sub·soil /ˈsʌbˌsɔɪl/
  下層土,底土(vt.)掘起…的底土
  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.
  Sub·soil, v. t. To turn up the subsoil of.
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  subsoil
       n : the layer of soil between the topsoil and bedrock [syn: undersoil]