Last edited 16 Oct 2020

Subsoil

Subsoil is the layer (or stratum) of soil immediately underneath the surface topsoil.

It is the weathered soil layer extending between the natural topsoil and the unweathered basal layer (geological parent material) below, or similar material on which topsoil can be spread. Subsoil has lower organic matter and plant nutrient content than topsoil. In most cases topsoils require a subsoil to perform one or a number of natural soil functions. Ref The HS2 London-West Midlands Environmental Statement, Glossary of terms and list of abbreviations, DETR 2013.

Typically, subsoil consists of the same variable mixture of minerals and small particles (e.g. sand, silt, clay) as the topsoil, but it has a much lower percentage of organic matter and humus (fine organic matter derived from the decomposition of plant and animal substances).

Subsoil that is clay-based provides the main source of material for cob, rammed earth, wattle and daub and other forms of earthen construction.

Subsoil tends to be paler than its overlying topsoil due to the lack of dark humus, and varies in colour from red to yellow.

The macro- and microscopic organisms that proliferate in topsoil are mostly lacking in the subsoil, although it may contain the deeper roots of some plants and trees. 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.

Beneath the subsoil is the substratum - a layer of residual bedrock, sediments or deposits.