Soil Classification & Distribution of Soils in the World

• Soil is the thin, living skin of the continents, rarely more than two or three metres deep. It is the medium in which climate, parent rock, relief, organisms and time are translated into the vegetation cover, and hence the biogeography, of a region.
  • It is the point of contact between the lithosphere and the biosphere: rock is weathered below, while plants and soil organisms work it from above.
• In the UPSC Geography Optional syllabus, soils sit where physical geography (weathering, pedogenesis) meets biogeography (zonal vegetation, biomes, ecosystem productivity).
• Soil classification is the grouping of soils with similar properties or a similar origin into named classes, so that soils can be mapped, compared and managed, and knowledge about one place can be transferred to another.
• Classifying something as heterogeneous as soil is inherently difficult. No scheme can be fully genetic (explaining why a soil formed) and fully morphological (describing exactly what it looks like) at the same time without losing some explanatory power or descriptive precision.

Approaches to Classifying Soil

Why soils are hard to classify

• Soils form a continuum. They change gradually across a landscape rather than falling into neat, discrete boxes, so every class boundary is drawn on a gradient.
• The unit is three-dimensional. A soil is a body with depth, the pedon, whose properties change from horizon to horizon; classification must decide which layers matter most.
• Many soils are polygenetic. Their profiles carry features inherited from older climates, so the present climate does not always explain them.
• Purposes differ. A pedologist wants to explain origin, a farmer or planner wants to know use and limitations; one scheme rarely serves both.

Genetic classification

• Genetic classification groups soils by their assumed mode of origin, chiefly the climate–vegetation combination believed responsible for a soil’s character.
  • This is the logic of the zonal system pioneered by the Russian pedologists Vasily Vasilyevich Dokuchaev and Konstantin Dmitrievich Glinka in the late nineteenth and early twentieth centuries.
  • Strength: it links soils directly to climate, vegetation and biomes, which makes it the natural bridge into biogeography.
  • Weakness: origin is inferred, not measured, so two workers can place the same soil in different classes.

Morphological classification

• Morphological (property-based) classification groups soils by observable, measurable properties of the profile itself: horizon sequence, colour, texture, base status and diagnostic layers, without presupposing an origin.
  • This is the philosophy of the USDA Soil Taxonomy, adopted internationally from the 1960s–70s, and of the World Reference Base for Soil Resources (WRB).
  • Strength: it is objective and replicable; the same soil gets the same name anywhere.
  • Weakness: it needs laboratory data and a heavy technical vocabulary.
• Other bases are used for practical ends:
  • Descriptive or regional classes by colour and place, such as the “red”, “black” and “alluvial” soils of India.
  • Technical or interpretive classes for use, such as the land capability classification of A. A. Klingebiel and P. H. Montgomery (1961), which ranks land from class I (few limitations) to class VIII (unfit for cultivation).

Basis	Genetic	Morphological
Groups soils by	Presumed origin (climate and vegetation)	Measured profile properties
Key units	Zonal, intrazonal, azonal; great soil groups	Diagnostic horizons; orders and reference soil groups
Main schemes	Dokuchaev–Sibirtsev–Glinka; Marbut; USDA 1938	USDA Soil Taxonomy; FAO–UNESCO legend; WRB
Strength	Intuitive; ties soils to biomes	Objective, quantitative, internationally comparable
Weakness	Subjective; relict soils misfit	Data-hungry; jargon-heavy

Development of soil classification

Year	Scheme or work	Contribution
1883–1886	Vasily Dokuchaev, Russian Chernozem and his genetic scheme	Soil as an independent natural body; normal, transitional and abnormal soils
1895–1899	Nikolai Mikhailovich Sibirtsev	Renamed the classes zonal, intrazonal and azonal
1914	Konstantin Glinka, Die Typen der Bodenbildung	Spread the zonal idea to the West; translated as The Great Soil Groups of the World (1927)
1927–1935	Curtis Fletcher Marbut	Pedalfers and pedocals; mature and immature soils
1938, revised 1949	USDA (Mark Baldwin, Charles Kellogg, James Thorp)	Zonal, intrazonal and azonal orders with great soil groups
1960–1975	Guy Donald Smith, the Seventh Approximation, then Soil Taxonomy	Property-based system of diagnostic horizons
1971–1981	FAO–UNESCO Soil Map of the World	First world soil map on one legend
1998–2022	World Reference Base (4th edition, 2022)	International correlation standard, 32 reference soil groups

• Both approaches remain live in India. The zonal system is the traditional vehicle for teaching soil–climate–vegetation correspondence. Soil Taxonomy is the scheme actually used by soil scientists, including the ICAR-National Bureau of Soil Survey and Land Use Planning (NBSS&LUP), Nagpur, for mapping and land-capability work.

The soil profile as the unit of classification

• Every scheme classifies the soil profile, the vertical section from the surface down to unweathered rock. A mature profile shows a sequence of horizons (O, A, E, B, C, R), each formed by addition, removal, translocation and transformation of material; the soil profile and its horizons are described in full separately.
• Organic vs mineral horizons: the O horizon (and, in some schemes, the upper A) is dominated by litter and humus; the E, B, C and R horizons are progressively mineral, reflecting less biological influence and more control by parent material with depth.
• Eluviation vs illuviation: eluviation washes soluble salts, clay and sesquioxides out of an upper horizon (typically E); illuviation re-deposits them at depth (typically B).
  • This single pair of processes explains the bleached E horizon of a podzol and the iron- and clay-enriched B horizon beneath it, and modern schemes turn such horizons into diagnostic horizons.

