Maarg Manthan · Topic 2.7

Relationship Between Rocks and Soils

Geography › Geology – Minerals and Rocks · Topic 2.7

Soil is formed from the weathering of rocks, so the parent rock decides the minerals, texture and nutrients of the soil. Basalt weathers into the black cotton soil (regur) of the Deccan, granite and gneiss into red soils, heavy rain on rocks into laterite, river deposits into alluvium, and limestone into reddish clay. Besides parent material, soil also depends on climate, relief, living things and time, so rock alone does not fix the soil.

5 min read · UPSC Prelims · Geography · Geology – Minerals and Rocks

At a Glance

Parent material Rock from which soil forms
Black soil From basalt (Deccan Traps)
Red soil From granite and gneiss
Laterite Heavy rain on rocks
Other factors Climate, relief, organisms, time
Exam link UPSC Prelims, Geography: Geology – Minerals and Rocks

Where Does It Fit?

This topic links the rocks of the macro to the soils studied in the macro on biogeography. It shows how weathering turns solid rock into the material on which plants grow. Previous topic: Relationship Between Rocks and Landforms. Next topic: Three Families of Rocks: Igneous, Sedimentary, Metamorphic.

How Does Rock Turn into Soil?

In short: weathering breaks rocks into small pieces and changes their minerals, and organic matter and living things then make it into soil.

Weathering: physical, chemical and biological processes break rock into fragments and new minerals, as in Weathering: Chemical, Physical/Mechanical and Biological Processes.
Regolith: the loose mantle of broken rock above bedrock.
Accumulation of humus: plants, microbes and animals add organic matter.
Layering: over time the material develops layers called horizons.

The rate of soil formation is slow. It can take hundreds to thousands of years to form a centimetre of soil, depending on the climate and rock, which is why soil lost through erosion is hard to replace. Soil that stays where it formed from the rock below is a residual soil. Soil carried by water, wind or ice and laid elsewhere is a transported soil, such as alluvium, loess and glacial soils.

Which Rocks Give Which Soils?

Parent rock	Soil	Features	Where found in India
Basalt (Deccan lava)	Black soil or regur	Clayey, holds moisture, swells when wet and cracks when dry; rich in lime, iron and magnesium; good for cotton	Maharashtra, Madhya Pradesh, Gujarat, Karnataka
Granite, gneiss, schist	Red soil	Sandy to loamy, red from iron oxide, low in nitrogen and humus	Tamil Nadu, Karnataka, Andhra Pradesh, Odisha, Chhattisgarh
Many rocks under heavy rain	Laterite	Leached, rich in iron and aluminium, poor in nutrients and acidic; hardens on drying	Western Ghats, Meghalaya, Odisha, Keralam (Kerala)
River sediments	Alluvium	Fertile, loamy, formed from material washed down from mountains	Indo-Gangetic plain, coastal plains and deltas
Limestone	Reddish clay (terra rossa)	Residue left after calcite is dissolved	Mediterranean lands; limestone regions
Wind-blown dust	Loess	Fine, yellowish, fertile	China, the American prairies

How Do Minerals in Rocks Affect Soil Quality?

Nutrients: feldspar and mica release potassium, apatite releases phosphorus, and basalt minerals release calcium, magnesium and iron.
Texture: quartz is hard to break down and gives sandy soil, while feldspar and mica weather into clay, so the mineral mix decides whether the soil is sandy, silty or clayey. Texture controls drainage and water holding, and so decides which crops can be grown.
Colour: iron oxides make soils red or yellow, and organic matter and titanium minerals darken them.
Acidity: limestone parent material gives alkaline soils, whereas granite and sandstone give acidic ones.

What Else Besides Rock Controls Soil?

