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

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

The same idea in a real exam question:

NDA · Sep 2021 · Q65Easy

Example 1 · Earth's Structure, Landforms and Geological Time · Soils

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

Concept 2 of 3: Soil-forming processes

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

The same idea in a real exam question:

NDA · Apr 2023 · Q62Moderate

Example 2 · Earth's Structure, Landforms and Geological Time · Soils

The process of Podsolization is predominantly found in :

Drill 2 more on soil-forming processes

Concept 3 of 3: Soil classification (soil orders)

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

The same idea in a real exam question:

NDA · Sep 2023 · Q124Hard

Example 3 · Earth's Structure, Landforms and Geological Time · Soils

Which one of the following is an organic soil?

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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