Subsoil is the layer of earth that sits directly beneath the topsoil you see at the surface. In soil science, it’s formally known as the B horizon, the middle layer in a three-part soil profile: surface soil (A horizon) on top, subsoil (B horizon) in the middle, and the rocky substratum (C horizon) at the bottom. It typically starts anywhere from 15 to 60 centimeters below the surface, depending on the landscape, and it plays a surprisingly important role in everything from plant growth to building foundations to how water moves underground.
How Subsoil Differs From Topsoil
The most obvious difference is color. Topsoil is dark because it’s rich in decomposed plant and animal material. Subsoil is lighter, often appearing tan, reddish-brown, or yellowish-orange depending on its mineral content. It contains far less organic matter because most biological activity happens closer to the surface.
What subsoil lacks in organic material, it makes up for in minerals and clay. As rainwater filters down through the topsoil, it dissolves minerals and carries tiny clay particles deeper into the ground. These accumulate in the B horizon over time, making subsoil denser and more clay-rich than the layer above it. This process, called leaching, is essentially gravity slowly sorting the soil profile into distinct layers over hundreds or thousands of years.
What Gives Subsoil Its Color
The reddish, orange, and yellow tones you see in exposed subsoil come primarily from iron oxide minerals. Different iron compounds produce different colors. Soils containing hematite tend to be distinctly red, with small hematite crystals producing a bright red tone. Goethite, another iron mineral, gives soil a yellowish-brown appearance. Orange subsoil often signals the presence of a mineral called lepidocrocite. These colors aren’t just cosmetic. Soil scientists use them as field clues to understand how the soil formed and how well it drains. A mottled mix of gray and orange patches, for example, typically indicates a layer that fluctuates between waterlogged and dry conditions.
Life Below the Surface
Subsoil isn’t lifeless, but it’s far less biologically active than topsoil. Research from Iowa measuring bacterial concentrations across soil depths found that the top 15 centimeters of soil contained roughly 159 billion bacterial gene copies per gram, while the deepest subsoil layers (150 to 180 centimeters down) held only about 12.5 million, a drop of roughly 10,000-fold. Many of the deepest samples fell below even that number.
The types of microbes also shift with depth. Surface soils host a diverse mix of bacterial groups, but deeper subsoil becomes increasingly dominated by a narrower set of organisms. Of the 58 bacterial genera that changed significantly across the soil profile, the vast majority decreased with depth. Only three increased, and the deepest layers were dominated by just one genus. Beneficial soil fungi that form partnerships with plant roots also decline substantially in the subsoil, which limits how effectively deep roots can scavenge nutrients.
Why Subsoil Matters for Plant Growth
Most of a plant’s accessible phosphorus sits in the topsoil, but nitrogen tells a different story. The most common form of plant-available nitrogen is nitrate, which dissolves easily in water and gets washed down into deeper soil layers with rainfall. Plants that can push roots into the subsoil capture nitrogen that shallow-rooted plants miss entirely. This is one reason deeper rooting is a major goal in crop breeding research.
Getting roots into the subsoil isn’t always easy. Acidic subsoils, generally those with a pH below 5, create a hostile environment for root tips. Aluminum becomes toxic at low pH and interferes with the uptake of calcium, magnesium, and potassium. Calcium is particularly critical because root tips need it to build new cell walls, and they can’t transport much of it internally from upper parts of the plant. The calcium has to come directly from the surrounding soil. When subsoil is both acidic and calcium-poor, root growth stalls. In arid regions, sodium, boron, and salt buildup in the subsoil can create similar barriers.
Roots that do penetrate compacted subsoil often rely on existing cracks and old root channels (called biopores) rather than forcing their way through solid material. Root growth slows when soil pressure reaches about 100 kilopascals and stops entirely at around 1,000 kilopascals.
How Subsoil Controls Water Movement
Because subsoil is denser and contains more clay than topsoil, it holds more water but lets it pass through more slowly. This has major consequences for drainage. A clay-rich B horizon can act like a bottleneck, slowing the downward flow of rainwater and influencing how quickly (or slowly) water reaches underground aquifers.
The balance of pore sizes in the subsoil determines its water-holding capacity. Organic matter in topsoil creates a mix of large and small pore spaces that absorb and release water efficiently. Subsoil, with less organic matter and more compaction, has fewer large pores. Compaction crushes bigger pore spaces into tiny ones, reducing both the total water the soil can store and the amount plants can actually extract. When subsoil is severely compacted, water that would normally soak in instead runs off the surface, increasing flood risk and degrading water quality in nearby streams and lakes.
Subsoil Compaction From Farming
Heavy agricultural machinery is the primary cause of subsoil compaction, and it’s been a growing concern for decades. Modern tractors and harvesters are significantly heavier than the equipment they replaced, and their weight pushes compaction below the plow layer into the subsoil where normal tillage can’t reach it.
The damage is cumulative and persistent. Research has shown that a single pass with a wheel load of just 5 metric tons can cause a permanent yield loss of about 2.5% due to subsoil compaction. Each additional pass along the same track worsens the soil’s mechanical properties. On lighter, waterlogged soils, the effect is even more dramatic: increasing soil density by just 0.1 metric tons per cubic meter can slash total pore space from 12% to 2%, reducing productivity by up to 15%. Unlike topsoil compaction, which can be broken up with plowing, subsoil compaction is extremely difficult to reverse and can persist for decades.
Subsoil in Construction and Engineering
For builders, subsoil is the material that ultimately supports a structure’s weight. Foundation design depends heavily on the subsoil’s bearing capacity, which is its ability to support a load without excessive settling or failure. Engineers calculate a factor of safety by dividing the soil’s ultimate bearing capacity by the actual load that will be placed on it. Building codes typically require a safety factor between 2.0 and 3.0 for standard foundations, while more demanding structures like bridge foundations may require a factor of 4.0.
Clay-heavy subsoils pose particular challenges because they expand when wet and shrink when dry, which can shift foundations over time. Sandy or gravelly subsoils drain better and are generally more stable, but they may not hold loads as well without compaction. Before any significant construction project, geotechnical surveys test the subsoil to determine its composition, density, moisture content, and load-bearing properties. The results dictate what type of foundation is needed and how deep it must go.