Subsoil

[Sarah]

The bed, or stratum, of earth which lies immediately beneaththe surface soil. Subsoil plow, a plow having a share and standardbut no moldboard. It follows in the furrow made by an ordinary plow,and loosens the soil to an additional depth without bringing it tothe surface. Knight.

[Becky]

the layer of soil that is under the surface level

[Kai]

The soil lying immediately under the surface soil.