NDA Geography · Earth's Structure, Landforms and Geological Time

Soils

Soil is the thin living layer that forms where weathered rock, climate, organisms, topography and time meet; soil-forming processes and a global classification organise its many types.

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Why this matters

5 PYQs. The earners are the soil-forming FACTORS (parent material, climate, organisms, topography, time — never 'human habitation'), the named soil PROCESSES (calcification, laterisation, podsolisation, gleisation), and a couple of classification facts (Histosols = organic soil).

Concept 1 of 3

How soil forms — the five factors

Intuition

Soil is what weathering leaves behind, slowly enriched with organic matter. Five natural factors control it: the PARENT material (the rock beneath), CLIMATE (rain and heat), living ORGANISMS, the TOPOGRAPHY (slope and drainage) and TIME. Human habitation is NOT one of the natural soil-forming factors — that's the classic trap.

Definition

The five soil-forming factors:

Parent material — the underlying rock that weathers into mineral grains.
Climate — temperature and rainfall drive weathering and leaching.
Organisms — plants, microbes and animals add humus.
Topography (relief) — slope controls drainage and erosion.
Time — soil deepens and matures over long periods.

Human habitation is NOT a soil-forming factor. Soil develops in distinct layers called horizons (O, A, B, C).

Worked example

Which of these is NOT a soil-forming factor: parent material, topography, climate, human habitation?

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 1Earth's Structure, Landforms and Geological TimeEASY

Which one of the following is not a soil forming factor?

[Q65 · Sep · 2021]

Concept 2 of 3

Soil-forming processes

Intuition

Once soil exists, named processes shape its profile. Translocation moves fine particles down from upper horizons (eluviation) and dumps them lower (illuviation). The dominant process depends on climate: dry climates concentrate lime (calcification), cold conifer forests strip and bleach the topsoil (podsolisation), and hot wet tropics leach everything but iron and aluminium (laterisation).

Definition

Translocation — fine particles washed DOWN by eluviation (removal from upper horizon) and deposited lower by illuviation.
Calcification — where evapotranspiration exceeds precipitation (dry climate), lime/calcium accumulates near the surface.
Podsolisation — in cool Taiga (coniferous) forests, acids bleach and leach the topsoil.
Laterisation — in hot wet tropics, silica is leached, leaving iron/aluminium oxides (red laterite).
Gleisation — waterlogged, poorly-drained soils.

Process classes: translocation, enrichment, removal, transformation.

Process	Where / what
Translocation	Eluviation (down) + illuviation (deposit) NDA 2025 — translocation = eluviation + illuviation.
Calcification	Evapotranspiration > precipitation (dry); lime builds up NDA 2023 — dry climate, evaporation exceeds rainfall → calcification.
Podsolisation	Taiga (coniferous) forest NDA 2023 — podsolisation is predominant in the Taiga forest.
Laterisation	Hot wet tropics (red laterite)

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 2Earth's Structure, Landforms and Geological TimeMODERATE

The process of Podsolization is predominantly found in :

[Q62 · Apr · 2023]

Drill 2 more on soil-forming processes

Concept 3 of 3

Soil classification (soil orders)

Intuition

The global soil-taxonomy splits soils into orders, each ending in '-sols'. A few are NDA-worthy: Histosols are the ORGANIC, peaty soils; Aridisols are dry-desert soils; Oxisols are heavily-weathered tropical soils; Vertisols are swelling clay soils.

Definition

Histosols — ORGANIC soils, rich in peat/decayed plant matter (waterlogged).
Aridisols — dry desert soils.
Oxisols — deeply weathered, iron/aluminium-rich tropical soils.
Vertisols — clay-rich soils that swell and crack (like India's black cotton soil).

Soil order	Character
Histosols	Organic (peaty) NDA 2023 — Histosols are the organic soil.
Aridisols	Dry desert soils
Oxisols	Weathered tropical soils
Vertisols	Swelling clays

Practice this conceptself-check · 2 quick reps

From the bank · past-year question

Example 3Earth's Structure, Landforms and Geological TimeHARD

Which one of the following is an organic soil?

[Q124 · Sep · 2023]

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

Soil-forming processes4 rows

Process	Where / what
Translocation	Eluviation (down) + illuviation (deposit) NDA 2025 — translocation = eluviation + illuviation.
Calcification	Evapotranspiration > precipitation (dry); lime builds up NDA 2023 — dry climate, evaporation exceeds rainfall → calcification.
Podsolisation	Taiga (coniferous) forest NDA 2023 — podsolisation is predominant in the Taiga forest.
Laterisation	Hot wet tropics (red laterite)

Soil classification (soil orders)4 rows

Soil order	Character
Histosols	Organic (peaty) NDA 2023 — Histosols are the organic soil.
Aridisols	Dry desert soils
Oxisols	Weathered tropical soils
Vertisols	Swelling clays

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