•      Soil makes up the outermost layer of our planet.

•      Topsoil is the most productive soil layer.

Figure 34: Soil- Outermost layer of Earth

•      This is the layer where plants grow and is the layer we see.


Formation of Soil:

•      The formation of soil happens over a very long period of time.

•      It can take 1000 years or more.

•      Soil is formed from the weathering of rocks and minerals.

Figure 35: Smaller pieces of rocks and minerals forms soil

•      The surface rocks break down into smaller pieces through a process of weathering which mixes with organic matter.

•      Over time this creates a thin layer of soil.


Composition of Soil:

•      Soil is a mixture of different things; rocks, minerals, and dead, decaying plants and animals.

•      Soil can be very different from one location to another, but generally consist of organic and inorganic materials, water and air.

•      The inorganic materials are the rocks that have been broken down into smaller pieces.

Figure 36: Soil constituents

•      The size of the pieces varies.

•      It may appear as pebbles, gravel, or as small as particles of sand or clay.

•      The organic material is decaying living matter.

•      These could be plants or animals that have died and decayed until they become part of the soil.


Soil Profile:

•      Soil profile refers to the layers of soil.

•      Soil is made up of distinct horizontal layers; these layers are called horizons.

•      They range from rich, organic upper layers (humus and topsoil) to underlying rocky layers (subsoil, regolith and bedrock).

Figure 37: Soil horizons


O is the Organic Matter:

•      The material formed by the decay of living organisms – (both plant and animal) is called organic matter.

•      The remains of these dead plants and animals become food for bacteria, fungi and algae in the soil. 


A is the Topsoil:

•      It contains fine particles of sand and clay, a lot of organic matter or humus, and air.

•      It is wet too.

•      As it contains humus, it gives all the necessary nutrients to the plants growing in it.


B is called the Subsoil:

•      It is the layer below the topsoil and contains large bits of rock.

•      It contains very little organic matter but has water.

•      The roots of some big trees grow down into the subsoil.


C is called the bedrock:

•      This layer has large pieces of rock.

•      There is not much water here.

•      It is quite deep inside the Earth.