Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil it is composed of a variable mixture of small particles such as sand, silt and/or clay, but it lacks the organic matter and humus content of topsoil. Below the subsoil is the substratum, which can be residual bedrock, sediments, or aeolian deposits. As it is lacking in dark humus, subsoil is usually paler in colour than the overlying topsoil. It may contain the deeper roots of some plants, such as trees, but a majority of plant roots lie within the surface topsoil.
Clay-based subsoil has been the primary source of material for adobe, cob, rammed earth, wattle and daub, and other earthen construction methods for millennia.
- Håkansson, Inge; Reeder, Randall C. (March 1994). "Subsoil compaction by vehicles with high axle load—extent, persistence and crop response". Soil and Tillage Research. 29 (2–3): 277–304. doi:10.1016/0167-1987(94)90065-5. Retrieved October 11, 2012. (subscription required)
- Adams, Fred; . Moore, B. L (January 1983). "Chemical Factors Affecting Root Growth in Subsoil Horizons of Coastal Plain Soils". Vol. 47 No. 1. Soil Science Society of America Journal. pp. 99–102. Retrieved October 11, 2012. (subscription required)
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