subsoil
Concise Oxford English Dictionary © 2008 Oxford University Press:
subsoil/ˈsʌbsɔɪl/
▶noun the soil lying immediately under the surface soil. ▶verb (usu. as noun subsoiling) plough (land) so as to cut into the subsoil.
– derivatives
subsoiler noun.
subsoiler noun.
'subsoil' also found in these Oxford entries:

