Maarg Manthan · Topic 7.2

Stages, Processes and Profiles of Soil Formation

Geography › Biogeography: Soils and Vegetation · Topic 7.2

Soil forms in stages as weathering turns rock into regolith, plants and microbes add organic matter, and water moves material up and down. The main processes are addition (of humus), loss (by leaching and erosion), transfer (eluviation, the washing out of material from upper layers, and illuviation, its deposition lower down) and transformation (changing of minerals). These produce a profile of layers called horizons: O, A, E, B, C and bedrock R. Regional processes include laterisation, podzolisation, calcification, salinisation and gleisation.

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

At a Glance

Horizons O, A, E, B, C, R
Eluviation Washing out from upper layers
Illuviation Deposition in lower layers
Tropical process Laterisation
Cold forest process Podzolisation
Exam link UPSC Prelims, Geography: Biogeography: Soils and Vegetation

Where Does It Fit?

This topic follows the one on soil characteristics and shows how soil develops through time. The processes and horizons described here explain the different kinds of soil that are classified in the next topic. Previous topic: Soil: Characteristics and Factors Responsible for Soil Formation. Next topic: Classification of Soils.

What Are the Stages of Soil Formation?

Weathering of the parent rock: physical, chemical and biological processes break the rock into a loose mass called regolith, as in Weathering: Chemical, Physical/Mechanical and Biological Processes.
Colonisation: lichens, mosses and then grasses and shrubs establish on the regolith. Their roots and acids break the rock further and they add dead matter.
Accumulation of humus: microbes and animals turn the dead matter into humus, which darkens the upper layer.
Development of horizons: water moves material downward and the soil separates into layers.
Maturity: a stable profile develops in equilibrium with the climate and vegetation. In very old soils, minerals are leached out until little is left.

The soil is described as young or immature when it has only a thin humus layer over weathered rock, mature when it has well-developed horizons, and old when it is deeply weathered and leached.

Which Processes Shape the Soil?

Process	What happens	Region
Leaching	Downward removal of soluble minerals by percolating water	Humid regions
Eluviation and illuviation	Fine particles and minerals are washed out of the upper horizon (eluviation) and deposited in the lower one (illuviation)	Humid regions
Laterisation	Silica is leached and iron and aluminium oxides remain, giving red, acid, infertile laterite	Hot, wet tropics
Podzolisation	Acid from coniferous litter leaches iron and aluminium from the upper layer, leaving a pale ash-grey layer	Cold, humid coniferous forests
Calcification	Lime accumulates in the lower horizon where evaporation exceeds rain	Semi-arid grasslands
Salinisation	Salts rise with evaporating water and collect at the surface	Dry, irrigated areas
Gleisation	Waterlogging reduces iron and gives grey, blue and mottled soil	Poorly drained areas, wetlands
Humification	Breakdown of organic matter into humus	Grasslands and forests

What Are the Layers of a Soil Profile?

Horizon	Name	Features
O	Organic layer	Leaf litter and partly decomposed matter on top, thick in forests
A	Topsoil	Dark, rich in humus and living things; where most roots grow; mineral and organic matter mixed
E	Eluvial layer	Pale layer from which clay, iron and humus have been leached; present in humid forest soils
B	Subsoil	Accumulation of clay, iron, aluminium or lime washed down from above
C	Parent material	Partly weathered rock with little organic matter
R	Bedrock	Unweathered solid rock

A vertical section through all the layers is called the soil profile, and soil scientists dig a pit to read it like a book of the soil’s history. The A and B horizons together are called the true soil or solum. Grassland soils have a thick, dark A horizon, while desert soils have thin or missing horizons. Forest soils have a thick O layer of leaf litter, and tropical soils are often deep with indistinct boundaries because mixing by termites and roots blurs the layers. Farmers care most about the depth of the A horizon, because it holds the nutrients, and about whether the B horizon has a hard, compact layer that stops roots and water from going deeper. Reading the profile also tells engineers about drainage before they build roads, houses or canals.

