Soil Formation

Introduction

Soil is an important natural resource found on the uppermost layer of the earth’s crust in which the terrestrial (land) plants grow. It comprises a mixture of small rock particles and humus (organic materials) that develop on the surface of the earth. It is also considered an important component of the ecosystem as it is responsible for the sustainability of life and the ecosystem on earth. Mineral particles, humus, air, and water are the major constituents of the soil. Thus, the substances present in the soil can exist in all three states: solid, liquid, and gaseous. A proper balance of all the three states in the soil is necessary for the growth of a plant.

Soil Profile 

Soil comprises various layers that lie parallel to the earth’s crust. Different soil layers in a vertical form parallel to the earth’s crust is known as the soil profile. ‘Soil profile’ is also known as soil Horizon, and it comprises three zones- Horizon A, Horizon B, and Horizon C.

1. Horizon A is the uppermost zone of the soil profile and consists of organic materials, nutrients, minerals, air, and water for the proper growth of plants.
2. Horizon B is the zone present between horizon A and horizon C. It is the transition zone between both horizons. This horizon is rich in minerals but poor in humus.
3. Horizon C is the zone between horizon B and horizon D. It comprises fragments of weathered rock. It is the layer from which the first stage of soil formation begins. It consists of little or no plant or animal life.
4. Horizon D is the last layer of the soil profile. It only consists of large rocks.

It takes thousands of years to make a parent rock into topsoil. The process of soil development is known as Pedogenesis.

Factors of soil formation 

The major factors responsible for soil formation include parent Rock material, relief or topography, climate, organic activity, and time. 

1. Parent material 

The Rocks from which the soil develops are referred to as parent material. It plays an important role in determining soil’s texture, composition, and colour or appearance.

Parent materials are also derived from the sediments formed from the weathering of the bedrock. Parent materials are also derived from the sediments  transported into an area by the erosion activities done by wind, water, or ice. The soils derived from the bedrock reflect the characteristics of the bedrock or the parent rock material. The process of forming soil by breaking down bedrock is known as weathering. Weathering can be physical (mechanical) or chemical. Weathering and erosion can change the soil characteristics. Sometimes the influence of some climatic factors can stow away the characteristics of the parent rock. In India, the parent material can be of various types.

Ancient crystalline and metamorphic rocks

It comprises the oldest rock system of India, which was formed over 4 billion years ago. They are mainly formed after the molten magma is solidified. These rocks give rise to the formation of red soil, known for its ferromagnetic properties.

Cuddapah and Vindhyan Rocks

This type of Rock system mainly comprises sedimentary rocks and is very thick. They are mainly found in central and south India. After the weathering process, they give rise to soils rich in calcium carbonate, and sometimes they also give rise to argillaceous soil, which comprises clay. They do not contain any metallic minerals as they are of sedimentary origin.

Gondwana rocks

They are very similar to Cuddapah and Vindhyan rocks, but they are not as old. After the weathering and erosion, they give rise to a less fertile soil type.

Deccan basalt

This type of rock is mainly found in the peninsular region of India. They are formed as a result of basaltic lava. After weathering and erosion, they give rise to the formation of black soil, which is also known as regular soil. This soil has a good moisture-retaining capacity and is black due to magnetite, magnesium, titanium, and aluminium oxides.

Tertiary and Mesozoic sedimentary rocks

This type of rock system mainly gives rise to the formation of alluvial soil. They are highly porous and lead to the formation of soil that contains fine silt and clay.

2.Relief/ topography 

Relief or topography modifies the development of soil. The soil formed on the regions of low relief is more fertile due to the deposition, whereas the soils formed on the steep slopes do not contain depositions and are very less fertile.

3. Climate 

The two important factors of climate that influence the formation of soil are precipitation (rainfall) and temperature.

