Topsoil

Topsoil is the upper layer of a soil profile, usually darker in colour (because of its higher organic matter content) and more fertile than subsoil, and which is a product of natural
biological and environmental processes. Ref The HS2 London-West Midlands Environmental Statement, Glossary of terms and list of abbreviations, DETR 2013.

Topsoil is typically between 5 - 20 cm (2 - 8 inches) deep. It can also be measured as the depth from the surface level to the subsoil, i.e. the distance to the first densely-packed layer of soil.

The topsoil is where the majority of the biological soil activity occurs, as it contains the highest concentration of organic matter and microorganisms, due to decomposition from animals, plants, and so on. The topsoil contains the highest concentration of plant roots, as plants acquire the majority of their nutrients from the topsoil layer.

The main elements that make up topsoil composition are:

• Organic matter.
• Mineral particles.
• Water.
• Air.

The soil’s strength and bearing capacity depends on its skeletal structure and decreases with the presence of organic matter.

Water run-off results in minerals from the topsoil migrating down into the subsoil which can lead to mineral deficiencies in the topsoil. The knock-on effect of this is that roots have to dig deeper for nutrients. During construction it can be necessary to strip topsoil, although it should be considered that by exposing the subsoil, the rate of erosion of soil minerals increases.

Construction work can lead to the organic matter of topsoil condensing and settling in different ways, such as in roadbeds and foundations.

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• Subsoil.
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