Soil Formation

The soil is one of our essential natural resources. It is important for the growth of vegetation we feed on. It holds plants firmly and provides nutrition. It is home to many microorganisms such as earthworms, rats and several other subterranean species on earth.

In this section, we will discuss soil formation and importance.

What is Soil?

It is the uppermost layer of Earth’s crust, formed by the continuous weathering of mountains over thousands of years. It is made up of four basic constituents; minerals, organic materials, air, and water. The three main components responsible for its texture are; sand, silt, and clay. Depending upon these three constituents the mineral texture of the soil varies. Leaves and organic constituents decompose to form the upper organic layer, known as humus. The humus content in soils plays a very important role in its fertility.

Soil Formation

Now let us look at the process of how soil formation happens.

Parent Material

The mineral from which the soil is formed is termed as the parent material. Rocks are the source of all soil minerals. The parent material is chemically or physically weathered and transported which then deposits to form layers of soils. Usually, the bedrock is the parent material but there have been cases wherein soil gets transported due to factors like the wind and water.

Soil Formation

Parent Rock

Now the actual process of formation of soil is a cumulative combination of a number of processes. Soil formation also known as Pedogenesis is first kicked off by weathering and variations come according to the weather conditions.

Carriers or Weathering Agents

  • Glacier:

    As glaciers move from one part to another, they push the soil further with them. The drifted material gets deposited miles away from the place of its formation. When the glaciers melt, huge mounds of soil are left behind, a part of which is carried by the stream.

  • Water:

    As rivers flow, the soil particles are transported along with the water. The smallest particles travel the farthest. Heavier particles, such as sand and rock get settled earlier. Soils deposited along the river banks are termed as alluvial soil, which is very rich in mineral content. Rainfall also plays an important role. Rainfall washes off the soils in exposed lands.

  • Wind:

    Air plays the most important role as it transports a huge amount of soil from one place to another. Loose soils are carried away by the wind from one place to another.

Weathering Processes

  • Freezing and Melting:

    Repeated freezing and melting results in the formation of cracks and crevices in rocks. In the presence of the sun, the surface of rock expands. Upon coming in contact with a water body, these pores get filled with water. As we know, water expands when frozen, which pushes the particles further apart, breaking it down. When ice melts again, the rock breaks into loose soil particles

  • Heating and Cooling:

    In places with extreme climatic conditions, such as the arctic circle or the arid region, the rocks are subjected to sudden expansion and contraction, which results in the loosening of their particles and increase in the air content. Over time, the effect significantly reduces a rock to loose soil.

  • Wetting and Drying: 

    The rocks swell when they are wet and shrink back when dry. Regular wetting and drying of rocks result in the loosening of its grains.

  • Grinding or Rubbing:

    As the sea waves pound to the rocks along the seashore, abrasion of the uppermost layer occurs along with its fragmentation into smaller rocks and further into smaller particles.

  • Organisms:

    The organisms such as earthworm live in the soil. They churn their way through it by eating it. This results in the production of nutrient-rich manure in the form of their excreta. Their movement in the soil helps in its mixing and aeration.

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Practise This Question

Why are plastic bags, pesticides and other chemicals considered bad for soil?