• Laterite soils are formed when granite rocks undergo weathering in a monsoon climate
• Black soils are formed due to high temperatures and lack of rainfall
• Sandy soils are formed due to the weathering of granite and sandstone in an arid climate
• Acidic soils are formed under cold weather conditions at higher altitudes

4. Organic activity 

Organic matter accumulation, profile mixing, and biogeochemical nutrient cycling come under organic activity. These organic activities are influenced by various living organisms, leading to modifying soil characteristics.

5. Time

The soils, which take a lot of time, lead to the formation of mature soil, whereas the soils formed in less time lead to the formation of immature soil. 

Process of Soil Formation 

Soil formation includes various processes such as

1. Additions- Most additions occur at the surface, including solar energy water controlled by climate and organic material.
2. Losses can occur both from the surface and from the deep subsoil. A huge amount of soil is lost due to erosion.
3. Translocation- When various types of physical movement of material occur within the soil, it is referred to as translocation. The material moved can be in any form of the matter and any direction. In very dry climates, salts are moved upwards in solution by capillary, whereas, in very cold climates, solid minerals are moved upward by frost actions.
4. Transformation- Transformation includes the changes of some of the soil constituents without any physical displacement. This can occur due to climate and organisms.

Types of Soil

1. Alluvial soil

• Alluvial soil covers about 40% of the total land area of India
• They are mainly found due to silt deposition by Indo-Gangetic-Brahmaputra rivers
• They are in mature soils having weak profiles
• They are rich in potash and poor in phosphorus
• They are porous and are best suited for agriculture

Wheat, sugarcane, rice, oilseeds, vegetables, and fruits can be easily grown

2. Red soil

• They are formed from igneous rocks and are seen mainly in areas receiving low rainfall
• They are red due to the presence of iron oxides
• Wheat, cotton, and pulses are easily cultivated
• They are deficient in phosphate, manganese, nitrogen, humus, and potash

3. Black soil 

• They are also known as regur or cotton soil
• They develop large cracks when dried; hence are self-ploughing soil
• They are rich in iron, calcium, potassium, aluminium, and magnesium

They are deficient in nitrogen, phosphorus, and organic matter

4. Desert soil

• They are mainly found in arid and semi-arid conditions
• They lack moisture and humus
• Only suitable for growing plants which need very less amount of water

5. Laterite soil

• They are found in areas of high rainfall and high temperature
• They are formed due to intense leaching
• Laterite soil is normally infertile and saline in nature
• They are poor in organic matter, nitrogen, phosphate, and calcium
• They are rich in iron oxide and potash

6. Saline soil

• They contain large amounts of sodium, potassium, and magnesium
• They are infertile and have poor humus content

7. Peaty/marshy soil

• They are found in areas of high humidity and rainfall
• A large amount of humus is present in peaty soil

8. Forest soil

• They are usually found in forest areas where sufficient rainfall is available 
• This soil has less humus content, and the soil is acidic

Conclusion 

Soil is a mixture of small rock particles and organic materials which support all life on earth. It is considered an important natural resource, and it is also considered an important component of the Earth that completes the biosphere. It comprises various layers in a vertical form, parallel to the earth’s crust which is referred to as soil profile and consists of horizon A, horizon B, horizon C, and horizon D. It takes thousands of years to make a parent rock into topsoil. The process of soil development is known as Pedogenesis. The major factors responsible for soil formation include parent rock material, relief (topography), climate, organic activity and time. All these factors play an important role in determining the texture, composition and colour or the appearance of the soil. Along with all these factors, various factors also contribute to the modification of soil. Some of the processes are additions, losses, translocation and transformation. 

As we know, various factors lead to the formation of different types of soil in India; eight major types of soil are found. Among these, 40% of India’s total land area is covered by Alluvial soil, which is best suited for growing crops. The second most important soil is black soil which is best suited for growing cotton. The decline of soil fertility is known as soil degradation or soil erosion. It is a gradual process in which the top layer of the soil is displaced. As we know, soil is an important natural resource, we must prevent soil erosion, and we should take certain measures to conserve it.