Thesolum (plural, sola) insoil science consists of thesurface andsubsoil layers that have undergone the samesoil forming conditions. The base of the solum is the relatively unweatheredparent material.
Solum and soils are not synonymous. Some soils include layers that are not affected bysoil 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 overlyingbedrock can be by itself the solum. A soil that consists only of recently depositedalluvium or recently exposed softsediment does not have a solum.
In terms ofsoil horizon designations, a solum consists ofA, E, andB horizons and their transitional horizons and some O horizons. Included are horizons with an accumulation ofcarbonates or more solublesalts if they are either within, or contiguous, to other genetic horizons and are at least partly produced in the same period of soil formation. The solum of a soil presently at the surface, for example, includes all horizons now forming. The solum of a soil is not necessarily confined to the zone of majorbiological activity. A solum does not have a maximum or a minimum thickness.
The lower limit should relate to the depth of rooting to be expected forperennial plants assuming thatsoil moisture conditions andsoil chemistry are not limiting. In some soils the lower limit of the solum can be set only arbitrarily and needs to be defined in relation to the particular soil. For example, horizons of carbonate accumulation are easily visualized as part of the solum in many soils in arid and semiarid environments. To conceive of hardened carbonate accumulations extending for 5 meters or more below the B horizon as part of the solum is more difficult.Gleyed soil material begins in some soils a few centimeters below the surface and continues practically unchanged to a depth of many meters. Gleying immediately below the A horizon is likely to be related to the processes of soil formation in the modern soil. At great depth, gleying is likely to be relict or related to processes that are moregeological thanpedological. Much the same kind of problem exists in some deeply weathered soils in which the deepest material penetrated by roots is very similar to the weathered material at much greater depth.[1]
In Scottish law, the solum is the area of ground that lies inside the walls or foundations of a building.