subsoil -
The layer of earth that is below the topsoil.
Wikipedia
Subsoil (also called substrata) is the layer of soil under the topsoil on the surface of the ground. The subsoil may include substances such as clay and/or sand that has only been partially broken down by air, sunlight, water, wind etc. , to produce true soil.
OmegaWiki Dictionary
Ω
subsoil Soil underlying surface soil, devoid of plant roots.
