Maarg Manthan · Topic 7.3

Classification of Soils

Geography › Biogeography: Soils and Vegetation · Topic 7.3

Soils are classified by their properties and origin. Early systems grouped them as zonal (matching the climate zone, such as podzol, chernozem and laterite), intrazonal and azonal soils. The modern system used worldwide is the United States Department of Agriculture's Soil Taxonomy, which has 12 orders: Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols and Vertisols. The FAO's World Reference Base is another widely used system. In India, soils are grouped by the Indian Council of Agricultural Research into eight major groups.

5 min read · UPSC Prelims · Geography · Biogeography: Soils and Vegetation

At a Glance

Old system Zonal, intrazonal, azonal
USDA Soil Taxonomy 12 orders
Black grassland soil Chernozem (Mollisol)
Tropical leached soil Oxisol (laterite)
Indian system ICAR, 8 major groups
Exam link UPSC Prelims, Geography: Biogeography: Soils and Vegetation

Where Does It Fit?

This topic follows the stages and processes of soil formation and sorts the soils into types. It gives the world picture, while the Indian soil types are covered in the macro on the physical geography of India. Previous topic: Stages, Processes and Profiles of Soil Formation. Next topic: Soil Erosion and Conservation.

What Are the Zonal Soils of the World?

Soil	Climate and vegetation	Features	Regions
Tundra soil	Cold, treeless tundra	Frozen, thin, waterlogged in summer	Arctic Canada, Siberia
Podzol	Cool, moist coniferous forest (taiga)	Grey, acidic, leached, infertile	Northern Canada, Scandinavia, Russia
Brown forest soil	Temperate deciduous forest	Fertile, rich in humus	Western and central Europe, eastern USA
Chernozem	Temperate grassland (steppe, prairie)	Deep black, humus-rich, lime-rich; very fertile; the ‘breadbasket’ soils	Ukraine and Russia, the Great Plains, the Canadian prairies
Chestnut and brown soils	Drier grasslands	Less humus and shallower than chernozem	Edges of steppes and prairies
Desert soil (sierozem)	Arid	Thin, pale, low in humus, rich in salts	Sahara, Arabia, Thar
Red and yellow podzolic soil	Warm, humid, subtropical forest	Acidic, leached, moderate fertility	South-eastern USA, southern China
Laterite	Hot, wet tropical forest	Red, deeply leached, infertile once the forest is cleared	Amazon, Congo, South-East Asia, Western Ghats

What Are the 12 Orders of the USDA Soil Taxonomy?

Order	Key feature	Typical region or Indian equivalent
Alfisols	Moderately leached, fertile, clay-rich subsoil	Humid temperate forests; some red soils
Andisols	Formed on volcanic ash; light, fertile	Volcanic regions, Japan, Indonesia
Aridisols	Dry soils with salts and lime	Deserts; Indian arid soils
Entisols	Young soils with no horizons	River floodplains, dunes; Indian alluvial soils in part
Gelisols	Soils with permafrost	Tundra and polar regions
Histosols	Organic soils such as peat	Bogs and marshes
Inceptisols	Young soils with weak horizons	Widespread, including the Indo-Gangetic plain
Mollisols	Dark, humus-rich grassland soils	Steppes and prairies (chernozem)
Oxisols	Deeply weathered, iron- and aluminium-rich	Humid tropics; laterite
Spodosols	Acid, leached, sandy soils under conifers	Podzols of the taiga
Ultisols	Strongly leached, acidic, red or yellow	Humid subtropics; red and yellow soils
Vertisols	Clay that swells and cracks	Black cotton soil (regur) of the Deccan

The names end in -sol, which comes from the Latin solum, meaning soil. The system is based on measurable properties and diagnostic horizons, not on climate. The food and agriculture organization of the United Nations uses a different system, the World Reference Base for Soil Resources, with 32 reference soil groups.

How Are Soils Used and Which Are Most Productive?

Most fertile: mollisols (chernozem and prairie soils), alluvium, vertisols and volcanic soils. They support the great grain regions of the world.
Poor soils: podzols, laterites and deserts, because of acidity, leaching or salts. They need much fertiliser or irrigation.
Fragile soils: tropical forest soils hold their nutrients in the living plants, not the soil, so clearance turns them infertile rapidly, as in Deforestation: Tropical and Temperate Forest Loss.

