Global Distribution of Soils: World Soil Types and Zones

Soils as a Mirror of Climate: the Zonal Concept

What the Global Distribution of Soils Means

The global distribution of soils is the pattern in which the world's major soil types are spread across the continents. Because soil forms under the combined action of climate, vegetation and rock, the types are not scattered at random but fall into broad, recognisable belts.

The single strongest control on this pattern is climate. Temperature and rainfall decide how fast rock weathers, how much humus builds up and how strongly water leaches the soil, so the soils change with latitude in step with the world's climatic zones.

Why the Zonal Pattern of Soils Matters

Why it matters is that these soil belts decide where the world can grow its food. The most fertile soils sit in a few grassland zones, while vast tropical and polar areas carry poor soils, so the soil map underlies the world's farming map.

The idea that soils form climate-controlled zones is the heart of soil geography. It was first set out by V.V. Dokuchaev, regarded as the father of soil science, who showed that soils vary geographically with climate, not just with the underlying rock.

Classifying the World's Soils: Genetic, USDA and FAO Systems

The Genetic Classification: Zonal, Azonal and Intrazonal Soils

The classical genetic classification, which groups soils by how they form, divides the world's soils into three great classes. Zonal soils are mature soils whose character reflects the climate of a broad zone, such as chernozem, podzol or laterite.

Azonal soils are young soils with little or no profile, such as fresh river alluvium or thin mountain lithosols, where there has not been time to form horizons. Intrazonal soils reflect a strong local factor, such as salinity or waterlogging, that overrides the regional climate.

The USDA Soil Taxonomy and its Twelve Orders

The modern USDA Soil Taxonomy classifies the world's soils by measurable properties rather than by assumed origin. It recognises twelve soil orders, arranged broadly from the least developed soils to the most highly weathered.

The orders run from raw Entisols and young Inceptisols, through climate-stamped orders such as Aridisols of the deserts, Mollisols of the grasslands and Gelisols of the permafrost, to the deeply weathered Oxisols of the humid tropics.

The FAO Soil Map and the World Reference Base

Because different countries used different schemes, the FAO and UNESCO produced a single Soil Map of the World, and this grew into the World Reference Base for Soil Resources, now the international standard for naming and mapping soils.

These global systems let scientists compare soils across borders in one common language. For the exam, the key point is that the genetic, USDA and FAO systems describe the same soils from different angles, not three different sets of soil.

The World Map of Major Soil Types

The Global Pattern of Zonal Soils on the Map

Mapped onto the globe, the zonal soils form broad belts that run with the climatic zones. Around the equator lie the red laterite soils of the rainforests, and the subtropical deserts carry thin, often salty soils with little humus.

Polewards come the dark, fertile chernozem of the mid-latitude grasslands, then the ashy podzols of the cold northern forests, and finally the frozen tundra soils of the polar margins. The same belts repeat, in mirror image, south of the equator.

The Major Zonal Soils of the World, Belt by Belt

Tundra Soils of the Polar Margins (Gelisols)

At the highest latitudes lie the tundra soils, classed as Gelisols where permafrost sits within a metre of the surface. The frozen subsoil blocks drainage, so the thin topsoil stays cold, waterlogged and peaty.

Because cold slows decay and weathering, these soils have weak structure and support only hardy vegetation. They cover large areas of the Arctic in northern Canada and Siberia, and are of little use for cultivation.

Podzols of the Boreal Forest (Spodosols)

South of the tundra, the great coniferous taiga forests grow on podzols, classed as Spodosols. Their name means under-ash, from the pale, ashy-grey layer that lies just below the surface where minerals have been stripped away.

In the cool, humid climate, acidic water washes iron and aluminium down the profile in a process called podzolisation, leaving a leached, sandy and rather infertile soil across Canada, Scandinavia and northern Russia.

