subsoil

Subsoil is the layer of soil that lies beneath the topsoil. In soil science, it is commonly identified as the B horizon, the mineral layer that has formed below the A horizon. Some definitions broaden subsoil to include parts of the lower A horizon or adjacent transitional horizons, but it generally excludes the organic-rich topsoil and the parent material beyond the solum.

Compared with topsoil, subsoil contains less organic matter and fewer soil organisms. It is typically more compact and dense, with a color influenced by accumulated clay, iron oxides, and other minerals leached from above. The texture and structure of the subsoil affect water movement and root growth, and its properties vary with climate, parent material, and time.

A key process shaping subsoil is illuviation, the translocation of clay, silt, and oxides from upper horizons into the subsoil, forming Bt or similar horizons with clay films and secondary minerals. In humid regions this results in a clay-rich subsoil; in arid regions calcareous, saline, or cemented subsoils such as duripans or calcic horizons may occur.

The subsoil influences plant performance by storing and transmitting water and nutrients and by providing or restricting rooting depth. It can limit drainage or become prone to compaction, affecting cultivation and irrigation. Management practices include subsoiling to break compacted layers, improving drainage, and adjusting soil amendments to address specific subsoil limitations.

In construction and land-use planning, subsoil characteristics help determine foundation design, drainage schemes, and suitability for different crops or structures.