The Zonal System and Its Classes

Origin and principle

• The zonal system is one of the oldest and most influential classifications of soil. Vasily Dokuchaev, studying the Russian plain from the tundra to the steppe, saw that soil belts run parallel to climatic and vegetation belts: his law of soil zonality.
  • He treated soil as the product of interacting factors of soil formation, above all climate and organisms acting on parent rock over time.
  • Nikolai Sibirtsev gave the three classes their names; Konstantin Glinka organised them into great soil groups; Curtis Fletcher Marbut carried the scheme to the United States, and the USDA 1938 classification made zonal, intrazonal and azonal its three orders.
• The three classes:
  • Zonal soils: well-developed, mature soils whose character is dictated primarily by regional climate and vegetation, largely independent of the local parent rock.
  • Intrazonal soils: also well-developed and mature, but shaped by a dominant local factor (poor drainage, calcareous parent rock, salinity) that overrides the regional climatic signature.
  • Azonal soils: immature, poorly developed soils lacking a distinct profile, because too little time has passed for horizons to differentiate or because a steep slope strips material as fast as it weathers.

Zonal soils

• Zonal soils follow the great climatic and vegetation belts, so they are described here in sequence from the poles towards the equator and from humid to arid regions. Each belt matches a climatic region and a biome.

Tundra soils

• Tundra soils are governed above all by ground ice and permafrost, which produce complex patterning through freeze–thaw churning (cryoturbation).
  • On stable slopes, slow decomposition under cold and waterlogging leaves a peaty surface layer over a grey, gleyed mineral horizon.
  • On unstable slopes, soils remain thin and poorly developed because of solifluction.
• Solifluction is the slow, viscous downslope flow of the saturated, thawed active layer over an impermeable frozen substratum in periglacial and tundra environments.
  • Impacts: solifluction lobes and terraces, thin and immature soil profiles on slopes, disruption of plant roots, and slope instability that constrains roads, pipelines, buildings and farming in high latitudes and high altitudes.
• Extremes:
  • In the most extreme, vegetation-free settings, such as the brown polar desert soils of Antarctica, soils are humus-poor mineral skeletons.
  • The birch-forested tundra margins of the northern hemisphere develop Arctic brown forest soils with a thick, dark organic A horizon.
• In USDA terms, tundra and permafrost-affected soils are Gelisols; in the WRB they are Cryosols.

Podzols (“ash soils”)

• Podzols form in the cool, moist climates immediately south of the tundra, characteristically beneath coniferous (taiga) forest. The name is Russian for “under-ash”, after the colour of the leached layer.
• Dominant process: cheluviation, the chemical complexing and downward movement of iron, aluminium and humus by organic acids from conifer needle litter.
  • The result is the podzol’s diagnostic feature: a bleached, ash-grey E horizon stripped of iron and clay, above an enriched B horizon.
• Three variants, by what has concentrated in the B horizon:
  • Humus podzol: humus itself is washed down and builds a dark, humus-enriched B horizon.
  • Iron podzol: iron oxide concentrates strongly in the B horizon, giving a rust-coloured band.
  • Gley podzol: iron cements into a thin, hard iron pan that impedes drainage, so the soil above becomes waterlogged and gleyed.
• Use: podzols are strongly acidic and naturally infertile; they need liming and heavy fertiliser for crops and are mostly left under forest.
• They correspond closely to the Spodosols of Soil Taxonomy and the Podzols of the WRB.

Brown earths (brown forest soils)

• Brown earths lie equatorward of the podzol belt, under milder climates that support temperate deciduous forest.
• Leaching still occurs but is far less intense than in podzols:
  • Free calcium carbonate is absent from the upper profile.
  • Sesquioxides (iron and aluminium oxides) are not segregated into a pale horizon but stay dispersed through the profile, giving the soil its uniform brown colour.
  • Humus (mild mull humus) is well mixed by earthworms and less acidic than in podzols.
• Distribution and use: lowland Britain, much of temperate western and central Europe, the north-eastern United States and parts of East Asia; long cleared for mixed farming.
• They correspond broadly to Alfisols (Udalfs) and some Inceptisols in Soil Taxonomy, and to Luvisols and Cambisols in the WRB.

Prairie soils

• Prairie soils occupy the transitional, increasingly humid zone between the chernozems and the brown earths of the forest margin.
• They share features of both: base-rich like chernozems, but showing the incipient leaching of forest soils, with no free carbonate layer.
  • This reflects their position at the grassland–woodland ecotone, as in the tall-grass prairie of Iowa and Illinois.
• They are now among the most productive maize and soybean soils in the world (Mollisols, Udolls; WRB Phaeozems).

Chernozems (“black earths”)

• Chernozems are the classic soils of steppe and prairie grassland under a semi-arid to sub-humid, strongly continental climate.
• Formation:
  • Light-to-moderate rainfall causes only incomplete leaching, so a calcium-carbonate-rich horizon forms at depth instead of being washed out.
  • Dense grass roots and rhizomes die back each year and feed a deep, dark surface layer, commonly up to about a metre thick; burrowing animals (ground squirrels, earthworms) mix it.
• Colour paradox: the humus content of the dark layer is usually only about 4–10%. The intense black owes as much to calcium-saturated clay–humus complexes as to the organic matter itself.
• Structure and fertility: a well-developed crumb (granular) structure, neutral reaction and high base status make them among the most naturally fertile soils on Earth.
• Parent material: typically loess (wind-blown silt), which explains their concentration in the Russian and Ukrainian steppe, the North American prairies and north-eastern China.
• Use: the world’s great wheat and maize belts; their main hazards are drought and wind erosion.

