Maarg Manthan · Topic 7.1

Soil: Characteristics and Factors Responsible for Soil Formation

Geography › Biogeography: Soils and Vegetation · Topic 7.1

Soil is the thin, loose layer of mineral particles, organic matter, water, air and living organisms that covers most of the land and supports plants. A good topsoil is roughly 45 per cent minerals, 25 per cent water, 25 per cent air and about 5 per cent organic matter. Its main properties are texture (the proportion of sand, silt and clay), structure, colour, acidity (pH), moisture and fertility. Five factors form it: parent material, climate, relief, organisms and time.

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

At a Glance

Composition of topsoil 45% mineral, 25% water, 25% air, 5% organic
Texture classes Sand, silt, clay, loam
Key properties Texture, structure, pH, fertility
Formation factors Parent material, climate, relief, organisms, time
Soil science founder V. V. Dokuchaev
Exam link UPSC Prelims, Geography: Biogeography: Soils and Vegetation

Where Does It Fit?

This is the first topic of the macro on soils and vegetation. It describes what soil is and what controls it, building on the rocks and weathering studied earlier, before the stages of formation and classification are taken up. Previous topic: Interlinking of Rivers Projects and Challenges with Ageing Dams. Next topic: Stages, Processes and Profiles of Soil Formation.

What Is Soil Made Of?

In short: soil is a mixture of rock particles, rotted plant and animal matter, water, air and living things.

Component	Share in good topsoil	Role
Mineral particles	About 45 per cent	Come from weathered rock; supply nutrients and structure
Organic matter	About 5 per cent	Humus from dead plants and animals; binds particles, holds water and nutrients
Water	About 25 per cent	Dissolves nutrients and carries them to roots
Air	About 25 per cent	Gives oxygen to roots and soil organisms
Living organisms	Small by mass, vital	Earthworms, fungi, bacteria and insects break down matter and recycle nutrients

One gram of fertile soil may hold billions of microbes. Soil is therefore a living system, and not just crushed rock. The way weathering turns rock into soil is described in Black soil From basalt (Deccan Traps) Red soil From granite and gneiss…">Relationship Between Rocks and Soils and Weathering: Chemical, Physical/Mechanical and Biological Processes.

What Are the Main Characteristics of Soil?

Texture: the proportion of sand (0.05 to 2 mm), silt (0.002 to 0.05 mm) and clay (below 0.002 mm). Sandy soil drains fast and holds few nutrients; clay holds water and nutrients but drains poorly; loam, a balanced mix, is best for most crops.
Structure: the way particles clump into aggregates, which can be granular, blocky, platy or prismatic. Good structure lets air and water move.
Colour: dark soils are rich in humus, red and yellow ones are rich in iron oxides, and grey or bluish ones are waterlogged.
Acidity (pH): most crops grow best between pH 6 and 7.5. Heavy rain makes soils acidic, and dry regions make them alkaline.
Moisture and drainage: depend on texture, structure and slope.
Fertility and cation exchange capacity: the ability to hold and supply nutrients such as nitrogen, phosphorus and potassium.

Which Factors Control Soil Formation?

Factor	Effect	Example
Parent material	Gives the minerals and texture; strongest in young soils	Basalt gives black soil; granite gives red soil
Climate	Rainfall and temperature control weathering and leaching; the most important factor	Heavy tropical rain gives laterite; dry climates give alkaline soil
Relief (topography)	Steep slopes have thin soil; flat land and valleys have deep soil; slope aspect changes moisture	Thin hill soils and deep river-plain soils
Organisms	Vegetation supplies humus; animals and microbes mix and decompose	Dark grassland soils (chernozem)
Time	Soil formation is slow; some centimetres take hundreds of years	Young alluvial soils against old, leached tropical soils

This is the factors-of-formation approach to soil science that goes back to the Russian scientist Vasily Dokuchaev in the 1880s, and was developed by the American Hans Jenny in 1941. As an analytical point, soil is formed by climate and living things acting on rock over time, so the soil map of a country follows its climate and geology. The processes and the vertical layers of soil are in Stages, Processes and Profiles of Soil Formation.

