Subsoils

Subsoils are the mineral layers of a soil profile that lie beneath the topsoil, typically corresponding to the B horizon, and above the parent material (C horizon). They form as materials such as clay, iron and aluminum oxides, and other constituents are translocated from upper horizons and accumulate in the subsoil. Subsoils generally contain less organic matter and fewer soil organisms than the surface layer and may have a different texture, density, and color, reflecting the minerals that have been deposited or concentrated there.

The properties of subsoils are shaped by climate, vegetation, parent material, and drainage. In many soils, illuviation (downward movement) of clay and oxides leads to a distinct, often harder and more compact layer with reduced porosity, which can impede root growth. Subsoils may also serve as a reservoir for water and nutrients, especially during dry periods, but available rooting depth is limited by structure and density. In arid and semi-arid regions, subsoils can develop cemented layers such as caliche, which further restrict rooting and drainage. Conversely, in well-drained soils with good structure, subsoils can help regulate moisture and nutrient storage.

Subsoils are important for agriculture, forestry, and construction. They influence root distribution, drainage, aeration, and the stability of foundations. Management practices that minimize subsoil compaction and maintain soil structure, such as controlled tillage and appropriate drainage, help maintain functional subsoil conditions. Subsoil characterization is common in soil surveys and is essential for crop planning and engineering considerations.