• The soil is made of stones, pebbles, sand, fine soil particles as well as organic substances.

Importance of Soil

  • The soil provides support plants by holding the roots firmly and helps in providing water and nourishment to the plants.
  • The soil Helps in growth of the plant and it is important for agriculture. Agriculture helps in satisfying the need of food clothing and shelter to all.
  • The soil is home to many organisms.
  • The survival of plants is very essential, because they are prime producers, as they can prepare their own food in the process called as photosynthesis. Animals are dependent on plants for survival. Thus animals are indirectly dependent on the soil for survival and nourishment.

Experiment to Understand Soil Profile:

  • We can perform this simple activity to show that the soil is composed of distinct layers.
  • Procedure: Take a little soil and break the clumps to powder it (don’t use crusher, do it by your hand). add a handful of this powdered soil in a transparent glass tumbler. Stir the suspension with a stick so that the soil gets dissolved. Let the suspension stand undisturbed for some time.

  • Observations: We see layers of particles of different sizes in the glass tumbler. There is a topmost layer of some dead rotting leaves or animal remains floating on water. This rotting dead matter in the soil is called humus. Below the layer of humus, there is a clear water level which contains dissolved part from the soil. Below water layer, there are layers of clay, sand, gravel and stones.

Soil Profile:

  • A vertical section through different layers of the soil is called the soil profile. Each layer differs in feel (texture), colour, depth and chemical composition. These layers are called as horizons. We usually see the top surface of the soil, To observe the layers below we have to study recently dug a ditch. Soil profile can also be seen while digging a well or during laying the foundation of a building. It can also be seen at a steep river bank.

Soil profile

  • The uppermost layer or horizon is thin and generally dark in colour due to the richness of humus. It is made up of living and decomposed and decomposing materials like leaves, plants and decaying or decayed dead terrestrial animals. The humus makes the soil fertile and provides nutrients to growing plants. This layer is called ‘O’ horizon.
  • The layer below ‘O’ horizon is called a topsoil or ‘A’ horizon. This layer is made up of minerals and decomposed organic matter and it is also very dark in colour. This layer is generally soft, porous and can retain more water.This is the layer that many plants roots grow in. This layer provides shelter for many living organisms such as worms, rodents, moles and beetles.
  • ‘E’ horizon is a mineral horizon present only in forested areas below ‘A’ horizon and above ‘B’ horizon.  It is a light coloured, leached (washed down or eluviated due to rainwater from ‘A’ level) horizon. Mainly it contains silicates.
  • The next layer is ‘B’ horizon and also referred as “subsoil”. This layer has clay and mineral deposits and less organic materials (humus) and more amount minerals than the layers above it. This layer is also lighter in colour than the layers above it. This layer is generally harder and more
  • Next layer is called the ‘C’ horizon also referred as “regolith”. It is made up of small lumps of rocks with cracks and crevices. A little bit of organic material is found here. Plant roots are not found in this layer. The layer also referred as parent material layer. Because from these lumps, the soil is formed.
  • Next layer is bedrock layer and also referred as ‘R’ horizon. This layer is hard and difficult to dig with a spade.

Formation of Soil:

  • The process by which rocks are transformed into the soil is called weathering. During this process, rocks are converted into smaller and smaller particles and ultimately they are turned into soil. Layers of this soil spread over the surface of the earth.
  • The weathering of rocks takes places due to effects of rain, rainwater, river, wind and continuous change of weather.

Soil Formation

  • Extreme heat, extreme cold and a large change in day and night temperatures cause the weathering of rocks.
  • Biological activities by rodents like rats and bandicoots cause weathering. They live in burrows.  Their burrowing activity causes weathering.  The growth of roots in the soil in crevices of rocks causes the weathering of rocks.

  • Plants, insects and animals live on soil, grow on soil and ultimately die and decay into the soil. It also adds to the weathering of soil.
  • It takes about 800 to 1000 years to form 2.5 mm thick layer of fertile soil.

Soil Types:

  • Weathering of rocks produces small particles of various materials, which include sand and clay.
    The relative amount of sand and clay depends upon the rock from which the particles were formed. 

Types of Soil on the Basis of Texture:

  • The size of particles, decide the texture of the soil. On the basis of texture, soils are classified into four types. viz. sandy soil, clayey soil, loamy and silt
  • Sandy Soil: If the soil contains a greater proportion of big particles it is called sandy soil. The particles are quite large and hence they cannot fit closely together. Thus there are large spaces between the particles which are filled by air. Hence sandy soil is highly aerated. Water can drain down through these spaces. Hence It has the least capacity of holding water. In general, particles of sandy soil are well aerated, light and dry.
  • Clayey Soil: If the proportion of fine particles is relatively higher, then it is called clayey soil. It has the highest capacity of holding water. The particles are very fine and hence they can fit closely together. Thus there are small spaces between the particles which are filled with water particles leaving almost no space for air. Hence clayey soil is ill aerated. Water cannot drain down through these small spaces. Hence It has the highest capacity of holding water. In general, particles of clayey soil are ill aerated, heavy and moist.
  • Loamy Soil:  If the amount of large and fine particles is about the same, then the soil is called loamy. Loamy soil is a mixture of sand and clay. Its water holding capacity is medium (right for proper growth of plants). Loamy soil also has humus in it and it is well aerated. The best topsoil for growing plants is loam.  
  • Silt: In this type of soil fine and medium-sized particle medium-sized particles are seen. Silt occurs as a deposit in river beds. The size of the silt particles is between those of sand and clay. Its water holding capacity is medium and is well aerated. 

Types of Soil on the Basis of Uses:

  • Depending upon the uses soil is classified into three types viz. China clay, clay and loamy soil.
  • China Clay:  It is also called Kaolin because it is found near the Kaolin mountain in China. It is white in colour. It is used for making crockery, tiles for bathrooms, tanks, and laboratory apparatus etc.
  • Clay: It is a special type of China clay. It is white in colour and is used for making idols and statues.
  • Loamy Soil: It is a special type of China clay. It is white in colour and is used for making idols and statues.
  • Note: Different types of oxides are used to decorate and paint earthenwares and porcelain articles. Copper oxide is used to obtain green colour and iron oxide is used to obtain red colour.

Constituents of Soil:

  • Soil contains organic and inorganic constituents.  Fine particles of quartz, felspar, mica, cabonates of metals, oxides and sulphides of iron are present in soil as inorganic part.
  • The organic constituents consist of particles from both the plant and the animal origin. Those from plant origin are bacteria, algae, fungi, decaying leaves, etc. Those from the animal origin include protozoa, worms, insects, shells and decaying bodies of dead animals.

Colour of Soil:

  • Colour is an important property of a soil. The soil is of different colours. It may be black, red, yellow or copper coloured. From the colour of the soil, we get an idea of the fertility, drainage, and other such properties.
  • The colour of soil depends on the texture, organic content and chemical substances like iron, quicklime that it may contain.

Uses of Soil:

  • Soil provides water and nutrients to plants.
  • The soil has the capacity to hold water. Due to this property water is collected in deep ponds and wells. Thus we can satisfy the need of water throughout the year.
  • The soil is used to make bricks, which are used for construction.
  • Everyday use vessels, earthenwares and pots are made from soil.
  • The soil is used for making idols and statues.

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