subsoil

sub- +‎ soil

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subsoil (countable and uncountable, plural subsoils)

1. The layer of earth that is below the topsoil.
   • 1954, Soil survey of Wise County, Virginia, page 28:

     In the few areas underlain by horizontally bedded limestone, the surface soil is dark gray to black and peatlike, and the subsoil gray and marllike.

subsoil (third-person singular simple present subsoils, present participle subsoiling, simple past and past participle subsoiled)

1. (transitive) To turn up the subsoil of.

• subsoiler