What is Soil?
Soil is a major part of the natural environment, alongside air
and water, and is vital to the existence of life on the planet.
Soil is the result of the process of the gradual breakdown of
rock - the solid geology that makes up the earth. As rock becomes
broken down through a variety of processes, such as weathering
and erosion, the particles become ground smaller and smaller.
As a whole, soil is made up from four constituents: mineral material,
organic material, air and water. There are considered to be three
main mineral parts to soil; ‘sand’, ‘silt’ and ‘clay’.
These parts give the soil its 'mineral texture'. In addition, as
leaves and other organic material fall to the ground and decompose
- there also forms an ‘organic’ layer. Soil scientists
(or pedologists) use a series of sieves to
separate out the constituent parts in order to characterise soil
by texture class.
Soils may be characterised in terms of the properties they inherit
from the underlying rock (the parent material) and the properties
resulting from alteration of the original parent material by soil
forming processes (also called ‘pedogenic’ processes).
The latter effects can be observed in the surface and subsurface
horizon layers. A good place to look at these layers is where a
construction project has exposed a cross-section of earth, for
instance where a new road is being built. The depth of soil varies
geographically, but a depth of 2 metres in the UK is usually taken
as the average depth to consolididated material (rock). Tropical
soils can be tens of metres deep!