Chestnut soils

• Chestnut soils occur on the more arid margin of the chernozem belt, under natural short-grass steppe.
• Compared with chernozems, the illuvial carbonate (calcic) layer sits closer to the surface and organic content is lower, reflecting the sparser grass cover of a drier climate; the surface is dark brown rather than black.
• Use: extensive grazing and dry-farmed wheat, with a constant risk of drought and wind erosion, as in the American Dust Bowl of the 1930s; Soil Taxonomy places them among the Mollisols (Ustolls), the WRB as Kastanozems.

Sierozems (grey desert soils)

• Sierozems occur in arid and semi-arid regions and can be understood as an extreme extension of chestnut soils: lime and gypsum are drawn even closer to the surface by strong upward capillary movement under intense evaporation.
• Vegetation is sparse and drought-adapted, so litter input is minimal and organic matter is very low, giving the characteristic pale grey colour.
• Red desert soils are their counterpart in hotter deserts, coloured by iron oxides, often with gravel or stony desert pavement on the surface.
• Use: despite low natural fertility, sierozems become highly productive once irrigated, because their high base status is intact and unleached; salinisation is the price of careless irrigation.
• They correspond to the Aridisols of Soil Taxonomy and to the Calcisols and Gypsisols of the WRB.

Mediterranean red and brown soils

• Under the winter-rain, summer-drought climate of the Mediterranean lands, soils are leached in winter and dry out in summer.
  • Clay moves down in the wet season, and iron oxides redden the profile in the hot, dry season.
  • Natural vegetation is sclerophyllous scrub and woodland (maquis).
• They are classed as Alfisols (Xeralfs) and support olives, vines, citrus and winter wheat, but are easily eroded on hill slopes.

Tropical black soils (regur)

• Tropical black soils are dark, clay-rich soils of savanna and other grass-covered tropical areas with a marked wet–dry seasonal climate.
• There are no clearly differentiated eluvial or illuvial horizons: the whole solum is uniformly rich in bases, especially calcium, which accounts for the dark colour along with fine humus and titaniferous minerals.
• Hallmark: intense dry-season cracking, caused by the swelling and shrinking of montmorillonite (smectite) clay with wetting and drying.
  • Surface material falls into the cracks, so the soil churns itself (self-mulching) and builds the hummocky gilgai micro-relief.
• India: known as regur or “black cotton soil”, concentrated on the Deccan lava plateau. Its high moisture-holding capacity suits rain-fed cotton, sorghum, soybean and pulses.
• They correspond to the Vertisols of both Soil Taxonomy and the WRB.

Ferralsols and laterites (tropical red and yellow soils)

• Soils of the humid tropics are commonly, if loosely, called “lateritic”. Strictly, laterite is a weathering product, an iron- and aluminium-rich crust that hardens on exposure, rather than a soil class in itself.
• Most soils of this environment are more precisely termed Ferralsols:
  • They are rich in sesquioxides of iron and aluminium, which give the red, brown or occasionally yellow colour (yellow where the iron is hydrated).
  • In a typical profile, the A horizon (often through the first metre) is acidic with low humus, while the B horizon, commonly fifteen metres or more deep, is predominantly clayey (kaolinite).
• Ferrallitisation: intense chemical weathering under high temperature and heavy rainfall leaches away silica and bases, leaving iron and aluminium oxides behind. The underlying process of laterisation is treated with soil genesis.
• The tropical paradox: Ferralsols are low in natural fertility, with few exchangeable bases and little humus, yet they carry the world’s most luxuriant vegetation.
  • Nutrients are held in the living biomass and recycled rapidly from the litter, not stored in the soil. Once the forest is cleared, fertility collapses within a few seasons, which is central to debates on tropical rainforest degradation and shifting cultivation.
• Use: plantation tree crops (rubber, oil palm, cocoa, tea, coffee); with heavy liming and fertiliser, the Oxisols of Brazil’s Cerrado have become a major soybean region.
• They correspond to the Oxisols and Ultisols of Soil Taxonomy and to the Ferralsols, Acrisols and Plinthosols of the WRB.

Chernozem and sierozem compared

Feature	Chernozem	Sierozem
Climate	Semi-arid to sub-humid steppe; continental	Arid desert and semi-desert
Natural vegetation	Tall and mixed grassland	Sparse desert scrub
Colour	Black or very dark brown	Pale grey
Organic matter	Moderate (about 4–10%) in a thick dark layer	Very low
Carbonate horizon	Deep in the profile	Very close to the surface; gypsum and salts common
Structure	Crumb (granular)	Weak; often a surface crust
Natural fertility	Very high, among the world’s best	Low naturally; high once irrigated
Equivalents	Mollisols; WRB Chernozems	Aridisols; WRB Calcisols, Gypsisols
Regions	Ukraine, Russian steppe, Prairies, NE China	Central Asian deserts, Iranian Plateau, American south-west

Intrazonal soils

• Intrazonal soils are well-developed, mature soils whose character is dictated by a local factor, such as relief, drainage or unusual parent material, strong enough to override the regional climatic signature. They occur as islands within several zonal belts. Three families are recognised.

Hydromorphic soils

• Hydromorphic soils have undergone gleying and are associated with marshes, swamps or any poorly drained site.
• Gleying is waterlogging and chemical reduction:
  • Water displaces air in the pores, and micro-organisms decomposing organic matter rapidly consume the remaining oxygen.
  • The resulting anaerobic (reducing) conditions turn iron into its soluble ferrous form, colouring the soil grey, blue or green, with rusty mottles where air re-enters along roots and cracks.
• Two positional types:
  • Groundwater gleys, where a high water table saturates the lower profile.
  • Surface-water gleys, where poor internal drainage waterlogs the upper soil whatever the depth of the water table.
• Peat (bog) soils are the extreme case: waterlogging so complete that dead plants barely decay and pile up as organic layers (Histosols).