Why Does Soil Matter?

Soil is the base of farming, and about 95 per cent of the food people eat is grown directly or indirectly in soil. It stores and cleans water, since rain filters through it to recharge groundwater, and it holds more carbon than the atmosphere and all plants together. It is also the home of a large share of the planet’s living species, from microbes to earthworms. Yet it is a slowly renewable resource: forming a centimetre of topsoil can take a century or more, while poor farming can wash or blow it away in a few seasons. Protecting the soil is therefore central to food security, climate action and the health of rivers, and it is the main reason the United Nations marks World Soil Day each year on 5 December.

Why Is This Topic Important?

Base Foundation for soils in India
Facts Texture and composition asked
Farming Soil decides crops

What Came Next?

The next topic explains the stages, processes and profiles of soil formation. Next topic: Stages, Processes and Profiles of Soil Formation.

Key People and Terms

Humus Dark decomposed organic matter
Loam Balanced mix of sand, silt and clay
Soil texture Proportion of sand, silt and clay
pH Measure of acidity or alkalinity
Parent material Rock or deposit from which soil forms
Pedology Study of soils

Exam Corner

Points to Remember

Topsoil: about 45% mineral, 25% water, 25% air, 5% organic.
Loam is the best texture for crops.
Dokuchaev is the father of soil science.
Five factors: parent material, climate, relief, organisms, time.
Climate is the most important factor.

Do Not Confuse With

Soil texture and structure Particle sizes versus the way they clump
Humus and organic matter Stable decomposed part versus all dead matter
Soil and regolith Layer with humus and life versus loose weathered rock

Memory Hook

Parent, climate, relief, organisms, time; five factors make the soil prime.

Mains Angle

Questions ask for the factors of soil formation and their effect on soil types. These points add depth.

Factors Five controls
Climate Dominant role
Management Soil fertility and conservation

A Question You May Face

An original practice question, not a past paper question.

“Explain the factors responsible for soil formation and the characteristics that decide soil quality.”

How to Answer

Define Soil and its components.
Properties Texture, structure, pH.
Factors Five controls.
Link Crops and soil types.

GS Relevance

Prelims: soil composition, texture, factors of soil formation. GS Paper 1: soils and their distribution; GS Paper 3: agriculture.

Frequently Asked Questions

What is soil?

Soil is the thin surface layer of the Earth's land made of weathered mineral particles, decayed organic matter, water, air and living organisms. It supports plant growth. A good topsoil is about 45 per cent minerals, 25 per cent water, 25 per cent air and 5 per cent organic matter.

What are the factors of soil formation?

Soil forms under the influence of five factors: the parent material from which it is derived, climate, relief or topography, organisms including vegetation, and time. Climate and parent material are generally the most important, while time decides how far the soil has developed.

What is soil texture?

Soil texture is the relative proportion of sand, silt and clay particles in the soil. Sand particles are the largest and clay the smallest. Texture determines drainage, water holding capacity and fertility, and a mixture of the three, called loam, is the most suitable for crops.

What is humus?

Humus is the dark, stable part of organic matter formed when dead plants and animals are decomposed by microbes. It binds soil particles, holds water and nutrients, improves structure and increases the fertility of soil. It is greater in grassland soils and in cool, moist climates.

Who is regarded as the founder of soil science?

The Russian scientist Vasily Dokuchaev is regarded as the founder of soil science. In the 1880s he showed that soil is a natural body that forms as a result of climate, living organisms, parent rock, relief and time, and that soils change with latitude and climate zones.

PYQ Practice — Statement Analysis

1 Loam is a mixture of sand, silt and clay.

True

It is generally the best soil for crops.

2 Time is not a factor in the formation of soil.

False

Soil formation takes a long time.

3 Clay particles are larger than sand particles.