• 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 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.