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SAI International School
Subject - Chemistry

Class - VII
Topic - SOIL: A Natural Resource
Sub-topic - Formation and Composition of Soil

Lesson Notes

Soil:
Soil is the topmost layer of the Earth that supports life. It is one of the most important
natural resources. It is a mixture of organic material, clay and rock particles. It also
contains air and water.

Formation of Soil:

Soil is formed by the process of weathering.

Weathering is the process of breaking down of rocks or other materials into smaller
particles due to long exposure to the different factors of atmosphere.

The agents of weathering are:

(i) Water, (ii) Wind & (iii) Temperature

Weathering process:

 Big rocks are broken into smaller pieces by various ways. Rain, alternate heating
and cooling effects of the Sun, very cold weather, roots of plants etc. may
contribute to it.
 The weathering is a slow and continuous process. Ultimately the rocks are
converted into soil which mixes with the dead remains of plants and animals to
form humus.

Composition of the Soil:

The composition of top soil varies from place to place. It is primarily composed of the
followings:

(i) Soil particles: Particles of sand, clay gravel etc. which differ in sizes are
present in the soil.
(ii) Humus: Soil contains humus. It is formed from the decayed remains of
plants and animals. It is dark, black and sticky. It makes the soil fertile. It
has the ability to hold water for plants to grow.

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