Calcimorphic soils

• Calcimorphic soils develop directly on calcareous parent material.
  • Rendzinas are thin, dark, organic-rich soils over chalk or soft limestone, classically on the chalk downs of Britain.
  • Terra rossa is a predominantly mineral soil typical of the Mediterranean region. Its upper horizons are clay-rich and vividly red, in sharp contrast to the pale limestone beneath; the colour comes from residual iron oxides left as the more soluble carbonate dissolves away.

Halomorphic (saline) soils

• Halomorphic soils occur mainly in deserts and poorly drained interior basins, where evaporation concentrates salts at or near the surface.
  • Solonchak (white alkali soils): form in depressions and show white salt crusts in dry periods; dominated by neutral salts such as sodium chloride and sulphate.
  • Solonetz (black alkali soils): the product of intense alkalinisation, with sodium carbonate, high pH, and a dark, dispersed clay surface over a hard, columnar subsoil.
  • Solodic soils (soloth): form when leaching under excess sodium removes clays and sesquioxides, producing a bleached, eluviated horizon that superficially resembles a podzol.
• India: the usar (reh, kallar) lands of the Indo-Gangetic plains, the coastal salt lands and the Rann of Kachchh are halomorphic soils.

Azonal soils

• Azonal soils are built largely by deposition from agents of erosion: fine rock particles transported from distant source areas rather than weathered in place.
• They are inherently immature and lack a well-developed profile, either because too little time has passed for horizons to form or because a very steep slope strips material as fast as it accumulates.
  • Alluvial soils on active floodplains show little or no profile development because each flood buries them under fresh sediment, as on the Indo-Gangetic plains.
    • In India, the new alluvium of the floodplains (khadar) is azonal; the older terrace alluvium (bhangar), with lime nodules (kankar), has begun to develop a profile.
  • Regosols are made of dry, loose dune sand or wind-blown loess that lacks a cohesive structure.
  • Lithosols are thin accumulations of imperfectly weathered rock fragments on steep slopes, where erosion removes material almost as fast as weathering produces it.
  • Fresh volcanic ash and glacial outwash are further azonal parent materials.
• Importance of azonal soils:
  • Floodplain and delta alluvium is renewed by each flood, deep, well-watered and easy to work. It feeds the rice and wheat bowls of the Ganga, Nile, Mekong and Yangtze and supports some of the highest rural population densities on Earth.
  • Volcanic ash soils weather quickly into fertile soils, which is why Java carries dense farming populations.
  • Lithosols and regosols are mostly left to forestry, grazing and water-catchment protection, and are highly erodible once disturbed.
• In Soil Taxonomy, most azonal soils are Entisols, and immature but slightly more developed soils are Inceptisols. In the WRB they are Fluvisols, Regosols, Arenosols and Leptosols.

Basis	Intrazonal soils	Azonal soils
Maturity	Mature, with a well-developed profile	Immature, with little or no profile
Controlling factor	A local factor overriding climate: drainage, lime, salt	Time too short, or slope or deposition too active
Location	Islands within zonal belts: marshes, limestone, salt basins	Floodplains, deltas, dunes, steep slopes, ash fields
Main types	Hydromorphic, calcimorphic, halomorphic	Alluvial soils, regosols, lithosols
Examples	Gleys, peat, rendzina, terra rossa, solonchak, solonetz	Khadar, dune sands, Himalayan slope soils
Use	Often limited: drainage or reclamation needed	Alluvium very fertile; slope soils fragile

Pedalfers and pedocals

• Curtis Fletcher Marbut divided the mature (zonal) soils into two broad groups by moisture regime and base status. This pedalfer–pedocal division, together with sub-varieties distinguished by vegetation, temperature regime and rainfall, forms the general classification of soils, a simplified capstone to the zonal system.
• The controlling balance is rainfall against evaporation:
  • Where precipitation exceeds evaporation, water percolates right through the profile. Bases (calcium, magnesium, potassium, sodium) are leached out and iron and aluminium remain: pedalfers (ped + Al + Fe).
  • Where evaporation exceeds precipitation, the wetting front stops within the profile and calcium carbonate precipitates at that depth: pedocals (ped + calcium).
  • The drier the climate, the shallower the carbonate layer. It lies deep under chernozems, nearer the surface under chestnut soils and at the surface under sierozems.
• Boundary: in the United States, Marbut drew it near the 98th meridian, at roughly 60–75 cm (25–30 inches) of annual rainfall, separating the humid east from the dry Great Plains.

Feature	Pedalfers	Pedocals
Formation environment	Humid regions under rich forest (or rainforest) cover	Arid and semi-arid regions under grass or scrub
Water balance	Rainfall exceeds evaporation; leaching through the whole profile	Evaporation exceeds rainfall; little or no leaching
Composition	Enriched in aluminium and iron (pedALFer)	Retain calcium carbonate (pedoCAL)
Base status	Deficient in calcium, magnesium and potassium, which are leached out	Retain most plant nutrients
Reaction	Acidic	Neutral to alkaline
Colour	Varies: grey and rust (podzol), brown (brown earth), red (laterite)	Varies: black (chernozem) to brown and grey (sierozem)
Diagnostic layer	Clay- or iron-enriched B horizon	Calcic (lime) layer, nearer the surface as aridity rises
Examples	Podzols, brown earths, red-yellow soils, laterites and ferralsols	Chernozems, chestnut soils, sierozems and other desert soils

• India: the laterites of the Western Ghats and Meghalaya are pedalfers; the desert soils of Rajasthan are pedocals; the black soils of the drier Deccan, with their kankar nodules, show pedocal traits.

