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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 info 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!