17. Soils 17.2 How Soil Is Formed


A number of factors contribute to the formation of soil. Factors such as climate, topography (the physical features of an area of land), parent material, organisms, time and human activity all play a key role in the formation of soil. Slight changes to any of these factors will result in different types of soil forming.


Millions of years ago, the earth was covered in huge rocks. Weathering helped break these rocks down into tiny mineral particles.


How are soils formed? Climate


Climate generates weather which creates frost, water and more for processes such as weathering.


Organisms break plant litter down into humus.


Organisms Topography


Soil takes a long time to form, in some cases hundreds of years per centimetre of depth.


2. How soils are formed 17.3 Soil Profiles


A soil profile is a vertical section of soil from its surface downwards. Soils have layers called horizons . Each layer is different from the one above and below it.


1 The A horizon is sometimes called the topsoil . It is the closest to the surface and most fertile layer of soil. It is usually dark in colour because it contains the most humus and plant litter.


2


The B horizon is called the subsoil . It is found just below the topsoil, has less humus and is lighter in colour than the topsoil. It may contain more stones than the topsoil as it is closer to the bedrock (C horizon).


3 The C horizon is also called the bedrock. It is a mixture of solid rock at its base and broken rock at its top.


Draw your own labelled diagram to describe the layers in a soil profile.


1 A horizon 2 B horizon Parent material Time


Human activity:  Improves soil by fertilising in farming  Damages soil by overuse.


Parent material is the type of rock that soils have formed from, e.g. if the rock was a limestone rock the soil would be very fertile.


3 C horizon 3. A typical soil profile 251


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