Critique of the zonal concept

• Despite its teaching value, the zonal classification has several well-recognised weaknesses.
• Zonal soil types recur across different climatic belts. Podzols, the “zonal” soil of cool continental climates, also form in maritime settings and even within the humid tropics wherever acidic sandy parent material and heavy leaching coincide (the giant podzols of Amazonian white sands).
• The azonal class is ambiguous. Azonal soils do not simply reflect insufficient time; they may result from persistent local factors (steep slope, continuous fresh deposition) that have arrested profile development over a genuinely long period.
• Profiles may reflect inherited, not present, climate. Many soils carry relict characteristics of earlier climatic regimes (palaeosols or “fossil soils”), such as old laterite crusts in now-dry lands or Pleistocene features in temperate soils.
• Climate is overweighted. On ancient, stable tropical shields, age and parent material explain soils better than present climate; on slopes, relief arranges soils in a repeating sequence, the catena, first described by Geoffrey Milne (1935) in East Africa.
• Classes are qualitative. Boundaries depend on judgement, not measurement, so correlation between countries was poor.
• Human influence is ignored. Paddy soils, terraced and irrigated soils and urban soils have no place in the scheme.
• These limitations motivated the shift to morphological, diagnostic-horizon-based systems: USDA Soil Taxonomy and the WRB.

USDA Soil Taxonomy

Origin and principles

• The United States Department of Agriculture, under Guy Donald Smith, built a classification founded on observed soil properties rather than presumed origin, through successive drafts called “approximations”.
  • The Seventh Approximation (1960) was published for testing; Soil Taxonomy followed in 1975 and its second edition in 1999.
  • The Andisols (1990) and Gelisols (1998) were added to the original ten orders, making twelve. The current Keys to Soil Taxonomy is the 13th edition (2022).
• Diagnostic horizons are the building blocks: layers defined by measurable limits of thickness, colour, organic carbon, base saturation, clay increase and so on.
  • Surface horizons (epipedons): for example the mollic (thick, dark, base-rich), umbric (dark but acid), ochric (pale, thin) and histic (peaty) epipedons.
  • Subsurface horizons: for example the argillic (clay-enriched), spodic (humus–iron–aluminium enriched), oxic (highly weathered oxides), cambic (weakly altered), calcic (lime), natric (sodium-rich clay) and albic (bleached) horizons.
• Soil moisture regimes (aquic, udic, ustic, xeric, aridic) and soil temperature regimes (cryic, frigid, mesic, thermic, hyperthermic) bring climate in as measured soil properties, not as an assumption.

The hierarchy and names

• Soil Taxonomy has six categories: order → suborder → great group → subgroup → family → series. Twelve orders branch into dozens of suborders, hundreds of great groups and, in the United States alone, tens of thousands of series.
• Names are built from formative elements, so each name describes the soil:
  • Typic Hapludalfs: alf = Alfisol (order); ud = humid moisture regime (suborder); hapl = simple horizons (great group); Typic = the central concept (subgroup).
  • Likewise Ustolls are Mollisols of a dry-season (ustic) climate and Udox are Oxisols of a humid climate.

The twelve orders

Order	Name meaning	Key diagnostic feature	Share of ice-free land	Rough classical equivalent
Alfisols	Al + Fe, from pedalfer	Clay-enriched (argillic) horizon; moderate to high base status	About 10%	Brown earths, Mediterranean soils, many red soils
Andisols	Japanese ando, dark soil	Formed in volcanic ash; light, porous, often dark and fertile	About 1%	Volcanic ash soils
Aridisols	Latin aridus, dry	Dry soils of deserts; weak horizons, lime or salt layers	About 12%	Sierozems, desert soils
Entisols	from recent	No significant horizon development; very young	About 18%	Azonal soils
Gelisols	Latin gelare, to freeze	Permafrost within 1–2 m; churned by cryoturbation	About 9%	Tundra soils
Histosols	Greek histos, tissue	Organic soils: peats and mucks	About 1%	Peat and bog soils
Inceptisols	Latin inceptum, beginning	Young soils with only weakly developed horizons	About 15%	Young alluvium, mountain soils
Mollisols	Latin mollis, soft	Thick, dark, base-rich mollic epipedon; grassland soils	About 7%	Chernozem, chestnut, prairie soils
Oxisols	French oxide	Highly weathered, oxide-rich oxic horizon; humid tropics	About 7.5%	Ferralsols, laterites
Spodosols	Greek spodos, wood ash	Bleached eluvial horizon over an iron–humus spodic horizon	About 4%	Podzols
Ultisols	Latin ultimus, last	Strongly leached, base-poor, with a subsurface clay horizon	About 8%	Red-yellow podzolic soils
Vertisols	Latin verto, turn	Self-mixing, cracking clays rich in smectite (montmorillonite)	About 2.4%	Regur, tropical black soils

• Entisols, Inceptisols and Aridisols are the most extensive orders; Andisols and Histosols the least.

Orders grouped by controlling factor

• Grouping the twelve orders by the factor each best illustrates (maturity, climate, parent material or organic-matter accumulation) is more instructive than listing them alphabetically.

