Global Distribution of Soils: World Soil Types, the 12 Soil Orders and Soil Zones

Soil Formation and the Soil Profile

Soil-Forming Factors: Passive and Active Controls

Five factors control the formation of soil, namely parent material, topography, climate, biological activity and time. They act together, and each changes how the others work. Soil formation, called pedogenesis, begins with weathering, because the weathered mantle is the raw material from which soil is built. Bacteria, mosses and lichens colonise the weathered debris first, and their dead remains add humus. Grasses, ferns, bushes and trees follow, and plant roots and burrowing animals make the mass porous and sponge-like until a mature soil forms.

The factors fall into two groups. Parent material, topography and time are passive controls: They supply the material, set how water drains and allow the processes to run, but they do not act on the soil themselves. Climate and biological activity are the active controls that do the work.

• Parent material: Residual soils form from rock weathered in place and transported soils form from deposits carried in by water, wind or ice. Young soils show strong links with their parent rock, and in limestone areas the link stays clear.
• Topography: Soils are thin on steep slopes and thick over flat uplands. On gentle slopes erosion is slow and water percolates well, so soil formation is very favourable.
• Climate: Moisture and temperature decide the rate of chemical and biological activity. Heavy rain moves soil components downward (eluviation) and deposits them lower down (illuviation). Where rain is very heavy, even most of the silica leaves the soil, which is called desilication. In dry climates evaporation exceeds precipitation, groundwater rises by capillary action, and the salts left behind form a crust called a hardpan.
• Biological activity: Dead plants provide humus, and bacteria fix nitrogen; Rhizobium in the root nodules of leguminous plants is the standard example. Ants, termites, earthworms and rodents rework the soil up and down.
• Time factor: Soils from recent alluvium or glacial till are young and show no horizons or only poorly developed ones. No absolute length of time can be fixed for a soil to mature.

Temperature works in two directions. Chemical activity rises in heat, falls in cold and stops in freezing conditions, so tropical soils show deeper profiles while the soils of the frozen tundra hold largely mechanically broken material.

Soil Profile and Horizons: O, A, E, B, C and R

A soil profile is the vertical section from the surface down to the unaltered rock, and a soil horizon is one layer in it that differs from the layers above and below in colour, texture and chemistry. Horizons are the result of soil-forming processes; a layer that has not undergone them is simply called a layer. Horizons are also what most classification systems use to define soil types.

Most soils show some or all of six horizons, each marked by a letter.

The E horizon and the B horizon are partners. Eluviation strips material out of E and illuviation deposits it in B. Not every soil has all six; young soils lack developed horizons, and soils that have been deeply ploughed or disturbed by earth-moving may lack them almost completely. Different countries use different horizon symbols, so a letter has meaning only inside its own system.

World Soil Types: Why Soils Follow Climate

Global Distribution of Soils: The Zonal Idea

Soils follow climate because soil forms from weathered rock under the work of water, heat and living things. Soil adjusts to the climate, landform and vegetation around it and changes internally when these change, which is why each great belt of climate carries its own typical soil.

The idea began on the steppes. In 1883 Vasily Dokuchaev studied the steppe soils of the Poltava region, noticed that peasants named soils by colour, and published Russian Chernozem. His work turned soil into a product of climate, vegetation, relief, parent material and time, and he is regarded as the father of soil science. Many Russian words he used, such as chernozem, podzol, gley and solonetz, became part of the international vocabulary of soil science.

Three contrasts show the work of climate.

• Wet tropics: Bacteria act fast and dead vegetation is oxidised quickly, so humus stays low while the profile is deep.
• Cold climates: Bacteria grow slowly, humus accumulates, and peat develops in sub-arctic and tundra climates.
• Dry climates: Evaporation exceeds precipitation, and salts left behind form hardpans.

Zonal, Intrazonal and Azonal Soils: The Genetic Classification

The older genetic classification of 1938 sorted soils into three groups by the factor that dominates their formation, which is climate, a local factor, or too little time. It remains in use because it explains why soils do not follow climate everywhere.

Intrazonal soils have more or less well-defined profiles that reflect a dominant local factor, and they split into three kinds.