Five factors work together: parent material, climate, relief, organisms and time. In the humid tropics, heavy rain removes silica and nutrients and gives laterite from nearly any rock, so climate can override the parent rock. Steep slopes have thin soils, while flat floors have thick soils. The same basalt gives different soils in dry and wet climates, which is why regur is common in the drier Deccan, but laterite forms on basalt in the wet Western Ghats. Time matters as well: young soils on fresh volcanic ash or river deposits are fertile, while very old soils in the wet tropics have lost most nutrients. The stages of soil formation and the soil types of the world are taken up in the macro on soils and vegetation, and the rock-related causes of landforms in Relationship Between Rocks and Landforms. As an analytical point, agriculture follows geology, since the choice of crop in a region depends on its soils, such as cotton on regur and tea on acidic hill soils.

Why Is This Topic Important?

Link Geology to agriculture
Indian soils Parent rock questions
Concept Residual versus transported soil

What Came Next?

The next topic classifies rocks into the three families: igneous, sedimentary and metamorphic. Next topic: Three Families of Rocks: Igneous, Sedimentary, Metamorphic.

Key People and Terms

Parent material Rock that gives rise to the soil
Regolith Loose weathered mantle
Residual soil Soil formed in place
Regur Black cotton soil
Laterite Iron-rich leached soil
Humus Decayed organic matter

Exam Corner

Points to Remember

Regur forms from basalt.
Red soil comes from granite and gneiss.
Laterite forms in heavy rainfall with leaching.
Alluvium is a transported soil.
Soil factors: parent material, climate, relief, organisms, time.

Do Not Confuse With

Residual and transported soil Formed in place versus carried
Regolith and soil Weathered rock versus a layer with humus and horizons
Laterite rock and laterite soil Hard ore-like rock versus soil that has formed on it

Memory Hook

Basalt to black, granite to red; heavy rain leaches laterite instead.

Mains Angle

Questions link soils to rocks and farming. These points add depth.

Link Rock, weathering and soil
Other factors Climate overrides rock
Use Crops and soil types

A Question You May Face

An original practice question, not a past paper question.

“Explain the role of parent rock in determining the characteristics of soils, with Indian examples.”

How to Answer

Process Weathering to soil.
Examples Regur, red soil, laterite.
Other factors Climate, relief, time.
Link Crops and land use.

GS Relevance

Prelims: parent rock of Indian soils, factors of soil formation. GS Paper 1: soils and distribution; GS Paper 3: agriculture.

Frequently Asked Questions

How are soils formed from rocks?

Soils form when weathering breaks rocks into fragments and alters their minerals into clays. Plants, microbes and animals then add organic matter called humus, and over time the material develops distinct layers called horizons. The parent rock, climate, relief, organisms and time all influence the result.

Which rock forms black soil?

Black soil, also called regur or black cotton soil, forms from the weathering of basalt, the lava rock of the Deccan Traps. It is clayey, holds moisture well, is rich in lime, iron and magnesium, and is well suited to growing cotton in Maharashtra, Madhya Pradesh and Gujarat.

Which rocks give rise to red soil?

Red soil develops from the weathering of ancient crystalline rocks such as granite, gneiss and schist. Its red colour comes from iron oxide. It is generally low in nitrogen, phosphorus and humus, and is found widely in Tamil Nadu, Karnataka, Andhra Pradesh, Odisha and Chhattisgarh.

What is the difference between residual and transported soil?

Residual soil is formed from the weathering of rock where it lies, so it keeps a close link with the parent rock. Transported soil is made from material carried by water, wind or ice and deposited elsewhere, as with alluvium of river plains, loess and glacial soils.

Why does laterite form in heavy rainfall regions?

In regions of heavy rain and high temperature, water draining through the rock leaches out silica and soluble nutrients, leaving behind iron and aluminium oxides. This process produces laterite, a reddish, acidic soil that is poor in nutrients and hardens when dry, as in the Western Ghats.

PYQ Practice — Statement Analysis

1 Black cotton soil is formed from basalt.

True

It is the soil of the Deccan lava region.

2 Laterite soils are rich in humus and nitrogen.

False

They are poor in nutrients.

3 Alluvial soil is a transported soil.