Soils characterised by maturity

• Entisols and Inceptisols can occur in virtually any climate or latitude, from equatorial to Arctic, because their defining feature is youth rather than a particular environment.
  • Entisols are mineral soils without distinct horizons. They count as “soil” chiefly because they support plant life, often on recently deposited material (floodplains, dunes, fresh volcanic ash).
  • Inceptisols show only weakly developed horizons, again usually because the soil is young.
  • The Entisols and Inceptisols of floodplains and deltas in warm, moist climates are, ironically, among the most productive soils in the world: favourable texture, nutrients renewed by fresh sediment and large water-holding capacity. The Indo-Gangetic and Brahmaputra plains are the classic Indian example.
• Alfisols and Spodosols are more mature and are told apart chiefly by climate and base status:
  • Alfisols have a clay-rich (argillic) horizon formed by illuviation and keep a relatively high base status. They span an extraordinary latitudinal range, from about 60°N in North America and Eurasia to equatorial South America and Africa.
    • Four suborders map onto climates: Boralfs (cold boreal forest; grey surface, brownish subsoil; now merged into Cryalfs), Udalfs (brownish, humid mid-latitudes), Ustalfs (brownish to reddish, warmer climates with a dry season) and Xeralfs (Mediterranean climate, cool moist winters and dry summers).
  • Spodosols have a pale, eluviated albic horizon above a dense, illuviated spodic horizon. They develop under cold needle-leaf forests and are strongly correlated with areas recently covered by Late Cenozoic ice sheets.
    • They are naturally poor for farming and strongly acidic, needing lime; this order is the USDA equivalent of the zonal podzol.
• Oxisols and Ultisols are the most weathered end of the maturity spectrum:
  • Oxisols develop in the moist equatorial, tropical and subtropical zones on land surfaces that have stayed geomorphologically stable for very long periods, so weathering has run almost to completion. They dominate vast areas of South America and Africa under rainforest and savanna: the USDA equivalent of the ferralsols and laterites.
  • Ultisols are similar but keep a subsurface clay (argillic) horizon. They occur through South-East Asia and the East Indies, eastern Australia, Central and South America and the south-eastern United States, and are highly vulnerable to erosion on steep slopes.
• The weathering sequence: under long humid weathering, a soil tends to pass from Entisol → Inceptisol → Alfisol (or Mollisol) → Ultisol → Oxisol, losing bases and silica at each step.

Soils characterised by climate

• Mollisols: soils of grasslands in sub-humid to semi-arid climates, with a thick, dark, base-rich surface (mollic epipedon).
  • Their loose crumb structure and high base status make them highly productive. They dominate the Great Plains, the Columbia Plateau and northern Great Basin of North America, the Pampas of Argentina and Uruguay, north-east China, and a great steppe belt from Romania across Russia and Siberia to Mongolia.
  • They are the USDA equivalent of the zonal chernozem–chestnut–prairie continuum.
• Aridisols: desert soils with weakly developed horizons, often with a subsurface accumulation of calcium carbonate, gypsum or soluble salts. They correspond to the dry tropical, dry subtropical and dry mid-latitude climates and to the zonal sierozems; with irrigation and management they can become fertile.
• Gelisols: soils of permafrost regions, churned by freeze–thaw action, generally on recent glacial parent material with slowly decaying organic matter; the zonal tundra soils.
  • Permafrost soils hold a very large store of frozen organic carbon, which thaw under present warming is beginning to release.

Soils characterised by parent material

• Vertisols develop from clay-rich, base-rich parent material that weathers to smectite clay, chiefly basalt and other basic rocks, but also limestone, marl and fine alluvium, in wet–dry climates under grassland or savanna.
  • They expand and contract with wetting and drying, opening deep cracks. Their black colour and high montmorillonite content mark them as the USDA equivalent of the zonal regur.
  • India’s Deccan Plateau, where basalt weathers into montmorillonite, is a defining Vertisol region of the world, along with the Gezira of Sudan, the Ethiopian highlands, eastern Australia and the Texas Blackland Prairie.
• Andisols form on relatively recent volcanic ash. They are typically dark, light, porous and fertile, and occur across a wide range of latitudes and climates wherever volcanism has deposited ash (Japan, Indonesia, the Andes, the East African Rift, New Zealand).

Soils high in organic matter

• Histosols are organic soils, peats and mucks, formed mostly in cool-to-cold climates with poor drainage. They occur in patches through the Spodosol-dominated northern lands and, in the tropics, as the peat swamps of Indonesia and Malaysia.
  • A very high proportion of organic matter is concentrated in a thick, dark upper layer. Peatlands cover only about 3% of the land yet hold roughly a third of all soil carbon, so draining or burning them releases large amounts of carbon dioxide.

Strengths and limitations of Soil Taxonomy

• Strengths: precise, quantitative definitions; the same soil receives the same name everywhere; names carry information; it serves mapping, land evaluation and technology transfer, and many countries, including India, use it.
• Limitations: it needs laboratory analyses and long-term soil climate data that many countries lack; its jargon is hard for non-specialists; its class limits were set largely on United States soils; and it is less intuitive than the zonal scheme for linking soils to vegetation and biomes.

FAO–UNESCO and the World Reference Base

The FAO–UNESCO Soil Map of the World

• The FAO–UNESCO Soil Map of the World (scale 1:5,000,000, published in ten volumes from 1971 to 1981) was the first map of the world’s soils on one common legend.
  • Its 1974 legend, prepared under Rudi Dudal, had 26 major soil groupings and 106 soil units. The revised legend of 1988 expanded this to 28 groupings and 153 units.
  • The names blend Russian terms (chernozem, podzol, solonchak, solonetz) with new coinages such as Ferralsols, Luvisols, Acrisols, Cambisols and Fluvisols.
• The world soil map below is drawn from a generalisation of this map’s dominant soil units.

The World Reference Base for Soil Resources (WRB)

• The WRB, developed under the International Union of Soil Sciences (IUSS) with FAO, succeeded the FAO legend as the international standard for naming soils and correlating national systems.
  • Editions: 1998 (30 reference soil groups), 2006 (32), 2014 with an update in 2015, and the current 4th edition (2022), released at the 22nd World Congress of Soil Science in Glasgow.
• Two tiers:
  • 32 Reference Soil Groups (RSGs), identified with a key in a fixed order.
  • Qualifiers (202 in all) added to the RSG name: principal qualifiers before it and supplementary qualifiers in brackets after it, for example Rhodic Ferralsol (Clayic).
• Unlike Soil Taxonomy, it does not use soil climate as a criterion, and it gives human-made soils their own groups.

