Formation And Types of Soil

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Soil Formation And Types and Properties of Soil

Formation And Types of Soil

"Soil Formation, Types, and Properties: Understanding the Essence of Earth's Foundation"

Soil, a complex and dynamic entity, is the fundamental support system for life on Earth. Its formation is a result of a slow and continuous process, influenced by various factors including climate, topography, parent material, organisms, and time. Over millennia, the interplay of weathering, organic decay, and geological processes gives rise to different types of soil.

These soil types—such as sand, silt, clay, and loam—each possess distinct properties that impact their ability to retain water, provide nutrients, and support diverse ecosystems. The texture, structure, pH levels, and composition of soils greatly influence their fertility and capacity to sustain plant life.

Understanding soil properties is vital for agriculture, construction, and environmental management. By grasping the nuances of soil composition, communities can effectively cultivate crops, manage water resources, and make informed decisions about land use and conservation practices.

Soil formation

Karl Terzaghi is known as the father of soil mechanics.

Soil/Soil deposit:

  • It is defined as an uncemented or weakly cemented accumulation of mineral particles, which are formed by the weathering (disintegration) of rocks.
  • The process of formation of soil is termed as 'pedogenesis'


It is the process of erosion, wear and tear of rocks, It can occur either physically or chemically.

Weathering of rocks


  • Impact and grinding by
    • Water
    • Air/ wind
    • ICE
  • Splitting action of ICE
  • Gravitational force
  • Plants and animals
  • No change in mineral content of soil


  • Oxidation (0,)
  • Reduction (H)
  • Carbonation (CO,)
  • Hydration (H,O)
  • Leaching by organic acid and H,0
  • Mineral content change from parent rock


If soil is formed by physical weathering, then there is no change in mineral content of soil but if it is formed by chemical weathering, then mineral content of soil differs from that of rock.

What is Leaching?

It is a process of extracting a substance from a solid material that is dissolved in a liquid.

What is Residual soil?

If the weathered material remains over the parent rock. It is termed as residual soil.

What is transported soil?

If the weathered material is transported. It is termed as transported soil.

The characteristics of soil depends upon the mode/agency of transportation, size of particles, shape and roundness, surface texture, and degree of sorting.

Transporting Agency / PropertiesWaterAirIceGravityOrganism (plants & animals)
SizeReductionConsiderable reductionConsiderable grinding & impactConsiderable impactMinor abrasion
Shapes and RoundnessRounding of sand & gravelHigh degree of roundingAngular, spherical particlesAngular non-spherical particleNo effect on shape
Surface TextureSmooth "shiny, polished"Impact produces frosted surfaceStriated surfaceStriated surfaceNo effect
SortingConsiderable sortingConsiderable sortingVery little sortingNo sortingLimited sorting
Different Modes of Transportation and their effect on the soil grain

important question

Q: Usually what could be the shape of a sand grain and a clay grain?

Sol: The majority of soils can be classified as either clays or sands. Sand grains generally have a rounded shape and clay particles have a flaky shape.

Size reduction and rounding shape due to transportation by water:

Effect of Transportation on shape of soil grain
Effect of Transportation on shape of soil grain

General cycle of soil formation:

