Soil and its Process of Formation

The word soil is derived from latin word solum meaning soil or land. Soil is the upper/weathered potion of earth crust. Typically soil is made up of parent material. Organic matter and living organism incorporated into it. The spaces between soil particles are filled with water and air.

Plants anchored in soil with help of roots. Soil is a source of nutrients for them. It also acts as great reservoir for the water. Plants absorb water from soil. It is also source of oxygen for the respiration of soil microorganism and roots of higher plants.

The solid portion of soil remain fairly constant in volume but the pore space between solid particles may be completely filled with water after rain.

Petrology is the science of rocks which form soil. Pedology is the study of soil in relation to their natural environment and deals with pedogensis, soil morphology and soil classification. Edaphology is the study of influences of soils on living things particularly plants. General sub fields of edaphology are agricultural soil sciences and environmental soil sciences.


Soil consists of small fragments of mineral matter.  These minerals are derived from solid rocks. Solid rocks are present beneath the surface of the earth. A rock is an aggregation of minerals. There are different aspects of soil formation like weathering, derivation of parent soil material and soil organic material.

The breakdown of rocks into mineral particles is called weathering. There are three main types of weathering:

  1. Mechanical weathering
  2. Chemical weathering
  3. Biological weathering

1. Mechanical weathering:

 The breakdown or disintegration of the rock by cracking through various agencies like water, temperature, wind and snow.  The rate of breakdown depends on the nature of the rock material and mechanical shocks. It is slow on igneous rocks like granite and basalt. Salty and lime stone rocks are broken down readily.


Fluctuations of temperature influence the disintegration of rocks. Rocks expand on heating and contract on cooling. Due to daily and seasonal changes in temperature, cracks are developed in rocks which are broken down into fragments.

Freezing and melting of water further increase weathering. Water expands on freezing and exerts a great pressure and breaks the rock.


Rock fragments of all kinds are brought down the slope with rain. When loaded with such rock pieces, the water has tremendous cutting power. Thus it cuts the rocks over which it flows.


Snow moves down the rocks from glaciers. It has grinding and abrasive action on the underlying rocks which are disintegrated. They also mix the mineral fragments of different types.


Wind carries small fragments of rocks from one place to another. The fragments strike on the rocks and cut the rocks. Moreover, the mineral particles blown with wind, strike against one another and are further disintegrated.

2. Chemical Weathering:

 New minerals are formed during chemical reactions. Some minerals are decomposed and disappear. It leaves more resistant minerals behind. Such processes are called chemical weathering. It commonly occurs in humid regions. Following processes take place during chemical weathering:

  • Hydrolysis
  • Hydration
  • Carbonation
  • Oxidation
  • Solvent action

It is process of decomposition with the help of water. Many minerals like feldspar and mica are broken down by hydrolysis.


In this case hydrogen and hydroxyl ions of water react with different compounds. For example hematite reacts with water to form limonite.


Water reacts with carbon dioxide to form carbonic acid. Carbonic acid reacts with rock.


Rocks containing iron undergo oxidation. It makes rocks more porous and breakable.

Solvent action

Water is universal solvent. It dissolves some of the mineral matter and cause weathering.

3. Biological Weathering:

Weakening and disintegration of rocks by plants, animals and microbes is called biological weathering.


Growing plant roots can exert stress or pressure on rock. Plant roots or microorganisms produce organic acids. These acids dissolve minerals and cause weathering.

Microbial activity

Microbes alter the chemical composition of soil. Certain bacteria produce ammonia, nitric acid, CO2. Similarly in lichen, fungi release chemicals and due to these chemicals, minerals are released


Burrowing and hoofed animals contribute to the disintegration of rocks. It can move rock fragment to the surface. It exposes the rock to more intense physical, chemical and biological processes. These processes indirectly enhance the process of rock weathering.


Parent material is the material from which soil is developed. Soil is classified into two types;

  1. Residual parent material
  2. Transported parent material

1. Residual parent material:

If the parent material remains at it place, it is called as residual parent material. Rock materials are exposed to atmosphere at same places. The intensity of weathering is greatest at these places. Complete physical decomposition and chemical alteration take place at the surface of a residual parent material. The mineral particles become larger with the increase of the depth. Therefore these particles are less altered chemically. They incorporate in bed rocks.

Residual parent material is composed of three types of rocks;

  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks

Igneous rocks are formed by the cooling of magma. Magma is hot mixture of elements. These rocks are present above or below surface of earth.


Different materials are deposited in lakes and oceans. These material forms layers. After passing a long geological time, rocks are formed. Such rocks are called sedimentary rocks.


These are formed of igneous or sedimentary rocks. These rocks are heated to intense pressure at considerable depths within earth, original minerals melt and forms new minerals. As a result new rocks are formed called metamorphic rocks.

2. Transported parent material:

If the parent material is transported to any other area by agencies, it is called transported parent material. They are composed of derived mineral particles. Various agencies like gravity, water, wind and ice bring these derived particles from their place of origin.

The transported soils vary according to the transporting agents and these agents operate one after the other. Therefore, the parents of this class does not resemble the underlying rocks.

The transported parent material or soil may be;

  • Colluvial
  • Alluvial
  • Glacial
  • Aeolian (Eolian)

The soil transported by gravity is known as colluvial soil. This transported deposited material is called talus. Such soil is present mostly in mountain regions.

Hillside deposits are colluvial in nature. Colluvial parent material is coarse and stony. It has least agricultural importance.


Soil transported by water is called alluvial soil. It has three general classes

  • Alluvial fans
  • Floodplain
  • Delta deposits
Alluvial fans  

Stream has narrow valley in mountain area. It then suddenly depends to a broader valley. Here minerals are deposited in the form of a fan, therefore called alluvial fan. These are found in mountains area

Flood plain

It is composed of flood affected valley. Its soil is rich in nutrients. It is suitable for agriculture.

Delta deposits

These are present near mouth of rivers. The river water has a large amount of suspended material. This material settles in this land. Delta are clay in nature and has fertile soil. It is excellent for agriculture.


Soil transported by glaciers is called glacial soil. Glacier carries material like boulders, gravel, sand and clay from area and deposits them to another area. It forms glacial tills.

It has two forms;

Basal till & Ablation till

Basal till

The material is carried in the base of glacier and is deposited under is called basal till.

Ablation till

The material is carried on or near the surface of glacier and is deposited on melting of glacier is called ablation till.


The soil transported by wind is called aeolian or eolian soil. Aeloian sediments consist of primarily sand or silt sized particles. These materials are extremely well sorted. Some rounding and frosting of mineral grains may be present in it.

Aeolian soil has two forms

Sand dunes & Loess

Sand dunes

Sand dunes are found in dry regions. Weathering of sandstones and other rocks produce sand. It is blown and deposited as dunes.

Sahara desert is example of sand dunes.


Loess is composed of windblown materials. It is primarily composed of silt. Some fine sand and clay particles are also present.

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