.2. Different History: American and European Paths

Soil Taxonomy was born out of the practical needs of the National Cooperative Soil Survey of the United States, which by the mid‑20th century had accumulated a vast amount of soil data on its territory and needed a unified system for mapping, cadastre, and interpretation. It was an internal task that eventually "grew" to global scale because the system proved successful (Buol et al., 2011; Smith, 1983).

WRB was born from the need for a unified legend for the Soil Map of the World (FAO‑UNESCO, 1974, 1988). The map had to cover all continents, and for this a language was needed that was understandable to soil scientists from different countries, even if their national classifications differed greatly. It was an international task from the start, and it was solved collectively with participation from many countries (Huang et al., 2012).

Thus, Soil Taxonomy is a system that "grew" from national experience and became international; WRB is a system "constructed" as international from the outset.

7.3. Different Methodological Principles

These differences are clear when compared (Buol et al., 2011; White, 2006; Weil & Brady, 2017):

Why did WRB abandon climatic criteria? In developing countries, long‑term observations of soil temperature and moisture are often lacking, and meteorological station data do not always accurately reflect soil microclimate. Climatic criteria would make the system inapplicable in these regions. WRB deliberately builds classification only on properties of the soil itself that can be observed or measured in the field or laboratory (IUSS Working Group WRB, 2014; Buol et al., 2006).

Why does Soil Taxonomy use climatic criteria? Because they proved to be powerful predictors of many other soil properties: type of weathering, leaching depth, organic matter accumulation, clay mineralogy. Climate is one of the five soil‑forming factors, and incorporating it into classification allows more accurate prediction of soil behavior. For the USA and other countries with well‑developed climatic networks, this was a natural choice (Soil Survey Staff, 1999; Buol et al., 2011).

7.4. Different Areas of Application

In practice, the choice of system depends on the task:

• Soil Taxonomy—for countries with developed soil survey services, for detailed mapping, for cadastre, for reclamation design, for scientific work where high identification accuracy is required. It is used in the USA and about 50 other countries (Buol et al., 2011; Weil & Brady, 2017).
• WRB—for international projects (databases, global maps, predictive models), for publications that should be understandable to scientists from different countries, for regions where complex laboratory analyses are not possible. It has become the standard for the European Union, FAO, ISRIC, and many international organizations (Huang et al., 2012; IUSS Working Group WRB, 2014).

Interestingly, several countries (e.g., Brazil, China, Australia) use both systems in parallel: the national one for internal needs, and WRB or Soil Taxonomy for international communication (Ganzhara et al., 2002; Gong & Zhang, 2007; EMBRAPA, 2006).

7.5. Differences in Nomenclature: Tradition vs. System

Nomenclature is also an important difference that often confuses people.

Soil Taxonomy uses artificial terms constructed from Latin and Greek roots. This was done deliberately to avoid confusion with old names that had different meanings in different countries (Heller, 1963; Buol et al., 2011). For example, "chernozem" in Russia means one thing; in the USA "Chernozem" was understood differently. Creating new terms (Alfisols, Mollisols, etc.) guaranteed that they would have only one, strictly defined meaning.

WRB, in contrast, retained many traditional names: Chernozems, Kastanozems, Podzols, Vertisols, Andosols, Histosols. This was done to make the system recognizable and more readily accepted by soil scientists accustomed to these terms. However, WRB gave these terms clear diagnostic definitions to avoid ambiguity (IUSS Working Group WRB, 2014; Huang et al., 2012).

Thus, Soil Taxonomy creates a "new language," while WRB "organizes the old one." Each approach has its advantages.

7.6. Why Has Harmonization Not Happened?

Ideally, one could develop a single international system that combines the best features of both. There have been and are attempts at harmonization. For example, WRB often borrows diagnostic horizons from Soil Taxonomy (though simplifying them), and Soil Taxonomy sometimes introduces new taxa under the influence of WRB experience (e.g., Gelisols were added to Soil Taxonomy in 1999 largely due to international discussions) (Soil Survey Staff, 1999; Huang et al., 2012).

But full harmonization faces serious obstacles:

1. Different philosophical foundations. Abandoning climatic criteria in Soil Taxonomy would be tantamount to rebuilding the whole system—too many taxa rely on them. Introducing them into WRB would make the system less accessible to developing countries (Buol et al., 2011).

2. Different depth of detail. WRB does not have categories analogous to family and series in Soil Taxonomy. Adding them would require radically changing WRB's structure, which is not in the developers' plans (IUSS Working Group WRB, 2014).

