Origin of Soil; Word Origin: Derived from Latin word ‘Solum’ meaning “floor”.

Definition of Soil; A dynamic natural body composed of minerals, organic materials, living organisms, air, and water, in which plants grow.

Branches of Soil Science

a) Edaphology

• Study of: Soil in relation to plant growth, nutrition, and crop yield.
• Word origin: Greek
  • Edaphos = Soil
  • Logos = Discourse
• Viewpoint: Soil is a natural medium for plant growth.

b) Pedology

• Study of: Origin, classification, and formation of soils.
• Word origin: Greek
  • Pedon = Soil
  • Logos = Study
• Father of Pedology: V.V. Dokuchaiev
• Viewpoint: Soil is a natural body.

Classification of Rocks Based on Mode of Formation

Rocks are classified into three major types:

1. Igneous Rocks

• Formed by the cooling and solidification of molten magma or lava.
  Examples: Granite, Basalt, Diorite, Gabbro, Syenite, 
• Types of Igneous Rocks:
  • Plutonic (Intrusive): Formed inside the Earth; cooling is slow → large crystals.Examples: Granite, Gabbro, Diorite, Syenite
  • Volcanic (Extrusive): Formed on the Earth’s surface; rapid cooling → glassy structure. Examples: Basalt, Andesite, Trachyte
• Note: Basalt is widely found in the Deccan Trap region of India.

2. Sedimentary Rocks

• Derived from fragmentary materials of other rocks by deposition and consolidation of particles.
  Formed by agents like water, wind, glacier, and gravity.
• Examples: Limestone, Dolomite, Shale, Sandstone,
• Conglomerate Cover ~80% of Earth’s visible surface but only ~5% of the Earth’s volume.

3. Metamorphic Rocks

Formed from igneous or sedimentary rocks by heat, pressure, or both, changing their texture and composition.

Examples & Origin:

• Gneiss (from Granite)
• Marble (from Limestone)
• Quartzite (from Sandstone)
• Slate (from Shale)
• Schist

Important Rock-Forming Minerals

Quartz (SiO₂)

• Most resistant to weathering
• Main component of acid rocks (e.g. granite)
• Gives rise to sandy soils
• Constitutes 36% of Earth’s crust
• Major component of sand fraction
• Acid rocks: Rich in free silica (e.g., granite)
• Basic rocks: Low in free silica (e.g., basalt)

Feldspars (K₂O·Al₂O₃·6SiO₂)

• ~48% of Earth’s crust
• Easily weathered → forms clay
• Potash Feldspar: Orthoclase (acid rocks)
• Plagioclase Group: Albite (Na), Anorthite (Ca)

Micas

• Micas (10% of Earth’s crust)
• Muscovite (White Mica): Potash-rich, more resistant
• Biotite (Black Mica): Magnesium-rich
• More resistant to weathering than feldspar

Olivine (FeMg)₂SiO₄

• Least resistant
• Weathers to serpentine and talc

Tourmaline; Source of boron, a micronutrient

Sources of Plant Nutrients in Rocks

Primary vs Secondary Minerals

• Primary Minerals (Coarse texture – sands, gravels): Examples: Quartz, Orthoclase, Mica, Hornblende, Augite, Olivine
• Secondary Minerals (Fine texture – clay, silt): Examples: Clay minerals, Goethite, Haematite, Calcite, Gypsum

Weathering Resistance of Minerals

Classification of Rocks by Silica Content

Rock Formation Cycle 

1. Igneous rock (solidification of magma/lava)
2. Weathering → Sediments
3. Sedimentary rock (compaction & cementation)
4. Heat & Pressure → Metamorphic rock
5. Melting → Magma → Cycle repeats

What is Weathering?

Weathering is the natural breakdown of rocks at or near the Earth’s surface by physical, chemical, or biological processes.
Weathering ≠ Erosion (movement of weathered materials)

Key Concept: Rock ➝ Fragmentation ➝ Chemical alteration ➝ Organic addition ➝ Soil formation

Classification of Weathering; There are 3 types of weathering processes:

(A) Physical or Mechanical Weathering

Breakdown of rocks without any chemical change. Dominant in arid, cold, and mountainous regions.
➡️ Leads to the formation of skeletal soils.

Agents of Physical Weathering:

1. Heat and Cold: Differential expansion of minerals → peeling/flaking
2. Freezing and Thawing: Water expands ~9% on freezing → force of ~150 tons/sq.ft.
3. Glaciers: High grinding power (40 lb/in² per 100 ft of thickness)
4. Streams & Rivers; Erosion power ∝ velocity⁶ → fine powder formation
5. Waves & Ocean Currents: Cause chipping and pounding of rocks
6. Wind with Sand Particles: Abrasive weathering common in deserts

(B) Chemical Weathering

Involves decomposition of minerals and formation of secondary products (e.g. clays, oxides).

Most active in:

• Humid tropical zones
• Warm & moist climates

Key Chemical Processes:

(C) Biological Weathering: Involves both physical and chemical weathering by living organisms.

Biological Agents:

• Lichens & Mosses: Secrete acids that break minerals
• Plant Roots: Penetrate cracks, produce CO₂
• Microorganisms: Decompose organic matter and release acids
• Animals & Earthworms: Burrow, churn, and aerate the soil

“Biological weathering is essentially physical and chemical weathering caused by biological agencies” – Joffe

Soil Development & Water Movement

Water movement influences mineral horizon development:

1. Humid Regions: Leaching → minerals move to lower layers
2. Arid Regions: Capillary rise → salt accumulation
3. Impermeable Subsoil: Lateral movement of water → accumulation on horizon

Weathering Susceptibility of Rocks

Chemical Composition of Earth’s Crust

Exam Pointers

• Kankar nodules form by carbonation and are found in Red and Black soils of India.
• Physical weathering dominates in deserts and alpine regions → Skeletal soils.
• Hydration and hydrolysis are key in forming clay minerals.
• Reduction occurs in anaerobic (waterlogged) soils.
• Oxidation colors soil red (Fe³⁺ → Fe(OH)₃).