undersoil

/ˈʌndərˌsɔɪl/
noun
  1. The layer of soil beneath the topsoil, often containing clay, sand, or rock.
    • Plant roots often struggle to grow if the undersoil is too compacted.
    • The gardener dug deep and found that the undersoil was mostly clay.
    • Construction workers tested the undersoil to see if it could support the building's foundation.
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