3. Political and organizational factors. Soil Taxonomy is under USDA, WRB under IUSS (International Union of Soil Sciences). These are different organizations with different decision‑making procedures. Harmonization would require lengthy and complex negotiations.

4. Inertia. Thousands of soil reports, maps, textbooks have been written using either Soil Taxonomy or WRB. Changing this is extremely costly and time‑consuming.

Therefore, at present, coexistence of the two systems with active correlation work is recognized as optimal. There are correlation tables between WRB Reference Soil Groups and Soil Taxonomy Orders (see, e.g., Weil & Brady, 2017, Appendix A; IUSS Working Group WRB, 2014). This allows scientists to "translate" names from one system to another.

7.7. For Students: How Not to Get Confused by Two Systems

Here are a few simple tips to help you navigate the world of soil classifications.

1. Accept that there are two, and that's normal. It is not an error, not a shortcoming, but a reflection of the complexity and multifaceted nature of the object (soil) and the multitude of tasks that classifications solve.

2. Understand the main purpose of each. Soil Taxonomy—for detailed identification, WRB—for international correlation and global generalizations. If your task is to understand the soil of your field, you will likely need the national system or Soil Taxonomy. If you are preparing a paper for an international journal, you need WRB (or both).

3. Don't memorize all names. Instead, learn to "read" diagnostic traits. What is a mollic epipedon? What is an argic horizon? What does the qualifier "Dystric" mean? When you understand the logic, names cease to be random strings of letters.

4. Use correlation tables. Textbooks (e.g., Weil & Brady, 2017, Appendix A; Huang et al., 2012, Chapter 32) contain tables showing which Soil Taxonomy Order corresponds to which WRB Reference Soil Group. Keep them handy.

5. Remember: classification is not an end in itself, but a tool. It is needed to better understand soil, predict its behavior, and make correct decisions. If you can describe the soil in understandable agronomic terms and make the right decision—you are already on the right track. Classification merely helps structure your knowledge.

7.8. Outlook: Convergence or Divergence?

In recent years, there has been a tendency toward convergence. WRB increasingly uses quantitative diagnostic criteria, approaching the style of Soil Taxonomy. Soil Taxonomy, in turn, has become more flexible and takes into account user feedback from other countries (e.g., the introduction of Andisols as a separate order, and then adjustment of its criteria) (Huang et al., 2012; Buol et al., 2011). In some countries, "hybrid" approaches are adopted, where the national system is built on Soil Taxonomy but with elements of WRB (as in China or Brazil).

Nevertheless, complete merger is not to be expected. Different goals require different tools. Perhaps in the future a more flexible, "modular" system will emerge, allowing the user to choose the level of detail and set of criteria depending on the task. But that is a distant future.

Meanwhile, it is important to remember: Soil Taxonomy and WRB are not competitors, but partners. Each is strong in its niche, and together they provide the soil scientist, agronomist, and ecologist with a full spectrum of tools for describing, understanding, and rational use of soils.

In the next, concluding section, we will move from theory to practice and answer the agronomist's main question: "How do I, as a farmer, use soil classification?"

8.1. Classification is Not an End, but a Means

Remember this as the main thesis: you should not know classification for the sake of classification. You should know it to:

• Understand the properties of the soil you work with.
• Predict its behavior under different uses.
• Make informed decisions about farming systems, fertilizers, reclamation, crop rotations.

Classification is like a library index: you don't memorize all the books, but you know how to find the one you need by its call number. Same here: you don't memorize all taxa, but you can "read" from the soil name its key properties and limitations (Buol et al., 2011; Eash et al., 2016).

8.2. You Don't Need to Memorize Names—You Need to Understand Diagnostic Traits

Instead of mechanically memorizing that "Typic Hapludalf" is one thing and "Aquic Hapludalf" another, understand the diagnostic horizons and regimes. They are the "bricks" from which the classification is built. If you understand what is:

• Mollic (dark, humus‑rich, with high base saturation)—you already know that this is a fertile soil, rich in organic matter, with good structure, and liming is probably not needed (Weil & Brady, 2017; Eash et al., 2016).
• Argillic (clay accumulation)—you know that such a soil has an illuvial horizon with increased clay content, which retains moisture and nutrients, but may create drainage problems under waterlogging (Buol et al., 2011).
• Aquic (water regime with saturation and reduction)—you know that the soil periodically becomes waterlogged, which may require drainage and influence crop selection (Eash et al., 2016).
• Kandic (accumulation of low‑activity clay, kaolinite)—you know that cation exchange capacity is low, nutrients are easily leached, and a well‑planned fertilizer system is needed (Weil & Brady, 2017).

