Subsoil

Part of speech: noun (n.)

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.

Part of speech: verb (v.)

To turn up the subsoil of.