• Calcimorphic soils: These develop on limestone. Rendzinas are thin soils with limited water-holding capacity; terra rossa soils are deep red and go with higher rainfall than rendzinas.
• Hydromorphic soils: These form in wetland conditions. Gley soils form where the pore spaces between grains stay saturated with water and hold no air; the lack of oxygen reduces the iron and gives a grey or blue colour with red flecks. Peat soils form where the vegetation cannot break down completely.
• Halomorphic soils: These form through salinisation.

Azonal soils are those where the parent material keeps moving or arriving. On Himalayan slopes weathered material keeps sliding down, so the time needed for soil formation never becomes available. On flood plains new alluvium arrives every year, so soils stay immature.

The 12 Soil Orders of USDA Soil Taxonomy

What Is a Soil Order? The USDA Soil Taxonomy

A soil order is the highest level of USDA soil taxonomy, which was developed by the United States Department of Agriculture and the National Cooperative Soil Survey. The system has six levels, which run from the most general to the most specific as order, suborder, great group, subgroup, family and series. It classifies by properties that can be measured, such as depth, moisture, temperature, texture, structure, organic matter and salt content, and every order name ends in -sol. The 12 orders of soil reflect major differences in how soils formed.

The twelve shares add up to 95 per cent, not 100, because the rest of the ice-free land is mostly mountain land, where the orders are mixed or no soil has formed. Published estimates differ by a few points, but in every one Entisols are the largest order.

The orders can also be read as stages of development. In order of increasing development they run Entisols, Inceptisols, Aridisols, Mollisols, Alfisols, Spodosols, Ultisols and Oxisols, while Histosols and Vertisols can appear at any stage. The sequence leaves out Gelisols and Andisols, which are defined by permafrost and by volcanic ash rather than by their stage of development.

Soil Orders of Cold and Wet Lands: Gelisols, Histosols and Spodosols

In the far north, cold and frost shape the soils, and waterlogging preserves plant remains in the wet lowlands. Three orders dominate these lands.

• Gelisols: Soils with permafrost within two metres of the surface, churned by freezing and thawing; they are found in the permafrost lands of Siberia, northern Canada, Alaska and Greenland. Organic matter gathers in the top layer, so most are black or dark brown, yet nutrients such as calcium and potassium leach easily above the frozen layer, so they are not very fertile.
• Histosols: Organic soils, formerly called bog soils, formed where waterlogging stops plant remains from decaying; most lie in Canada, Scandinavia, the West Siberian Plain, Sumatra, Borneo and New Guinea.
• Spodosols (podzols): Acid soils of coniferous and boreal forests, where acids leach iron and aluminium down from an ash‑grey upper layer, usually on quartz-rich sands.

The name podzol is Russian for under-ash soil. Peasants ploughing virgin ground turned up a pale, ash‑like leached layer, the E horizon, low in iron, aluminium and humus. The iron and aluminium washed out of that layer collect below it, which is why a podzol shows the E and B horizons so sharply.

The largest single tropical peatland in the world lies in the Cuvette Centrale of the Congo Basin, mainly in the Democratic Republic of the Congo. Peat is the material of Histosols, plant remains that waterlogging has kept from decaying. By country, Indonesia holds more tropical peatland and mangrove forest than any other, with peat in Kalimantan and Sumatra. Histosols and Gelisols store large quantities of organic carbon, which is why the peat lands matter for the climate.

Soil Orders of Grasslands and Deserts: Mollisols, Aridisols and Vertisols

The drier interiors of continents carry soils that keep their nutrients because little water washes through them. Grass roots add organic matter; desert heat adds salts.

• Mollisols: Deep, dark, fertile soils with a thick organic-rich top layer, the mollic epipedon, built by grass roots; they are typical of the North American Great Plains and Central Asia.
• Aridisols: Dry desert soils with fewer than 90 consecutive moist days in the growing season, little organic matter and layers of caliche or duripan; they dominate the deserts and dry shrublands.
• Vertisols: Clay-rich soils that swell when wet and crack deeply when dry; major areas include eastern Australia, the Deccan Plateau of India and parts of Sudan, Ethiopia and Chad.

