Composition and Formation of Soil | Learning Concepts | Science
Job Alert : To view our Careers Page Click Here X (+91) 8-888-888-999
ORCHIDS The International School


Concept: Components and Formation of Soil

What is soil?

Soil is an essential natural resource. It forms the uppermost layer of the earth.

Importance of soil:

  • It provides support to the roots of plants for their growth.
  • Soil also provides nutrients to plants.
  • It is a natural habitat for many living organisms like fungi, bacteria, earthworms, ants etc.
  • Soil is needed for growing crops.
  • It acts as a reserve for water.

Formation of soil:

  • The formation of soil occurs over a long period by the process of weathering.
  • Weathering means the wearing and tearing of large rocks (also called the parent rock) into smaller fragments due to different environmental factors.
  • Weathering can occur via physical weathering, chemical weathering and biological weathering.

Components of soil:

  • Soil is mainly formed of sand silt and clay.
  • The amounts of each of these components decide the texture and type of soil.
  • The other major components of soil are air, moisture, humus (organic matter) and minerals.

Properties of soil:

1. Soil texture:

It refers to the relative proportions of sand, silt, and clay particles present in a soil sample. Sand, silt and clay have different particle sizes and feel different when touched. So the varying amounts of these materials determine the texture of the soil. Sandy soils are coarse in texture, while clay-rich soils are fine-textured.

  • Sand: Sand particles are of considerable size with much space between them.
  • Silt: Silt particles are finer than sand and have less air space between them.
  • Clay: Clay particles are very dense and fine, leaving no room for air between the particles.

2. Colour:

  • The colour of soil depends on the amount of organic matter present in it.
  • Darker soil has more organic content in it.
  • Some minerals found in the soil also determine its colour. For example, soil rich in iron oxide is usually red.
  • Nature-1

3. Density:

  • The density of soil depends on its constituents.
  • The soil in the upper layers of the soil profile is less dense than the lower layers.
  • The reason is that as one moves to the deeper layers of soil, the organic content decreases and the mineral content increases.

4. Pressence of air:

  • Different gases occupy the space present between the soil particles.
  • Atmospheric gases like oxygen, carbon dioxide and nitrogen are found in soil.
  • Oxygen is used by the plant roots and soil microbes.
  • Carbon dioxide is respired by plants, animals, bacteria, fungi, etc.
  • Nitrogen is used by leguminous plants indirectly.

5. Percolation:

  • The movement of water through the soil is called percolation.
  • The rate of percolation (i.e., time taken by water to move through the soil) varies from soil to soil. It mainly depends on the concentration of constituents of the soil.
  • For instance, the more the number of clay particles, the less is the percolation rate whereas the more the number of sand particles, the higher is the rate of percolation.
  • Nature-1

6. Water-holding capacity:

  • The maximum amount of water that the soil can hold is called its water-holding capacity.
  • This property differs for different soil types.
  • Clayey soil has the highest, whereas sandy soil has the least water-holding capacity.

Soil profile:

The vertical arrangement depicting the layers of a particular soil type is called a soil profile. It shows the following horizons:

1. Horizon A: Topsoil

  • It is the uppermost layer of the soil profile.
  • The soil in this layer is dark in colour and very fertile due to the presence of organic matter called humus.
  • The growth of plants is confined to this layer.
  • Topsoil is generally porous with a high water-holding capacity and provides shelter to many microbes.

2. Horizon B: Subsoil

  • It is the second layer of the soil profile.
  • It is less dark than the topsoil as it contains minimal or no amount of humus.
  • This layer is rich in minerals and iron oxides.
  • The subsoil is relatively compact and less porous.
  • No microorganisms live here.

3. Horizon C: Substratum

  • This layer lies below the subsoil.
  • It comprises fragments of stones and rocks formed due to weathering of the parent rock.

4. Parent rock:

  • The parent material which lies at the base is called the parent rock.
  • It gets converted into soil gradually over a long period by the process of weathering.
  • This layer is non-porous with the presence of large rocks.

New Words:

Humus: The organic material formed due to the decomposition of animal and plant waste.

Compact: When the particles are held close together close.

Did You Know?

  • One teaspoon of soil contains more number of organisms than the number of people on earth.
  • Weathering of rocks for almost 500 years leads to the formation of 1 inch of topsoil.
  • The soil particles hold 0.01% of the total water present on earth.

  • -

    Admission Enquiry

    A Journey To A Better Future Begins With Us