Class 3 Science

Rocks and Soil – Class 3

Rocks and Soil

In the beginning the earth was full of big rocks. The Sun heated up the rocks and the rain cooled them. This heating and cooling over thousands of years broke down the big rocks into smaller rocks. The smaller rocks continued to break down and formed tiny pieces of soil. If you look at the rock closely through a magnifying glass, you will see small grains of different colours in the rocks. The small grains that make up rocks are called minerals. Some rocks are made up of only one mineral while other rocks are made up of several minerals.

Some rocks are very hard. Granite is a hard rock. Diamond is the hardest rock. Some rocks are not so hard such as coal. Chalk is a soft rock which breaks easily.

Soil

Do you know how soil is formed? The force of water and wind, and changes in weather break the rocks into very small pieces called soil. When plants and animals die, their bodies rot and mix with the soil. Rotten plants and animals are called humus. Humus is dark brown in colour. It is very good for the growth of plants.

Thus, soil is made up of two main parts – small pieces of rocks and rotted pieces of plants and animals.

Soil found in different places have different colours. Some are light brown, some are dark brown. In some places red soil is also found. These soils also feel different. These soils are different in colour and feel because they are made from different rocks.

What does soil contain?

Take some soil from a garden and put it in a transparent bottle. Now pour some water into it. Put the lid and shake the bottle. Let the bottle to rest for some minutes. After few minutes we can see different layers in the water. Let see what are these layers.

  • At the bottom will be small stones called gravel.
  • Above them will be a layer of soil containing larger particles. These particles are of sand. We can see the particles separately.
  • Above the sand is a layer of soil containing very fine particles. We can’t see these particles separately. This layer is called clay.
  • At the top we can see some dark brown pieces of humus.

Types of Soil

Different types of soil are clayey soil and sandy soil.

Potter using Clayey Soil

Potter using Clayey Soil

Soil which contain clay is called clayey soil. Clayey soil contains very small particles and is very smooth to touch. Potters uses clayey soil as it is soft and can be easily made into whatever shape he wants. Clayey soil is very sticky and water cannot go through this soil easily. It therefore holds a lot of water.

Plants cannot grow properly in clayey soil as it has no spaces between the particles to hold air.

Have you observe the soil in the ocean beaches?

Beach sand

Beach sand

The beaches has sandy soil. It feels rough and very loose.  The soil which contains a lot of sand is called sandy soil. It has large particles with lots of spaces between them. Water goes through these spaces very easily. Therefore, sandy soil cannot hold much water. Very few plants can grow in sandy soil.

Loam is a mixture of sand and clay. It can hold both water and air. It also has humus in it. Plants grow well in loam.

Importance of soil

We grow plants in soil. Plants take water and mineral salts from the soil for its growth. Different plants requires different types of soil for it proper growth. For example, rice grows well in clayey soil whereas bajra grows well in sandy soil. Many plants grow well in loamy soil. We can make soil fertile by adding manures and fertilizers to soil. So, there will not be any food for us if there is no soil.

We use soil as a building material. We make many useful items such as cement, bricks, tiles, glass and pots from soil. Sand is used to build houses. By digging deep into the soil we get useful resources such as petrol and gas.

Click Next button for Exercises on the chapter – Rocks and Soil for Class 3.

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