India's black soils of the Deccan belong to the Vertisols, and the arid soils of the dry west fall among the Aridisols.

Soil Orders of Forests and the Tropics: Alfisols, Ultisols and Oxisols

Where rain and warmth last long enough, weathering runs deep and leaching strips out bases. Three orders show the sequence from moderately to extremely weathered.

• Alfisols: Clay-enriched forest soils with enough moisture and warmth for at least three months of plant growth; they are found in the Ohio basin, western Europe, European Russia and the drier parts of Peninsular India.
• Ultisols: Acid, highly weathered soils of humid temperate and tropical regions, poor in calcium, magnesium and potassium; they include the red clays of the southern United States. Their red and yellow colours come from iron oxide that has accumulated.
• Oxisols: The most weathered soils, rich in iron and aluminium oxides but low in silica, best known in tropical rain forest within 25 degrees north and south of the equator, and found almost exclusively in tropical areas of South America and Africa.

The richest forests on land stand on some of the poorest soils. Rain passing through the litter on the forest floor is acidified and leaches minerals from the upper layers, and in the heat bacteria oxidise dead vegetation so fast that little humus is left. The forest keeps its nutrients in living plants, so clearing it leaves bare, poor ground. The same intense leaching under tropical rain forms India's laterite soils.

Young and Volcanic Soil Orders: Entisols, Inceptisols and Andisols

The largest orders are the least developed. Entisols and Inceptisols together cover a large part of the world's ice-free land, because so much land is young, steep, sandy or freshly deposited.

• Entisols: Recently formed soils without developed horizons, common on river and beach sediments and volcanic ash; some are only an A horizon on bedrock.
• Inceptisols: Young soils with some subsurface horizon but little eluviation and illuviation; common in rocky or mountainous areas and on river, coastal and marine sediments, but not in deserts or permafrost.
• Andisols: Young volcanic ash soils, usually very fertile, about 1 per cent of ice-free land, mostly around the Pacific Ring of Fire in Chile, Ecuador, Japan, Java and New Zealand; wet rice on Java's Andisols supports some of the densest populations in the world.

World Soil Types Map: Soil Zones Belt by Belt

Tundra Soils and Permafrost: Fragile but Economically Important

Tundra soils sit on permafrost. In the Arctic tundra the ground is permanently frozen below a depth of 25 to 90 cm, which stops trees from growing; in summer only the top layer thaws, and because water cannot sink through the frozen ground it forms marshes, lakes and bogs.

• A carbon store: Tundra soils hold large amounts of biomass stored in the permafrost, making them a carbon sink; warming thaws them and releases greenhouse gases, which feeds further warming.
• Fragile ground: Thawing permafrost could change which species survive within decades or centuries, and buildings and roads built on frozen ground are at risk as it thaws.
• Economic value: The Arctic tundra is rich in petroleum, natural gas and uranium; the Yamalo-Nenets region is the centre of Russia's natural gas production, and nomadic reindeer herders such as the Nenets live on the tundra.

Chernozem and Grassland Soils: The World's Breadbaskets

Some of the richest farm soils lie under the mid-latitude grasslands. Chernozem, the black earth, carries a high share of humus, 4 to 16 per cent, stores moisture well and gives high yields. It forms two great belts. The Eurasian steppe belt runs from Ukraine and Russia to Kazakhstan and Mongolia-Manchuria, and the North American belt runs from the Canadian Prairies through the Great Plains of the United States. Chernozem covers about 230 million hectares.

• Why so fertile: Grass roots add organic matter year after year, building the thick dark top layer.
• Why still fragile: Long cropping depletes the humus, so even chernozems need fertiliser.
• In classification: Chernozems are Mollisols in USDA soil taxonomy and a Reference Soil Group of their own in the World Reference Base.

The grassland soils change as the climate dries. Kastanozems, the chestnut soils, are lighter in colour than chernozems and are related to the Mollisols; they are rich in humus and carry a characteristic brown surface layer, and they are found in relatively dry zones with 200 to 450 mm of rain a year. Beyond them, in still drier country, lie the desert soils, the Aridisols.

