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]

The Collaborative International Dictionary of English v.0.48:

Subsoil \Sub"soil`\, v. t.
   To turn up the subsoil of.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subsoil
    n 1: the layer of soil between the topsoil and bedrock [syn:
         subsoil, undersoil]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "subsoil":
   acres, alluvion, alluvium, arable land, clay, clod, crust, dirt,
   dry land, dust, earth, freehold, glebe, grassland, ground, land,
   landholdings, lithosphere, marginal land, marl, mold, real estate,
   real property, region, regolith, sod, soil, subaerial deposit,
   terra, terra firma, terrain, territory, the country, topsoil,
   woodland