The Collaborative International Dictionary of English v.0.48:

Subsoil \Sub"soil`\, n.
   The bed, or stratum, of earth which lies immediately beneath
   the surface soil.
   [1913 Webster]

   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.
      [1913 Webster]