Soils can be defined as an aggregate that is naturally occurring with or without organic content which can be separated by light mechanical agitation. Soils are a composition of mineral grains. Rocks are aggregates composed for mineral grains that is bound together by means of cohesive force.

The weathering process occurring with time will decrease the cohesiveness within the grains and result in the disintegration of the big rock masses into small particles called the soil. Hence the soil formation is a result of weathering of parent rock.

Formation of Soils

The soils are formed either by

  1. Mechanical Weathering
  2. Chemical Weathering

1. Mechanical Weathering

Mechanical weathering or physical disintegration of the parent rocks can occur due to the following processes:

Temperature changes: All types of rocks may not have same thermal coefficient that they behave differently for different temperatures. This will cause unequal expansion and contraction with change in temperature. When this process is repeated several times, the rock will start to disintegrate and soils are formed.

Wedging action of ice: The rocks consist of pores that gets filled up with water. During freezing temperature, the ice freezes and the volume increases. This volume change will make the area in expand.This creates cracks and further expansion will propagate the cracks. This will lead to broken pieces and finally convert as soil

Spreading of roots of plants: The roots of trees and shrubs grow in the cracks and fissures of the rocks, forces act on the rocks. The segments of the rock are forced apart and disintegration of rocks occurs.

Mechanical Weathering of Rocks - tree root intrusion

Fig.1: Mechanical Weathering of Rocks by tree root spreading. Image Courtesy: University of Space Research Organisation, Ken De Pue

Abrasion: Movement of water, wind and glaciers over the surface of rock, will result in abrasion and scouring. It results in the formation of soils.

The physical disintegration of rocks won’t bring any change in the chemical composition. The properties of soil will be similar to that of a parent rock. The soils formed by physical disintegration are gravels, sand and coarse-grained soils.

2.Chemical Weathering

Easy transformation of hard rocks to soft and erodible materials is conducted through the process of chemical weathering or chemical decomposition. Below mentioned are the principal types of chemical decomposition happening in rocks.

Hydration: In hydration, water combines with rock minerals and results in the formation of a new chemical compound. The chemical reaction causes a change in volume and decomposition of rock into small particles.

An example of hydration reaction that is taking place in soils is the hydrolysis of SiO2

SiO2+ 2H2O —>  Si(OH)4

Carbonation: It is a type of chemical decomposition in which carbon dioxide in the atmosphere combines with water to form carbonic acid. The carbonic acid reacts chemically with rocks and causes their decomposition.

Sedimentary rocks which contain calcium carbonate are the products of chemical reaction of rocks by carbonation.

Oxidation: Oxidation occurs when oxygen ions combine with minerals in rock. Oxidation results in decomposition of rocks.

Solution: Some of the rock minerals form a solution with water when they get dissolved in water. Chemical reaction takes place in the solution and the soils are formed.

Hydrolysis: It is a chemical process in which water gets dissociated into H+ and OHions. The hydrogen cations replace the metallic ions such as calcium, sodium and potassium in rock minerals and soils are formed with a new chemical composition.

Chemical Weathering of Granite Rock By Hydrolysis

Fig.2. Chemical Weathering of Granite Rock By Hydrolysis; Image Courtesy: Radford Univeristy

Most of the clay minerals are derived from chemical decomposition. Plastic properties in soils are imparted by clay minerals. Hence clay soil are major products of chemical decomposition of rocks.

Different Types of Soils

As mentioned above, the soils can be formed by physical or chemical weathering processes. The size of individual particle can vary from a small colloidal state to large boulder forms. Hence, this concludes that all the products of rock weathering can be considered as soils.

1. Types of Soils Based on Grain Size

  • Cobbles : These are larger than pebbles and smaller than boulders. The size is between 64 to 256mm.
  • Gravels: The diameter is between 4.75 and 76.2mm
  • Clay: Soil grains finer than 0.002mm
  • Silt: Soil grain with size ranging from 0.075 to 0.002mm
  • Sand: Particles seen by naked eye and size less than 4.75mm
Clay, Sand, Silt and Loam

Fig.3: Clay, Sand, Silt and Loam ; Image Courtesy: Wilson Bros Gardens

2. Types of Soils Based on Origin

The soils can be classified into two based on origin as:

Residual Soils: These types of soils will remain at the place of their formations. They are located near the weathered parent rock. The two factors governing the depth of the residual soils are the exposure time and climatic conditions. These are stiff and stable in temperature zones. The size of residual soil is indefinite.

Transported Soils: As the name say, these are soils that are transported from the area of weathering to some other location by means of transportation agents like wind, water, ice or gravity. Based on which they are classified as shown in below table.

Table.1: Different Transported Soils and their Transporting Agent

Sl. NoSoil TypeTransporting AgentProperties
1Alluvial SoilRunning WaterCoarser and Finer Particles
2Lacustrine SoilWater – Deposited at quiet lakesCoarser and Finer Particles
3Marine SoilsSea waterCoarser and Finer Particles
4Aeolian SoilsWindSilt – low density and high compressibility
5Colluvial SoilsGravitational Force
6Glacial SoilGlaciersFiner Particles, boulders

The soils formed at a place may be transported to other places by agents of transportation, such as water, ice, wind and gravity.

Organic Soils & Inorganic Soils: The soils that have organic origin comes under the category of organic soils. These are formed by the growth and decomposition of the plants, shells of organisms, inorganic skeletons etc. Sometimes inorganic content by some source can make the soil inorganic.

Different Soils Used in Practice

  1. Bentonite : The clay formed by the decomposition of volcanic ash. Properties of clay are exhibited highly. This contain high amount of montmorillonite.
  2. Kaolin: This is also called as china clay which is pure form of white clay. The main application are in clay industries
  3. Boulder Clay: This is acombination of sedimented deposited clay (glacial clay) with unsorted rocks and pulverised clay.
  4. Varved Clays: Consist of layers of silt and flat clays.
  5. Shale : This material have a state between clay and slate
  6. Marl : This material is a combination of clay, loam and calcareous sands
  7. Peat : Fibrous aggregate with decomposed fragments of vegetable matter. Very compressible in nature and used for supporting of structures.
  8. Loam : This material is a combination of silt, clay and sand