Loess: Soil Built from Wind-Blown Silt

Loess is a pale yellow, silt-sized sediment formed by the accumulation of wind-blown dust, and about 10 per cent of the Earth's land area is covered by loess or similar deposits. Loess is made of angular grains of quartz, feldspar and mica, usually with 20 per cent or less clay, and it is often loosely cemented by calcium carbonate. Deposits can be more than a hundred metres thick in north-western China and tens of metres thick in parts of the Midwestern United States.

Because its grains are angular and loosely held, loess stands in steep vertical banks for years and is easy to dig, which is why cave dwellings are cut into it. For the same reason it erodes quickly once its plant cover is stripped. Loess is a parent material rather than a soil order, so the soils that develop on it can belong to many orders. It shows that wind can build land as well as strip it.

World Soil Classification: USDA Orders and the FAO World Reference Base

FAO World Reference Base: 32 Reference Soil Groups

The World Reference Base for Soil Resources is the international system for naming soils and making the legends of soil maps. Its current fourth edition, of 2022, is edited by a working group of the International Union of Soil Sciences, and its first level has 32 Reference Soil Groups. A second level adds qualifiers, 202 in total, so that a soil name carries more detail. The WRB replaced the FAO/UNESCO legend of the Soil Map of the World as the international standard and borrows from Soil Taxonomy, the FAO legend and French and Russian concepts.

The two systems overlap but do not match one to one: USDA uses measured properties to define orders, while the WRB names groups that suit a world soil map.

The WRB arranges its 32 groups by what dominates each soil.

• Thick organic layers: Histosols.
• Strong human influence: Anthrosols and Technosols.
• Permafrost and limits to root growth: Cryosols, Leptosols, Solonetz, Vertisols and Solonchaks.
• Iron and aluminium chemistry: Gleysols, Andosols, Podzols, Plinthosols, Planosols, Stagnosols, Nitisols and Ferralsols.
• Pronounced accumulation of organic matter in the topsoil: Chernozems, Kastanozems, Phaeozems and Umbrisols.
• Accumulation of salts and carbonates: Durisols, Gypsisols and Calcisols.
• Clay-enriched subsoil: Retisols, Acrisols, Lixisols, Alisols and Luvisols.
• Little profile development: Cambisols, Fluvisols, Arenosols and Regosols.

Soil Degradation and Conservation

What Is Soil Degradation? Erosion, Salinisation and Loss of Fertility

Soil degradation is the loss of topsoil by erosion and the loss of fertility through over-use, acidification, salinisation or chemical contamination. It is serious because soil is renewed only very slowly. It can take up to 1,000 years to produce just 2 to 3 cm of soil, and 95 per cent of the world's food comes from soil. About 33 per cent of the Earth's soils are already degraded, and over 90 per cent could become degraded by 2050. Erosion can cut crop yields by up to 50 per cent, and land degradation and erosion together threaten the well-being of at least 3.2 billion people.

Erosion is a natural process, but human activity has made it roughly 28 times faster than the natural rate. Intensive agriculture, deforestation, roads, acid rain, climate change and urban sprawl are among the chief human causes. The processes of soil formation and erosion go on together and are normally in balance; the balance breaks when deforestation, over-grazing, construction and mining strip the cover.

Water Erosion and Wind Erosion: Causes and Stages

Water erosion proceeds in four stages, each more severe than the one before.

1. Splash erosion: A falling raindrop makes a small crater and throws soil particles out; this is the first and least severe stage.
2. Sheet erosion: Water flowing over a broad surface carries the loosened particles away.
3. Rill erosion: The flow concentrates into small, short-lived channels.
4. Gully erosion: Runoff gathers into narrow channels and removes soil to a considerable depth, as in the ravines of the Chambal basin.

Wind erosion works by deflation, the lifting and removal of loose fine particles, and by abrasion, the wearing action of wind-driven grains. Deflation works through three mechanisms. In surface creep larger grains roll or slide, in saltation particles bounce across the surface, and in suspension fine particles travel long distances. Saltation likely accounts for 50 to 70 per cent of deflation, suspension for 30 to 40 per cent and creep for 5 to 25 per cent. Where deflation removes the fine particles and leaves a sheet of rock fragments, the surface is called desert pavement, as in the stony regs of the Sahara.