Soil maps are made by field survey and by satellite data. The Soil Survey of India and the National Bureau of Soil Survey and Land Use Planning prepare soil maps of the country, and the Food and Agriculture Organization compiled a Soil Map of the World. Such maps guide the choice of crops, irrigation and fertiliser, and show where land is vulnerable to erosion or salting. A single field can contain more than one soil type, and so large-scale maps are needed for farm planning, while small-scale maps give only the general pattern. As an analytical point, the soil map and the climate map of the world match closely, since climate and vegetation are the strongest soil-forming factors. India’s soils are grouped differently, as in Types, Textures and Distribution of Soils in India, and the way soil is lost is in Soil Erosion and Conservation.

Why Is This Topic Important?

Map Chernozem and podzol are asked
Taxonomy Soil orders
Link Climate and soils

What Came Next?

The next topic describes soil erosion and its conservation. Next topic: Soil Erosion and Conservation.

Key People and Terms

Chernozem Black grassland soil
Podzol Leached grey forest soil
Mollisol Dark grassland soil order
Oxisol Deeply weathered tropical soil
Vertisol Swelling clay soil
Soil Taxonomy USDA system of 12 orders

Exam Corner

Points to Remember

Soil Taxonomy has 12 orders.
Chernozem is the black soil of temperate grasslands.
Vertisols include India’s black cotton soil.
Podzols form under coniferous forest.
Oxisols are the soils of the wet tropics.

Do Not Confuse With

Chernozem and regur Temperate grassland black soil versus tropical black soil on basalt
Podzol and laterite Cold leached soil versus hot leached soil
Zonal and intrazonal soils Climate-controlled versus locally controlled

Memory Hook

Twelve orders, ending in sol; chernozem black, podzol pale, laterite red.

Mains Angle

Questions ask for the world distribution of soils in relation to climate. These points add depth.

System Zonal soils and orders
Distribution Follows climate
Fertility Which soils feed the world

A Question You May Face

An original practice question, not a past paper question.

“Classify the soils of the world and explain how the distribution of major soil types is related to climate and vegetation.”

How to Answer

Systems Zonal and Soil Taxonomy.
Types Podzol to laterite.
Link Climate and vegetation.
Use Fertile and poor soils.

GS Relevance

Prelims: soil orders, zonal soils, world distribution of soil types. GS Paper 1: soils and their distribution.

Frequently Asked Questions

How are soils classified?

Soils are classified by their properties and origin. The older system groups them as zonal, intrazonal and azonal soils, and the modern USDA Soil Taxonomy has 12 orders, such as Alfisols, Mollisols, Oxisols and Vertisols. The FAO uses the World Reference Base, and India uses a classification of eight major groups.

What is chernozem soil?

Chernozem is a deep black, humus-rich and lime-rich soil formed under temperate grasslands, such as the steppes of Ukraine and Russia, and the prairies of North America. It is among the most fertile soils in the world and supports large wheat and maize production. Its USDA name is a Mollisol.

What is a podzol?

A podzol is an acidic, infertile soil formed in cool, humid coniferous forests, where acids from decaying needles leach iron, aluminium and humus from the upper layers. It has a pale, ash-grey leached horizon below a thin organic layer, and is found in the taiga of Canada, Scandinavia and Russia.

How many orders are there in the USDA Soil Taxonomy?

The USDA Soil Taxonomy has 12 soil orders. They are Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols and Vertisols. The classification is based on measurable properties and diagnostic soil layers, and not on climate.

Which soils are the most fertile?

Chernozem and other prairie soils, river alluvium, black cotton soils of volcanic origin and soils formed on volcanic ash are among the most fertile in the world. They support the main grain-producing regions. Podzols, laterites and desert soils are poor in nutrients and need fertilisers or irrigation to be productive.

PYQ Practice — Statement Analysis

1 Chernozem soils are found under temperate grasslands.

True

They are dark and very fertile.

2 Podzols are formed in hot, humid tropical forests.

False

They form in cool coniferous forests.

3 The USDA Soil Taxonomy has 12 soil orders.