Source : Webster's Revised Unabridged Dictionary (1913)

Subsoil \Sub"soil`\, 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.