Soil Formation Cycle
Soil Formation Cycle

types of soil

  • i) Alluvial soil: It is the soil which is deposited from suspension in running water. It is found along the banks of rivers (Northern Region of India).
  • ii) Lacustrine soil: It is the soil which is deposited from suspension in fresh, still water of lake.
  • iii) Marine soil: It is the soil which is deposited from sea water.
  • iv) Aeoline soil/sand dunes: It is the soil which is transported by blowing
    • Rounded shape, frosted texture.
    • Found in the arid or semi-arid region (Rajasthan and Gujarat)
  • v) Glacial soil: It is the soil which is transported by Ice
    • Found in Jammu and Kashmir.
  • vi) Colluvial soil/talus soil: It is formed due to transportation by gravity.
    Found in mountainous region (Uttarakhand and Himachal Pradesh).
  • vii) Loess soil: It is uniformly graded wind-blown silt, slightly cemented by calcium compound or Montmorillonite. It's cementing action and becomes soft, compressible and collapsible. So, it is also termed as collapsible soil.
  • viii) Marl soil: It is finely grained calcium carbonated soil of marine origin, which is formed due to the decomposition of aquatic plants and bones of animals.
    • Marine soil (Found in coastal area)
  • ix) Tuff soil: A fine grained slightly cemented volcanic ash that is transported by wind or water
  • x) Bentonite soil: It is chemically weathered volcanic ash, consisting high percentage of montmorillonite.
    • It is highly plastic, possess high shrinkage and swelling nature and low shear strength.
    • Generally used as lubricant in drilling operations.
  • xi) Black cotton soil: It is a residual soil, formed from basalt that contains a high percentage of montmorillonite.
    • It is suitable for "cultivating" cotton.
    • It shows high swelling/shrinkage, high compressibility, and low shear strength.
  • xii) Laterite soil: It is the soil formed due to leaching (washing out of silicious compound and accumulation of Fe2O3 and Al203). It is generally found in hilly areas having humid climate Ex. Northeast, Western Eastern ghats.
  • xiii) Muck Soil: It is a mixture of fine particles of inorganic soil and black decomposed organic matter. Found in marshes / swampy area and the area where over flooding of river happens.
  • xiv) Peat soil: It is highly organic soil which almost "entirely consisting" of black decomposed vegetative matter in different stage of decomposition.
    • It is highly compressible.
    • Cumulose soil - Muck soil + Peat soil
  • xv) Loam soil: It is a mixture of clay + sand + silt in definite proportion.
  • xvi) Gumbo soil: It is black, sticky and highly plastic soil.
Soil map of India
Soil map of India

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important question

Q: What is the difference between bentonite and black cotton soil?

Sol: Bentonite and black cotton soils are similar in their composition but the basic difference between them is that in Bentonite soil, weathering happens after volcanic eruption while black cotton soil is weathered basalt.

Q: What is soil formation and what are the primary factors influencing it?

Sol: Soil formation is the process through which rock materials break down into smaller particles over time. Factors influencing soil formation include climate, parent material, biological activity, topography, and time (known as the CLORPT factors).

Q: How does weathering contribute to soil formation?

Sol: Weathering, both physical and chemical, breaks down rocks into smaller particles. Physical weathering involves the physical breakdown of rocks, while chemical weathering involves chemical alterations that decompose or alter the minerals present, contributing to soil formation.

Q: Explain the role of organic matter in soil formation.

Sol: Organic matter, such as decayed plant and animal material, affects soil formation by adding nutrients and enhancing the soil structure. It aids in the development of soil horizons and increases soil fertility.

Q: What are the different soil horizons and how are they formed?

Sol: Soil horizons are layers of soil parallel to the soil surface, each with different properties. They are typically formed due to the processes of leaching, translocation, and accumulation of materials within the soil profile.

Q: What are the primary classifications of soil based on particle size?

Sol: Soils are classified into three main groups based on particle size: sand, silt, and clay. Additionally, there's loam, a soil type that consists of a balanced mixture of sand, silt, and clay.

Q: How does soil texture differ from soil structure?

Sol: Soil texture refers to the relative proportions of sand, silt, and clay in the soil, while soil structure refers to the way soil particles are organized and grouped together.

Q: Discuss the engineering significance of soil classification.

Sol: Soil classification is crucial in civil engineering as it influences the soil's behaviour in response to loads and its ability to support structures. It helps in determining soil characteristics, settlement, shear strength, and other engineering properties.

Q: Explain the Unified Soil Classification System and its relevance in engineering.

Sol: The Unified Soil Classification System categorizes soils based on their physical properties for engineering purposes. It assists in understanding the behaviour of different soil types and aids in making decisions regarding construction and design.

Q: How does soil type impact construction and foundation design?

Sol: Soil type influences the bearing capacity, settlement characteristics, and overall stability of foundations. Understanding the soil type is crucial for selecting appropriate foundation types and construction methods.

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