Origin of Soil

Exposed rocks undergo erosion and erosion through physical and chemical processes. The eroded material is transported by agents such as water and wind, which alter and disrupt the Earth’s equilibrium, causing significant movements and upheavals in the Earth. This repetition exposes more rocks, continuing the geological cycle.(Basic introductio of Soil)

If the soil remains near its site of formation, just above the parent rock, it is called residual or sedimentary soil. The engineering properties of residual soil vary depending on its origin. If soil is transported to a new location, it is called transported soil.

Formation of Soil

Soil formation involves geological processes like weathering, transportation, deposition, and upheaval, driven by physical and chemical disintegration of rocks.

1. Weathering of Soil

Physical Disintegration

Temperature changes: Minerals in rocks have different coefficients of thermal expansion, causing stress and separation of particles due to temperature fluctuations.

Ice wedging: Water freezing in the pores and cracks of a rock expands, increasing stress that causes the rock to break.

Spread of plant roots: Plant roots penetrate into the cracks of the rock and exert force, causing the rock to gradually break into pieces.

Abrasion: Water, wind, and glaciers cause friction and abrasion on rock surfaces, resulting in soil formation.

In physical dissolution, the chemical composition remains unchanged, leading to the formation of coarse-grained soils such as gravel and sand.

Chemical Decomposition

Hydration: Minerals react with water, forming new chemical compounds, changing volume, and decomposing rocks into smaller particles.

Carbonation: Carbon dioxide in the atmosphere combines with water to create carbonic acid, which then reacts with rocks, causing decomposition.

Oxidation: Oxygen ions combine with minerals, akin to rusting, leading to rock decomposition.

Solution: Some minerals dissolve in water through chemical reactions, forming soil.

Hydrolysis: Water dissociates into H+ and OH- ions, replacing metallic ions in rock minerals, resulting in soil with altered chemical composition.

Chemical decomposition yields clay minerals, imparting plasticity to the soil. Clayey soils primarily form through chemical decomposition.

2. Transportation of Soil

Soil is transported by natural agents like running water (rivers), moving ice (glaciers), and blowing wind to new locations. The fragmented material weathered from rocks and transported to a different location is referred to as transported soil.

Classification Based on Transporting Agent

Water-Transported Soil

Alluvial Deposits: Layers of sand, silt, and clay carried swiftly by running water, forming alluvial soil. Examples include alluvial cones and deltas.

Lacustrine Deposits: Soil carried by rivers into lakes, with coarse particles settling at the lake’s edge and fine-grained particles settling in the center, forming layers with seasons. These are called lacustrine deposits.

Marine Deposits: Soil transported and deposited in the deep sea by sedimentation of soil particles. Marine deposits have low shear strength, high compressibility, and contain a significant amount of organic matter, especially marine clays.

Wind-Transported Soil

Fine-grain soil eroded and deposited by wind, forming aeolian deposits. Accumulations of such wind-deposited sand create dunes.

Loess: Silt deposited by wind, with low density, high compressibility, and poor bearing capacity when wet. It can experience volume reduction with increased moisture, known as collapse compression due to the collapse of soil structure.

3. Glacial Deposits

Soil formed from large masses of ice (glaciers) compacting snow. Glaciers, moving slowly, deform and scour surfaces. Melting glaciers deposit the material, termed as till or moraine. Glacial deposits typically contain angular particles.

Gravity Deposits

Material transported over short distances by gravity, termed colluvial soil or talus. These soils form in mountain valleys due to gravitational forces causing sediments to creep or slide down slopes.

Swamp and Marsh Deposits

In water-saturated areas with fluctuating water tables and vegetation, swamp and marsh deposits form. These soils are soft, high in organic content, and may contain muck or peat derived from aquatic plant decomposition. They are not suitable for construction due to their lightweight, high compressibility, and unpleasant Odor.

In India, there are several major types of soil deposits, each with its own characteristics, distribution, and significance. The primary soil deposits in India include

Alluvial Soil

Alluvial soil is one of the most widespread and agriculturally significant soil types in India. It is found in the Indo-Gangetic plains, river valleys, and deltas, including regions near the Ganges, Yamuna, Brahmaputra, and other major rivers. It is fertile and ideal for growing various crops.

Black Soil (Regur Soil)

Black soil, also known as “black cotton soil,” is found mainly in the Deccan Plateau, particularly in states like Maharashtra, Madhya Pradesh, and Gujarat. It is known for its dark color and high clay content. When properly irrigated, it can be very productive for cotton and other crops.

Red and Yellow Soil

Red and yellow soils are prevalent in the central and southern parts of India, covering states like Tamil Nadu, Karnataka, Andhra Pradesh, and parts of Maharashtra. They are characterized by their reddish to yellow color and are suitable for crops like millets, pulses, and groundnuts.

Laterite Soil

Laterite soil is found in the western ghats, parts of Karnataka, Kerala, and Odisha. It is formed by the leaching process and is typically rich in iron and aluminum oxides. Although not very fertile, it can be suitable for tea, coffee, and rubber plantations.

Arid and Desert Soil

Arid and desert soils are found in arid regions of Rajasthan, Gujarat, and Haryana. They are sandy and less fertile, suitable for drought-resistant crops and xerophytic vegetation.

Mountain and Forest Soil

Mountain and forest soils are found in the hilly and mountainous regions of India, including the Himalayas and the Western Ghats. They are generally acidic and suitable for the growth of forests.

Peat and Marshy Soils

Peat and marshy soils are found in the coastal regions, especially in the deltas of rivers like the Ganges and Brahmaputra. They are rich in organic matter and are suitable for rice cultivation.

Coman Types of Soil Terminology

Coman Types of Soil Terminology

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