subsoil

n. layer of earth that lies beneath the topsoil

For the album by Biosphere, see Substrata (album)Subsoil is the layer of soil under the topsoil on the surface of the ground. The subsoil may include substances such as clay and has only been partially broken down by air, sunlight, water, etc., to produce true soil. Below the subsoil is the substratum, which can be residual bedrock, sediments, or aeolian deposits, largely unaffected by soil forming factors active in the subsoil.
See more at Wikipedia.org...

(v. t.)
To turn up the subsoil of.
  

(n.)
The bed, or stratum, of earth which lies immediately beneath the surface soil.
  

Noun
1. the layer of soil between the topsoil and bedrock
(synonym) undersoil
(hypernym) soil, dirt