What Are Zonal, Intrazonal and Azonal Soils?

Dokuchaev’s early system grouped soils by how far they reflect climate and vegetation.

Zonal soils: mature soils whose properties reflect the climate and vegetation of a wide zone, such as podzol, chernozem and laterite.
Intrazonal soils: soils whose character comes mainly from a local factor, such as bad drainage (bog and marsh soils), salt (saline soils) or limestone.
Azonal soils: young soils without developed horizons, such as alluvium, sand dunes and mountain screes.

As an analytical point, the profile records the history of the soil, and so the same rock gives different soils under different climates, as in Black soil From basalt (Deccan Traps) Red soil From granite and gneiss…">Relationship Between Rocks and Soils. The soil types arising from these processes are classified in Classification of Soils.

Why Is This Topic Important?

Processes Leaching and laterisation are asked
Profile Horizon names are asked
Link Soil types

What Came Next?

The next topic describes the classification of soils. Next topic: Classification of Soils.

Key People and Terms

Horizon Distinct soil layer
Solum A and B horizons together
Leaching Washing out of soluble minerals
Eluviation Removal from an upper layer
Illuviation Accumulation in a lower layer
Pedogenesis Formation of soil

Exam Corner

Points to Remember

Horizons: O, A, E, B, C, R.
A horizon is topsoil; B is subsoil.
Laterisation: hot and wet tropics.
Podzolisation: cold coniferous forests.
Calcification: semi-arid regions.

Do Not Confuse With

Eluviation and illuviation Moving out versus moving in
Leaching and erosion Dissolved loss downward versus physical removal by agents
Zonal and azonal soils Climate-controlled versus undeveloped soil

Memory Hook

A is for top, B is for below, C is for parent rock, R for the bedrock below.

Mains Angle

Questions ask for the processes and profile. These points add depth.

Stages Rock to regolith to soil
Processes Climate-specific
Profile Layers and what they show

A Question You May Face

An original practice question, not a past paper question.

“Describe the stages and processes of soil formation and explain the profile of a mature soil.”

How to Answer

Stages Weathering to maturity.
Processes Leaching to calcification.
Profile Horizons O to R.
Types Zonal, intrazonal, azonal.

GS Relevance

Prelims: soil horizons, processes such as leaching, laterisation, podzolisation, calcification. GS Paper 1: soils.

Frequently Asked Questions

What is a soil profile?

A soil profile is a vertical section through the soil, from the surface down to the parent rock, which shows its layers or horizons. The main horizons are the organic layer O, the topsoil A, the leached layer E, the subsoil B, the weathered parent material C and the bedrock R.

What is the difference between eluviation and illuviation?

Eluviation is the removal of fine particles, minerals and humus from an upper soil horizon by percolating water. Illuviation is the deposition of these materials in a lower horizon. The two processes together produce the contrast between the leached upper layers and the enriched subsoil.

What is leaching?

Leaching is the downward removal of soluble minerals and nutrients from the soil by water percolating through it. It is strongest in areas of heavy rainfall, and it leaves the upper soil poor in nutrients, as in tropical laterite soils, which are rich in iron and aluminium oxides but poor in plant nutrients.

What is laterisation?

Laterisation is a process of soil formation in hot, humid tropical regions in which heavy rain leaches away silica and soluble bases and leaves behind iron and aluminium oxides. The result is a red, acidic, nutrient-poor soil called laterite, which hardens like brick when it dries.

What is podzolisation?

Podzolisation is a soil-forming process in cold, humid, coniferous forests. Acids from decaying needles leach iron, aluminium and humus from the upper layer, leaving a pale ash-grey horizon, and deposit them lower down. The resulting acidic, infertile soil is called a podzol.

PYQ Practice — Statement Analysis

1 The A horizon is the topsoil that is rich in humus.

True

Most roots grow there.

2 Laterisation takes place in cold coniferous forests.

False

It takes place in the hot, wet tropics.

3 Calcification occurs in semi-arid regions.