SUBSOIL
subsoil, undersoil
(noun) the layer of soil between the topsoil and bedrock
Source: WordNet® 3.1
Etymology
Noun
subsoil (countable and uncountable, plural subsoils)
The layer of earth that is below the topsoil.
Verb
subsoil (third-person singular simple present subsoils, present participle subsoiling, simple past and past participle subsoiled)
(transitive) To turn up the subsoil of.
Source: Wiktionary
Sub"soil`, n.
Definition: The bed, or stratum, of earth which lies immediately beneath
the surface soil. Subsoil plow, a plow having a share and standard
but no moldboard. It follows in the furrow made by an ordinary plow,
and loosens the soil to an additional depth without bringing it to
the surface. Knight.
Sub"soil`, v. t.
Definition: To turn up the subsoil of.
Source: Webster’s Unabridged Dictionary 1913 Edition