Grouping in the WRB key	Reference Soil Groups
Thick organic layers	Histosols
Strong human influence	Anthrosols, Technosols
Limited rooting (permafrost, shallow rock, sodium, shrink–swell, salt)	Cryosols, Leptosols, Solonetz, Vertisols, Solonchaks
Iron and aluminium chemistry (gleying, volcanic, podzolic, plinthic, stagnant water, tropical)	Gleysols, Andosols, Podzols, Plinthosols, Planosols, Stagnosols, Nitisols, Ferralsols
Organic-rich, base-rich topsoil (grasslands)	Chernozems, Kastanozems, Phaeozems, Umbrisols
Accumulation of silica, gypsum or lime	Durisols, Gypsisols, Calcisols
Clay-enriched subsoil	Retisols, Acrisols, Lixisols, Alisols, Luvisols
Little or no profile differentiation	Cambisols, Arenosols, Fluvisols, Regosols

Correlation of the main schemes

Classical (zonal) soil	USDA order	WRB group	Indian example
Tundra soil	Gelisols	Cryosols	None mapped; alpine patches only
Podzol	Spodosols	Podzols	Patches in the Himalayan conifer belt
Brown earth	Alfisols (Udalfs), Inceptisols	Luvisols, Cambisols	Himalayan forest soils
Chernozem, chestnut, prairie	Mollisols	Chernozems, Kastanozems, Phaeozems	Tarai of Uttarakhand (Mollisols)
Sierozem, desert soil	Aridisols	Calcisols, Gypsisols, Arenosols	Thar Desert
Regur (tropical black)	Vertisols	Vertisols	Deccan Plateau
Ferralsol, laterite	Oxisols, Ultisols	Ferralsols, Acrisols, Plinthosols	Western Ghats, Meghalaya
Halomorphic soils	Aridisols, natric Alfisols	Solonchaks, Solonetz	Usar lands, Rann of Kachchh
Hydromorphic, peat	Histosols, aquic suborders	Histosols, Gleysols	Kari soils of Kerala
Alluvial (azonal)	Entisols, Inceptisols	Fluvisols, Cambisols	Indo-Gangetic plains
Lithosol, regosol	Entisols	Leptosols, Regosols, Arenosols	Himalayan slopes, Thar dunes

Criterion	Zonal system	USDA Soil Taxonomy	WRB
Basis	Presumed genesis (climate and vegetation)	Measured diagnostic horizons and soil climate	Measured diagnostic horizons, properties and materials
Origin	Dokuchaev, Sibirtsev and Glinka, Russia (late 19th century)	USDA under Guy Donald Smith (1960–1975)	IUSS and FAO (1998; 4th edition 2022)
Categories	Zonal, intrazonal, azonal; great soil groups	12 orders in six categories	32 reference soil groups plus qualifiers
Strength	Intuitive; links directly to biomes and vegetation	Rigorous, replicable, detailed for mapping and land capability	International correlation; simpler; includes human-made soils
Weakness	Same soil recurs in different climates; azonal class ambiguous; ignores relict features	Technical vocabulary; needs soil-climate data	Only two levels; less detail for local mapping

Distribution of Soils in the World

Soil type	USDA equivalent	Principal world regions	Natural vegetation
Tundra soils	Gelisols	Arctic fringe of North America, Scandinavia, Siberia; Antarctic margins	Moss, lichen, dwarf-shrub tundra
Podzols	Spodosols	Canada, Scandinavia, northern Russia and Siberia (taiga belt); patches in maritime western Europe	Coniferous (boreal) forest
Brown earths	Alfisols (Udalfs)	Western and central Europe, north-eastern United States, parts of East Asia	Temperate deciduous forest
Chernozems	Mollisols	Ukraine and the southern Russian steppe, North American prairies, north-eastern China, Pampas margins	Tall and mixed-grass steppe and prairie
Chestnut soils	Mollisols (drier subtype)	Drier margins of the Russian and North American steppe belts, Central Asia	Short-grass steppe
Sierozems	Aridisols	Central Asian deserts, Iranian Plateau, interior Australia, American south-west	Sparse desert scrub
Mediterranean soils	Alfisols (Xeralfs)	Mediterranean basin, coastal California, central Chile, south-west Australia, Cape region	Sclerophyllous scrub (maquis) and woodland
Tropical black (regur)	Vertisols	Deccan Plateau, Sudan and Ethiopia, northern and eastern Australia, Texas Blackland Prairie	Savanna, tropical grassland
Ferralsols, laterites	Oxisols, Ultisols	Amazon and Congo basins, Western Ghats and north-east India, South-East Asia, coastal West Africa	Tropical rainforest and moist savanna
Alluvial soils	Entisols, Inceptisols	Indo-Gangetic and Brahmaputra plains, Nile delta, Mekong and Yangtze deltas, Mississippi valley	Cropland (naturally riparian forest and grassland)
Andisols	Andisols	Andes, Indonesia, Japan, East African Rift volcanoes	Variable; often dense montane forest
Histosols (peat)	Histosols	Canadian and Siberian bogs, Indonesian and Malaysian peat swamps, northern European fens	Bog, fen, peat swamp forest

