Substratum

Substratum [ Horticulture - Ecology - Pedology ]
( Plural: Substrata or Substratums )

Underlying base: any stratum, base, layer, or element lying underneath another.

In horticulture:	Any materials allowing the growth of roots of a plant. See also: Potting medium, soil
In biology and ecology:	A nonliving surface, layer or material on which an organism grows or is attached.
In microbiology:	The solid surface over which a cell moves, or upon which a cell grows
In pedology and agronomy:	A layer of earth beneath the surface soil; The part f the soil below the solum; the subsoil.