Subsoil is the layer of material found directly beneath the organic-rich topsoil (A horizon). This underlying layer, generally designated as the B horizon, serves as the essential link between the biologically active surface and the deeper, poorly weathered parent material (C horizon). It is a transition zone where materials washed down from above are deposited, and where deep-reaching plant roots anchor and access resources. Subsoil characteristics result from long-term weathering and the movement of water, minerals, and organic compounds through the soil column.
The Mineral Matrix
The bulk of the subsoil is composed of inorganic mineral particles, which determine its texture and structure. These particles originate from the weathering of the parent material and are classified by size into sand, silt, and clay. Subsoil layers often have a higher concentration of clay compared to the topsoil above them.
This increased clay content results from illuviation, where fine clay particles are transported downward by percolating water from the A horizon and accumulate in the B horizon. This accumulation gives the subsoil a denser, finer texture and a distinct structure, often forming blocky or prismatic shapes. The specific chemical composition of the subsoil, including the types of silicate minerals, is dictated by the geology of the underlying parent material.
In addition to clay, the subsoil accumulates iron and aluminum oxides leached from the upper layers. These oxides often coat the mineral grains, imparting the characteristic reddish or yellowish-brown colors frequently observed in the B horizon. These compounds are less soluble than many other minerals. The accumulated minerals contribute significantly to the density and stability of the subsoil structure, providing physical support for the entire soil profile.
Water Dynamics and Air Pockets
The subsoil structure dictates how water moves through and is stored in the soil profile. Porosity, the spaces between mineral particles, determines the amount of water and air the subsoil holds. Due to high clay and oxide content, subsoil has lower total porosity than topsoil, but a greater capacity to retain water because of its small pore size.
The dense, clay-rich B horizon causes water to drain more slowly than through coarser topsoil. This high water retention capacity makes the subsoil an important reservoir, providing moisture to deep-rooted plants during drought. However, slow water movement can lead to saturated conditions if drainage is inadequate, sometimes forming a compact layer called a hardpan.
Proper aeration, the presence of air within pore spaces, is necessary for healthy root function and soil organisms. When the subsoil becomes saturated, the lack of oxygen restricts root growth, limiting a plant’s ability to access stored water and nutrients. The balance between water retention and aeration is a direct consequence of the subsoil’s texture and structure.
Biological and Chemical Characteristics
The subsoil is a less biologically active environment compared to the topsoil due to reduced organic matter content. Organic carbon, which fuels microbial life, is significantly lower in the B horizon because plant residues are difficult to incorporate or transport to this depth. Consequently, the diversity and population density of bacteria and fungi are also lower.
Despite reduced microbial life, the subsoil contains accumulated nutrients and chemical compounds. Soluble ions, such as calcium, magnesium, and potassium, are often leached from the A horizon and accumulate here. These nutrients are sometimes less available to plants because they are held tightly by the high concentration of clay or locked into the mineral structure.
The chemical environment of the subsoil is highly variable, with pH levels often differing from the surface layer. Accumulation of basic cations can lead to a higher pH in some regions, while leaching results in a more acidic subsoil in others. Deep-rooted organisms, including fungi and burrowing animals, help mix and aerate the subsoil, providing channels for water and root penetration.