The Dust Bowl of the 1930s shows what bare soil and wind can do. In the semi-arid High Plains, where less than 510 mm of rain falls a year, farmers had deep-ploughed the virgin topsoil and displaced the deep-rooted native grasses that held soil and moisture. When drought came in three waves, in 1934, 1936 and 1939‑40, the unanchored soil turned to dust and the wind carried it away in huge clouds.

Salinisation and Waterlogging: Soil Damaged by Irrigation

Soil salinisation is the build-up of salts in the soil, and it is caused by natural processes such as mineral weathering and by human ones such as irrigation. Salts accumulate near the surface when water evaporates faster than rain can flush them out, so the soil of dry regions is naturally salty, and irrigation can add to the load because almost all irrigation water contains some salt. Proper irrigation management supplies enough drainage water to leach the added salts away; where drainage is disrupted, salts accumulate.

In drylands, salinity can also rise from below. When the water table stands within two to three metres of the surface in clayey soils, capillary action lifts salts from the groundwater to the surface. Clearing trees for farming is a major cause of the rising water table, because more rainwater then enters the aquifer. The Aswan High Dam in Egypt, built in 1970, is the standard example of disturbed drainage. After its construction the groundwater level stayed continuously high and the arable land became salinised.

When sodium predominates, the soil becomes sodic, as in many of the saline and alkaline soils of India. Sodic soils have very poor structure, which limits or prevents water infiltration and drainage. Gypsum is the most common compound used to reclaim sodic soil, and salt-tolerant plants such as halophytes help as well.

Desertification: Land Degradation in Drylands

Desertification is the gradual degradation of fertile land into arid desert through a combination of natural processes and human activities. The United Nations Convention to Combat Desertification (UNCCD) defines it as "land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities." The immediate cause is the loss of most vegetation, driven by drought, climatic shifts, tillage for agriculture, overgrazing and deforestation for fuel or construction materials. Unprotected dry soil surfaces blow away with the wind or are washed away by flash floods, leaving infertile lower layers that bake in the sun and become an unproductive hardpan.

The older idea that desertification means the physical expansion of deserts has been rejected; the process is the degradation of the land itself. Drylands occupy about 40 to 41 per cent of the Earth's land area and are home to more than 2 billion people, at least 90 per cent of whom live in developing countries. The areas most affected are the Sahel in Africa, the Gobi Desert and Mongolia in Asia, and parts of South America. Effects include sand and dust storms, food insecurity and poverty. Mitigation includes improving soil quality, managing grazing, and tree-planting by reforestation and afforestation.

India measures its own position in the Desertification and Land Degradation Atlas of India, published by ISRO's Space Applications Centre at Ahmedabad. About 97.85 million hectares, or 29.77 per cent of the country's geographical area, are affected by land degradation and desertification, and India's drylands extend across around 228 million hectares. The United Nations marks 17 June each year as the World Day to Combat Desertification and Drought.

Soil Conservation: Methods that Work with Slope, Wind and Water

Soil conservation is the prevention of loss of the topmost soil layer by erosion, and of reduced fertility from over-use, acidification, salinisation or chemical contamination. The methods follow the cause of the loss.

• Contour ploughing: Furrows follow the contour lines, which slows the flow of water down the slope; ploughing up and down the slope instead forms channels for quick runoff and erosion. Contour ploughing can raise yields by 10 to 50 per cent, partly because more soil is retained.
• Terracing: Steps cut into a slope make nearly level areas, and terrace cultivation restricts erosion; the western and central Himalayas have well-developed terrace farming.
• Strip cropping: Strips of grass are left to grow between crops, which breaks up the force of the wind.
• Shelter belts and windbreaks: Rows of trees at the windward edge of a field slow the wind; shelter belts have contributed significantly to stabilising sand dunes and the desert in western India.
• Sand fences: Fences of wood or wire make drifting sand settle in a chosen place, helping to stabilise dunes.
• Cover crops and crop rotation: Legumes and other cover crops blanket the soil year-round, act as green manure and replenish nitrogen.
• No-till farming and green manures: These soil-enhancing practices minimise erosion and can revive damaged soil.

