Solum

The solum (plural, sola) in soil science consists of the surface and subsoil layers that have undergone the same soil forming conditions. The base of the solum is the relatively unweathered parent material.

Solum and soils are not synonymous. Some soils include layers that are not affected by soil formation. These layers are not part of the solum. The number of genetic horizons ranges from one to many. A surface layer that is 10 cm thick overlying bedrock can be by itself the solum. A soil that consists only of recently deposited alluvium or recently exposed soft sediment does not have a solum.