Humus is formed by dead and decaying bits of plants and animals. It is found in the topsoil which is the uppermost layer and is dark coloured due to the presence of humus.

Earlier the surface of the earth was covered with rocks and there was no soil on earth. The sun heated the rocks, and the rain made them cold. This continued for thousands of years. Due to the changes in the weather, the rocky surface began to crack. The fast flowing rain and river water entered these cracks and broke the rocks into smaller pieces. These pieces further became fine due to the action of air and water, and thus soil was formed. This process is called weathering.