Soil Orders in India and the Soil Types of the Subcontinent

Soil Orders in India: Inceptisols to Mollisols

India classifies its soils by the same USDA taxonomy. To study its soils and make them comparable at the international level, the Indian Council of Agricultural Research classified Indian soils into USDA orders, working through the National Bureau of Soil Survey and Land Use Planning.

• Largest: Inceptisols, 39.74 per cent, and Entisols, 28.08 per cent, together over two-thirds of India.
• Next: Alfisols, 13.55 per cent, the forest soils of the drier Peninsula, and Vertisols, 8.52 per cent, the black soils of the Deccan.
• Smaller: Aridisols 4.28, Ultisols 2.51 and Mollisols 0.40 per cent; other soils make up 2.92 per cent.

India's soils are also grouped by genesis, colour, composition and location into alluvial, black, red and yellow, laterite, arid, saline, peaty and forest soils.

Black, Red and Laterite Soils: Parent Rock and Climate

Three Indian soils show a world principle at work, namely that soil follows parent rock and climate.

• Black soil: Also called regur, and known as black cotton soil because it suits cotton. It is typical of the Deccan Trap region, which is made up of lava flows, so the parent rock is basalt. It is very fine and clayey, holds moisture well, is rich in calcium carbonate, magnesium, potash and lime, but is generally poor in phosphoric content. It develops deep cracks in hot weather, which aerate it, and it is sticky when wet.
• Red and yellow soil: It develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern parts of the Deccan plateau. It gets its reddish colour from the diffusion of iron in crystalline and metamorphic rocks, and looks yellow when the iron is in a hydrated form.
• Laterite soil: It develops under a tropical and subtropical climate with alternate wet and dry seasons, as the result of intense leaching by heavy rain. It is deep, acidic and generally deficient in plant nutrients, and it is prone to erosion. Red laterite soils in Tamil Nadu, Andhra Pradesh and Kerala suit cashew nut, and with conservation, tea and coffee grow well on them in the hills.

The Deccan Traps were formed when lava extruded in fissure eruptions around 66 to 65 million years ago, and they cover a very large area, which is why one rock type produces such a wide belt of black soil.

Sources

• NCERT: Fundamentals of Physical Geography (Class XI), Geomorphic Processes
• NCERT: Contemporary India II (Class X), Resources and Development
• NCERT: India: Physical Environment (Class XI, earlier edition), Soils
• FAO Soils Portal: World Reference Base for Soil Resources
• FAO: World Soil Day
• FAO: Global Symposium on Soil Erosion, key messages
• Wikipedia: USDA soil taxonomy
• Wikipedia: Soil horizon
• Wikipedia: Zonal soil
• Wikipedia: Vasily Dokuchaev
• Wikipedia: Pedogenesis
• Wikipedia: Gelisol
• Wikipedia: Histosol
• Wikipedia: Podzol
• Wikipedia: Mollisol
• Wikipedia: Chernozem
• Wikipedia: Vertisol
• Wikipedia: Oxisol
• Wikipedia: Alfisol
• Wikipedia: Ultisol
• Wikipedia: Aridisol
• Wikipedia: Entisol
• Wikipedia: Inceptisol
• Wikipedia: Andisol
• Wikipedia: Tundra
• Wikipedia: Permafrost
• Wikipedia: Yamalo-Nenets Autonomous Okrug
• Wikipedia: Peat
• Wikipedia: Cuvette Centrale
• Wikipedia: Loess
• Wikipedia: Deccan Traps
• Wikipedia: Soil erosion
• Wikipedia: Wind erosion
• Wikipedia: Dust Bowl
• Wikipedia: Soil salinity
• Wikipedia: Desertification
• Wikipedia: Soil conservation
• Wikipedia: Sand fence
• Wikipedia: World Reference Base for Soil Resources
• Wikipedia: Rendzina
• Wikipedia: Terra rossa
• Wikipedia: Gleysol
• Wikipedia: Kastanozem
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• Press Information Bureau: Desertification and Land Degradation Atlas of India (17 June 2021)
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• UPSC: Civil Services Examination question papers