Geography: Earth's Interior and Its Material: Weathering and Soil, Gradation, Soil and Its Formation

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Weathering and Soil

  • The different types of land features are produced in areas of different types of climate through the process of weathering. Weathering plays an important role in the formation of soil which provides basis for agriculture and world’s food supply.

  • Mechanical weathering of the surface rocks disintegrates the rock and converts it into a fine powder. These small particles are deposited in layers with the help of water. Biotic weathering produces humus. This organic matter is formed through the action of plants and animals which helps in the formation of soil. Various processes of weathering help in giving different colours and properties of soil.

Weathering and Soil Gradation, Soil and Its Formation

Weathering and Soil Gradation, Soil and Its Formation

Weathering and Soil Gradation, Soil and Its Formation


Exogenetic forces are constantly working to bring about levelling or gradation of the land. They attempt to achieve a balanced condition between erosion and deposition which indicates a graded position. Agents of gradation like rivers, glaciers, winds, sea waves, and underground water perform their task with the help of the triple action of weathering, erosion and deposition. The levelling down of elevated portions of the earth’s surface is done by erosion. The filling up of depressions is done by deposition of the eroded material transported by the external agents of gradation. The endogenetic forces of the earth give rise to major landforms on the earth surface and the exogenetic forces level them down.

The work of gradation has two components:


When rocks are removed by scraping, scratching, and cutting due to the process of erosion, thereby lowering the elevation of the land, it is called degradation. Degradation includes the work of weathering that is the movement of scarped and scratched material aided by the force of gravity. It also includes the work of erosion implying the transportation of the rock material by agents of gradation.


Deposition is the filling up of low-lying areas of depression by eroded material. It starts when the agents of gradation lose their force or have obstruction in their way. As a result, eroded material is deposited in depressions which not only creates new landforms but also modifies the existing ones.

Soil and Its Formation

Soil is the uppermost layer of the land surface that plants use and depend on for nutrients, water and physical support.

Factors of Soil Formation

The five factors, which control the formation of soil are parent rock, relief, time, climate and plant and animal organisms. The former three are called the passive factors while the latter two are the active factors.

Parent Rock

A soil is derived from the underlying rock or the parent rock material containing different minerals. The parent rock gets broken into tiny pieces and is decomposed slowly by physical and, chemical weathering. The parent rock influences the rate of soil formation, the chemical composition, colour, texture, structure, mineral content and fertility.


Steep slopes are subjected to more rapid run off surface water than the gentle slopes. Hence, there is less infiltration of water on steeper slopes, which retards soil forming processes. Rapid run off on steep slopes often erodes their surface faster than soil can develop. It is because of this that the mountainous topography develops coarse, thin and infertile soil and the plain areas have rich well-developed fertile soils.


A well-developed soil results as an end product of physical, chemical and biological processes operating collectively for a very long period of time.


It is by far the most important factor in the sense that over a long period of time it not only tends to reduce the difference caused by the parent material but also influences biological activities within the soil. Due to this factor two different parent materials may develop the same type of soil in one type of climatic region. For example, granite and sandstone have developed into sandy soil in dry Rajasthan desert. On the other hand, two different types of soils may develop from the same parent material in two climatic regions. For example, the crystalline granites have developed laterite soils in monsoon regions and non-laterite soils in sub humid regions.

Plant and Animal Organisms

Dead plants and animals contribute to the organic content of the soil. The process of decay, added by bacterial action, transforms organic matter into humus. Humus is responsible for the fertility of the soil. It also enhances water retention capacity of the soil. This organic material helps the soil to support plant life. It also prevents greater evaporation of soil moisture by its thick canopy, thus allowing soil to mature and become fertile.

Soil Horizons

Soil horizon is a layer of soil which lies more or less parallel to the surface and has fairly distinctive soil properties. They are distinctive layers found in soils that differ in physical or chemical composition, organic content or structure. The display of horizons on a cross section through the soil is termed as soil profile. The main types of horizons are – O, A, B, and C. The A horizon is the upper most horizon, rich in organic matter. Next is the E horizon from which the clay particles and oxides of aluminium and iron are removed by downward seeping water, leaving behind pure grains of sand or coarse silt. The B horizon receives the clay particles, aluminium and iron oxides, as well as organic matter washed down from the A and E horizons. Beneath the B horizon is the C horizon, consists of the parent mineral matter of the soil.

Soil Erosion

Soil erosion is the removal of soil at a greater rate than its replacement by natural agencies of water, wind etc.

Types of Soil Erosion

Soil erosion is of four types:

Wind Erosion

Winds carry away vast quantity of fine soil particles and sand from deserts and spread it over adjoining cultivated land and thus destroy their fertility. It takes place in and around all desert regions of the world. In India, over 1 lakh kilometres of land is under Thar Desert, spread over the parts of Gujarat, Haryana, Punjab and Rajasthan. These areas are subject to intense wind erosion.

Sheet Erosion

Water when moves as a sheet takes away thin layers of soil. Such type of erosion is most common along the riverbeds and areas affected by floods. With time, the soil is completely exhausted due to removal of topsoil and becomes infertile.

Rill Erosion

It includes the removal of surface material usually soil by the action of running water. The processes create numerous tiny channels or rills a few centimetres in depth, most of which carry water only during storms.

Gully Erosion

When water moves as a channel down the slope, it scoops out the soil and forms gullies which gradually multiply and in the long run spread over a wide area. The land thus dissected is called bad lands or ravines. In India, the rivers of Chambal in Uttar Pradesh and Yamuna in Madhya Pradesh are famous for their ravines.

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