Chernozem and Chestnut Soils of the Grasslands (Mollisols)

The mid-latitude grasslands carry the world's most fertile soils, the black chernozem, grouped with the Mollisols. Deep grass roots and a semi-arid climate build up a thick, dark, humus-rich topsoil that holds moisture well.

Chernozem holds about 4 to 16 per cent humus and gives high yields, which is why the steppes of Russia and Ukraine, the North American prairies and the South American pampas are great grain belts. Drier margins carry browner chestnut soils.

Desert and Mediterranean Soils of the Subtropics (Aridisols)

The subtropical deserts carry Aridisols: thin, pale, stony soils with very little humus, because plant cover is sparse. Weak rainfall and strong evaporation often leave a crust of salt or lime at or near the surface.

By area they are among the most extensive soils on Earth, found across the Sahara, Arabia, Central Asia and inland Australia. In the nearby Mediterranean climate, the weathering of limestone yields the reddish terra rossa soils.

Red, Yellow and Laterite Soils of the Humid Tropics (Oxisols)

The hot, wet tropics carry the deeply weathered Oxisols and the leached Ultisols, of which the best known are the laterite soils. Long, heavy rain strips out silica and bases, leaving a soil rich in iron and aluminium oxides.

This gives laterite its rusty-red colour; it hardens on exposure and was even cut as building brick at Angkor Wat. Despite the lush forest above, these soils are poor and infertile, and they spread across India, Africa, South America and Southeast Asia.

Azonal, Intrazonal Soils and the Pedogenic Regimes

Azonal and Intrazonal Soils Across the Zones

Not every soil fits the zonal belts. Azonal soils, such as the alluvial soils of river valleys and deltas and the thin lithosols of steep mountains, are too young or too disturbed to have developed a full profile.

Intrazonal soils form where a local factor overrides the climate: saline and alkaline soils in poorly drained drylands, and peat or bog soils in waterlogged ground. Both classes cut across the broad climatic zones rather than following them.

The Five Great Pedogenic Regimes

The zonal soils are the work of a few great soil-forming regimes, each tied to a climate. Laterisation in the hot wet tropics and podzolisation in the cold northern forests both leach the soil, but leave very different products behind.

Calcification builds the lime-rich chernozem of the grasslands; salinisation crusts the desert soils with salt; and gleization produces the airless, grey, waterlogged soils of the tundra and bogs. Climate decides which regime dominates.

Soil Fertility, World Food Belts and UPSC Relevance

Which Soils Feed the World

What is the significance of this global soil distribution is clearest on the farm. The grassland chernozems and Mollisols are the world's breadbaskets, while the tropical laterites and the polar tundra soils carry most of its poorest land.

This is why a handful of grassland belts grow much of the world's wheat and maize, while huge tropical and desert regions, despite their warmth or their size, struggle to support intensive farming.

World Soils, Indian Soils and the Exam

Contemporary linkages run from these belts to today's biggest debates. Climate change and the spread of desertification threaten the fertile margins, while the slow loss of grassland chernozem to erosion is a worldwide food-security worry.

In the Indian context, the same world types reappear as the country's laterite, black and alluvial soils, the subject of a separate study. For the exam, the world distribution is tested as a set of precise, high-yield points.

• Soils form climate-controlled zones; the genetic system groups them as zonal, azonal and intrazonal.
• The USDA Soil Taxonomy recognises twelve soil orders.
• Chernozem (Mollisols) of the temperate grasslands are the world’s most fertile soils.
• Laterite (Oxisols) of the humid tropics are deeply leached, iron-rich and infertile.
• Podzols (Spodosols) form under boreal forest by the process of podzolisation.

Sources and Further Reading

• Wikipedia: Soil classification
• Wikipedia: USDA soil taxonomy
• Wikipedia: Chernozem
• Wikipedia: Podzol
• Wikipedia: Laterite
• FAO: World Reference Base for Soil Resources
• NCERT: Fundamentals of Physical Geography (Class XI)
• United States Geological Survey (USGS)
• NASA Earth Observatory
• Indian Council of Agricultural Research