When you know these "bricks," you read the soil name rather than memorize it. For example:

Fine‑loamy, mixed, active, mesic Typic Hapludalfs (Weil & Brady, 2017; Buol et al., 2011)

What does this tell the agronomist:

• Alf → Alfisol → has an argillic horizon, base saturation >35%, so the soil is not very acid, with good buffering.
• Ud → Udalf → udic moisture regime: humid most of the year, no long droughts.
• Hapl → Hapludalf → minimal development of other horizons (no fragipan, duripan, etc.), so the profile is relatively simple.
• fine‑loamy → texture: loamy, medium, not too sandy and not too clayey.
• mixed → mineralogy: mixed, no dominance of one mineral.
• active → high exchange activity (CEC/clay >0.6) → good ability to retain cations (Ca²⁺, Mg²⁺, K⁺).
• mesic → moderate temperature (8–15 °C) → temperate climate.

Now you can already make a prediction: this is a fertile, well‑drained, medium‑loamy soil in a temperate humid climate, with good nutrient‑holding capacity, without major limitations for most crops. And this without a single laboratory analysis—just from the name.

8.3. Use Classification as a "Label" for Quick Assessment

In practice, an agronomist rarely has time and opportunity to conduct a full range of analyses for each field. But if you know the soil classification (from a soil map or previous studies), you already have starting information:

• What is the likely soil texture?
• What is its water regime?
• Are there limiting horizons (fragipan, petrocalcic, duripan)?
• What are the acidity and base saturation?
• What is the risk of salinization or solonetzization?
• How sensitive is the soil to erosion?
• Does it require liming or gypsum application?

Of course, classification does not replace analyses. But it helps generate hypotheses and direct the search. Instead of analyzing everything, you can focus on those properties that are most likely to be problematic for that taxon (Eash et al., 2016; White, 2006).

8.4. Example: How Classification Helps Choose a Fertilizer Strategy

Consider a concrete example. You have two soils:

1. Typic Hapludalf (loamy, humid, Alfisol)

2. Aquic Paleudult (loamy, waterlogged, Ultisol)

What can you do knowing only these names?

For Typic Hapludalf:

• This is Alfisol → base saturation >35% → pH likely around 5.5–6.5 → liming may not be required or only in small doses.
• Udic regime → adequate moisture → good crop growth, but denitrification possible under excessive nitrogen in wet conditions.
• Loamy texture → medium water‑holding capacity, good buffering.
• Mixed mineralogy, active clay → holds potassium and magnesium well; losses due to leaching are small.
• Recommendation: moderate nitrogen doses considering moisture, potassium fertilizers at medium rates, liming according to pH.

For Aquic Paleudult:

• Ultisol → base saturation <35% → soil is likely acid → liming required.
• Aquic → periodic waterlogging → risk of nitrate leaching, denitrification, and possible aeration problems for sensitive crops.
• Pale → old, highly weathered → low‑activity clays (kaolinite) → low cation retention, especially potassium and magnesium → more frequent application of these elements required.
• Recommendation: mandatory liming (possibly high rates), split nitrogen application, higher potassium doses (accounting for possible leaching), selection of crops tolerant to temporary waterlogging (rice, some corn varieties).

Compare this volume of information with what you would get simply by seeing "Typic Hapludalf" and "Aquic Paleudult" on a map without understanding their meaning. Classification gives direct access to accumulated knowledge about soil properties and behavior (Eash et al., 2016; Weil & Brady, 2017; Buol et al., 2011).

8.5. Use National Classification and WRB as Complementary Tools

In Russia, you will most often encounter genetic names: sod‑podzolic, chernozems, chestnut, grey forest, etc. (Ganzhara et al., 2002; Shishov et al., 2004). These are not "outdated" terms—they carry a huge amount of information accumulated by the Russian school of soil science. For a Russian agronomist, this is the native language in which you can quickly assess soil and make decisions. For example, "typical chernozem" immediately conveys that the soil is rich in humus, structural, highly fertile, but may require moisture‑conserving technologies in dry years.

However, for international communication (publications, participation in projects, reading foreign literature), it is useful to know WRB (or Soil Taxonomy). For example, typical chernozem often corresponds to Chernozem in WRB, and sod‑podzolic to Retisol or Luvisol (with qualifiers). Knowing the correlations, you can easily "translate" your knowledge into an international language (IUSS Working Group WRB, 2014; Weil & Brady, 2017, Appendix A; Huang et al., 2012, Chapter 32).