• Broad regularities in the pattern:
  • Latitudinal zonation dominates at the continental scale. The pole-to-equator sequence tundra → podzol → brown earth or chernozem → chestnut and sierozem → ferralsol recurs on every major landmass, compressed or expanded by continentality and relief, much as the Köppen climate belts do.
  • Continentality distorts the latitudinal pattern. Interior landmasses (Central Asia, the North American interior) develop drier chestnut, sierozem and Aridisol belts at latitudes where coasts carry forest soils; the soil belts of North America run north–south across the Great Plains, following rainfall rather than latitude.
  • Parent material can override climate locally. The Deccan’s Vertisols and the scattered Andisol patches show that one rock type (basalt, volcanic ash) can produce a recognisable, mappable soil across widely different climatic zones.
  • Fluvial and deltaic azonal soils matter most to settlement. Though geologically “immature”, the Entisols and Inceptisols of major floodplains support some of the highest rural population densities on Earth, because sediment renewal offsets the lack of profile development.
  • Mountains repeat the zonal sequence vertically. On the Himalaya, forest soils give way upslope to podzolised soils, alpine meadow soils and thin, frost-churned soils below the snowline.
  • The southern hemisphere lacks the cold belts. There is almost no land at 50–70°S, so it has no broad podzol or tundra-soil zone outside Patagonia and Antarctica.
  • Ancient shields carry the most weathered soils. The stable Brazilian and African shields hold the great Oxisol expanses because their surfaces have weathered for millions of years.

Economic significance of the major soils

• Mollisols (chernozem, prairie and chestnut soils) are the world’s granaries: the wheat and maize belts of Ukraine and southern Russia, the North American Midwest and Prairies, the Pampas and north-east China.
• Alluvial Entisols and Inceptisols carry wet-rice civilisations in the Ganga, Brahmaputra, Mekong, Yangtze and Nile valleys and deltas.
• Vertisols suit rain-fed cotton, sorghum, soybean and pulses, because they store water into the dry season, but are hard to till when wet or dry.
• Oxisols and Ultisols support plantation tree crops and shifting cultivation; with lime and fertiliser they now carry large soybean and maize areas in the Brazilian Cerrado.
• Alfisols of the temperate and subtropical lands are general-purpose mixed-farming soils; Mediterranean Alfisols carry olives, vines and citrus.
• Aridisols grow irrigated cotton, wheat and dates (Central Asia, the Nile and Indus valleys), at the risk of salinisation.
• Spodosols, Gelisols and Histosols are mainly forest, grazing or peat lands. They matter today as the largest soil carbon stores, so their protection is part of climate policy.

Podzol and ferralsol compared

Feature	Podzol	Ferralsol (laterite)
Climate	Cool, humid (boreal)	Hot, humid (tropical)
Vegetation	Coniferous forest	Tropical rainforest and moist savanna
Dominant process	Cheluviation (organic-acid leaching)	Ferrallitisation (intense chemical weathering)
What is removed	Iron, aluminium, humus moved down within the profile	Silica and bases removed from the profile
Diagnostic horizon	Bleached E over an iron–humus B horizon	Deep, clayey, oxide-rich B horizon
Colour	Ash-grey above, rust below	Red or yellow throughout
Reaction and fertility	Strongly acid; poor	Acid; poor once the forest is cleared
USDA order	Spodosol	Oxisol, Ultisol

India’s soils

• India offers a compact illustration of almost every principle above. The Indian Council of Agricultural Research (ICAR) groups soil types in India by genesis, colour and regional occurrence into eight major classes, which map onto both the zonal categories and the USDA orders.

Indian soil type (ICAR)	Approximate zonal and USDA parallel	Region
Alluvial soils	Azonal; Entisols, Inceptisols	Indo-Gangetic and Brahmaputra plains; deltas of the Mahanadi, Godavari, Krishna and Kaveri
Black soils (regur)	Zonal (tropical black); Vertisols	Deccan lava plateau: Maharashtra, Madhya Pradesh, Gujarat, parts of Karnataka, Telangana and Andhra Pradesh
Red and yellow soils	Zonal (transitional to ferralsols); Alfisols, Ultisols	Eastern and southern Peninsula, Chhattisgarh, Odisha, Jharkhand, parts of Tamil Nadu
Laterite and lateritic soils	Zonal (ferralsol); Ultisols, some Oxisols	Western Ghats, Kerala, Karnataka coast, parts of Odisha, north-east India
Arid and desert soils	Zonal (sierozem); Aridisols, Entisols	Western Rajasthan (Thar), parts of Gujarat (Kachchh) and Haryana
Saline and alkaline soils (usar, reh)	Intrazonal (halomorphic); Solonchak and Solonetz analogues	Patches in Punjab, Haryana, Uttar Pradesh, Gujarat and coastal deltas
Peaty and marshy soils	Intrazonal (hydromorphic); Histosol and gley analogues	Kerala (kari and kayal lands), coastal Odisha, the Sundarbans, parts of Bihar
Forest and mountain soils	Zonal, changing with altitude; Inceptisols, Alfisols, Spodosols	Himalayan ranges, Western Ghats uplands, north-eastern hill states

• NBSS&LUP has mapped Indian soils formally on USDA Soil Taxonomy:
  • Eight of the twelve orders occur. Inceptisols (about 29% of the area), Entisols (about 24%) and Alfisols (about 24%) together cover over three-quarters of the country.
  • Vertisols (about 8%), Aridisols (about 4.5%) and Mollisols (about 2.4%) follow. Ultisols and Oxisols occupy only small areas in the wettest uplands, and Andisols and Spodosols are virtually absent.
  • This is a practical demonstration of how the “traditional” descriptive scheme and the “modern” diagnostic scheme describe the same physical reality from two directions.
• India’s pattern follows the general rules: parent material explains the Deccan black soils; rainfall separates the leached laterites of the Western Ghats from the pedocal desert soils of the Thar; rivers spread the azonal alluvium of the plains; and altitude arranges the Himalayan forest and mountain soils.
• Pressures: soil health, salinity and erosion now drive soil policy, through the Soil Health Card scheme and watershed programmes; soil erosion and land degradation are the subject of their own note.