Origin of Soils

Soils are formed by weathering of rocks and decomposition of organic matter.

There fore, soils may be organic or inorganic (depends on source of origin).

Organic soils are called ‘Cumulose soil'

Examples: Peat, Humus, muck etc.

Geological Cycle: Weathering of rocks —Transportation — Deposition — Upheaval.

Types of weathering:

1. Physical                                
2. Chemical

Physical weathering: 

• It is due to physical effects like temperature, abrasion ,wedging action of ice, penetration of plant roots etc.
• Physical weathering results in no change in chemical composition of particles.
• It produces coarse grained and non cohesive soils. 
• Examples: Gravel, Sands.

Chemical weathering:

• It is due to chemical actions (oxidation, hydration, carbonation, solution, leaching, hydrolysis, etc.
• Original rock minerals are transformed into clay minerals.
• It results in fine grained and cohesive soils.
• Examples: Clays.

 Sedimentary soils:

These soil particles created at one location, transported and finally deposited in another location.

Source of Transportation / Deposition	Types of Soil
River	Alluvial soil
Lakes	Lacustrine Soil
Sea	Marine Soil
Wind	Aeolian Soil e.g- Sand Dunes, Loess
Gravitation	Colluvial Soil e.g Talus
Glacial	Glacial deposit soil e.g- drift, till

Residual soils (Sedentary soils):

These are soils which remain on the parent rock without getting transported.

Examples: Black cotton soils, Laterite soils.

• Gravitational forces are predominant in gravels and sands.
• Surface forces, chemical forces, electrical forces are predominant in clays.

Commonly used soil Designations:

v

Varved Clay	Contains alternate thin layers of clay and silt It is generally a lacustrine deposit
Moorum	It is gravel mixed with red clay
Loam	Mixture of sand, silt and clay (Approximately in equal proportion)
Bentonite	It is a decomposed volcanic ash Contains high percentage of clay mineral. montmorillonite. Exhibits high degree of shrinkage and swelling.