8.6. Classification and Soil Maps: a Field Navigator

Soil maps are classifications applied to the landscape. On large‑scale maps (1:10,000 and larger), you see specific soil polygons with indices corresponding to certain taxa (Ganzhara et al., 2002, Chapter 6). This allows you to see field heterogeneity, plan variable‑rate fertilizer application, select crops for different areas, and design reclamation systems.

An agronomist who can "read" a soil map and understand classification indices gains a powerful advantage: he knows his field in advance, even before going out with a shovel and auger. He can plan not "blindly" but with consideration of real soil diversity.

8.7. Why It's Important to Distinguish Taxon from Mapped Unit

Here a caveat is in order, often overlooked. A taxon (e.g., series) is a conceptual class, not a specific area on the ground. On a soil map, you see map units named after the dominant taxon, but within them there are almost always other soils (inclusions) (Buol et al., 2011; Weil & Brady, 2017; Eash et al., 2016).

For example, a map may say "Sod‑podzolic medium‑loamy," but within that polygon there may be patches of sandy loam variants, weakly gleyed varieties, or even fragments of other soil types. Therefore, for detailed planning (e.g., for orchard planting or irrigation projects), you cannot rely solely on the map unit name—you need additional auger holes or test pits within the field to verify the real heterogeneity (White, 2006; Eash et al., 2016).

Classification gives general understanding, but in practice, verification on site is always needed.

8.8. What to Do if You Don't Know the Exact Classification?

If you don't have a ready soil map and cannot conduct a full classification, you can still use classification principles for rapid field assessment:

• Dig a soil pit to a depth of 1–1.5 m.
• Describe the horizons: color (Munsell), texture (field feel), structure, presence of carbonates (with HCl), pH (indicator paper or field pH meter), signs of gleying, clay coatings, etc.
• Compare with the diagnostic horizons we listed (mollic, argillic, spodic, calcic, etc.).
• Based on this, you can already determine the order (using a simplified key) and get a general idea of the properties. This will not replace a full classification but will give a working hypothesis.

8.9. Soil Evaluation and Agro‑Production Grouping—Applied Tools Based on Classification

Of particular interest to the agronomist are soil evaluation (bonitation) and agro‑production grouping—classifications created specifically for agricultural purposes (Ganzhara et al., 2002, Chapters 7–8).

• Bonitation is a point‑based assessment of soil quality (potential fertility) based on its properties (humus, thickness, pH, texture, erosion, etc.). Scores allow comparison of different soils by their productive potential, justification of rent rates, and planning of reclamation.
• Agro‑production grouping—combining soils into groups that are similar in crop cultivation conditions and require uniform agronomic and reclamation measures. This is a direct "translation" of classification information into the language of agronomic recommendations (Ganzhara et al., 2002, Chapter 7).

These tools are based on classification, but they are already "tuned" to the needs of farming. If you work on a farm, you will most likely need these—and to use them, it is important to know how soil properties (reflected in classification) affect plant growth and development.

8.10. Main Point: Classification Helps a System of Thinking

And finally, the most important thesis. Soil classification is not only a reference of properties. It is a way of systematic thinking. It trains you to:

• See soil as a system of interconnected horizons, not as a homogeneous mass.
• Understand that soil properties are not random but are regularly determined by soil‑forming factors.
• Predict changes in properties under altered use (e.g., what will happen to soil under plowing, drainage, irrigation, long‑term fertilization).
• Perceive the field as a mosaic, not a monolith, and plan differentiated practices.

It is precisely this systematic thinking that distinguishes a good agronomist, who makes informed decisions, from one who acts by rote.

8.11. Conclusion

Let us summarize everything we have discussed in this lecture.

Soil classification is not just an academic system. It is a practical tool that helps the agronomist:

• Identify the soil.
• Predict its properties and behavior.
• Justify decisions on fertilizers, reclamation, crop selection.
• Communicate with colleagues in a common language (both national and international).

You don't need to memorize taxa—you need to understand diagnostic traits and the logic of the system. If you master this, you will be able to "read" the soil from its name and make the right decisions in the field.

And remember: soil classification is not the ultimate truth, but an evolving system that reflects the current state of our knowledge. It will change—and that is normal. What matters is not what the soil is called, but what you know about it and how you use that knowledge.