The B horizon, often called subsoil, forms primarily through a process called illuviation: the gradual accumulation of materials like clay, iron, aluminum, and organic matter that water carries downward from the soil layers above. This process works over thousands of years, slowly building a denser, more colorful layer beneath the topsoil.
Illuviation: The Core Process
Illuviation is the defining mechanism behind B horizon formation. As rainwater percolates through the upper layers of soil, it picks up fine particles of clay, dissolved minerals, and organic compounds and carries them deeper underground. When these materials reach the B horizon, they deposit and accumulate there over time. Think of it like a filter: the upper layers lose material, and the B horizon catches it.
The materials that collect in the B horizon include silicate clays, iron and aluminum oxides, humus (decomposed organic matter), carbonates, gypsum, and silica. These can accumulate alone or in combination, which is why B horizons look so different from one soil type to another. In some soils, the B horizon is rust-red from iron buildup. In others, it’s stained dark brown by organic matter or appears pale from calcium carbonate deposits.
Where the Material Comes From
The B horizon can’t accumulate material without a source, and that source is the soil above it. The A horizon (topsoil) and the E horizon (when present) lose material through a complementary process called eluviation. Eluviation is essentially the opposite of illuviation: it’s the removal of clay particles, organic matter, and iron and aluminum oxides by water moving downward through the soil profile. The A and E horizons become depleted and often lighter in color, while the B horizon below becomes enriched and more colorful.
This two-part relationship between eluviation above and illuviation below is fundamental to how soil scientists understand soil development. You can often see the contrast with your own eyes in a road cut or trench: a pale, washed-out E horizon sitting directly on top of a dense, brightly colored B horizon.
Other Processes That Shape the B Horizon
Illuviation isn’t the only process at work. B horizons also develop through in-place weathering, where minerals within the horizon itself break down chemically and transform into new clay minerals, iron oxides, or hydroxides. This process, sometimes called argillization, can create clay-rich B horizons even without significant material washing in from above. Over time, these chemical changes produce the blocky or prismatic soil structure that characterizes well-developed B horizons. As the soil expands and contracts with wetting and drying cycles, it fractures into distinct geometric shapes rather than remaining loose or granular like topsoil.
In drier climates, another variation occurs. Carbonates dissolve near the surface where some moisture is available, move downward, and then reprecipitate as solid deposits within the B horizon when the water evaporates. This creates the hard, whitish calcium carbonate layers common in soils across the western United States.
What the B Horizon Looks and Feels Like
Because of all this accumulated material, the B horizon is typically firmer and denser than the topsoil above it. It’s usually lighter in color than the dark, organic-rich A horizon but noticeably brighter than the relatively unchanged parent material (C horizon) below. Colors range from yellowish-brown to deep red depending on which minerals have accumulated. Iron-rich B horizons tend toward reds and oranges, while those dominated by organic matter lean brown or dark gray.
The structure is also distinctive. Rather than the crumbly, granular texture of topsoil, B horizons generally develop blocky or prismatic aggregates. If you pull apart a chunk of B horizon soil, it tends to break along defined planes into angular or rounded blocks. This structure develops as clay accumulates and the soil repeatedly swells when wet and shrinks when dry.
How Climate Controls B Horizon Development
The rate and character of B horizon formation depend heavily on climate, particularly rainfall and temperature. More precipitation means more water moving through the soil profile, which accelerates both eluviation and illuviation. This is why soils in the eastern United States, where rainfall is higher, generally have thicker and more developed profiles than those in the arid West.
Temperature matters too. Warmer conditions increase microbial activity, which speeds the breakdown of organic matter and the chemical weathering of minerals. Moving from north to south across the United States, rising soil temperatures drive faster decomposition of humus, altering what organic material is available to move into the B horizon. In cooler, wetter climates like the Pacific Northwest or northern Europe, organic compounds and iron are more likely to accumulate in the B horizon because they dissolve and travel more readily in acidic soil water.
The combination of these factors means B horizon formation is not a fixed timeline. Thick, clay-enriched B horizons in temperate regions can take thousands to potentially millions of years to develop fully, while thin or weakly developed B horizons may appear in much younger soils. A young volcanic soil in Hawaii and an ancient soil in the southeastern United States will have dramatically different B horizons, reflecting their very different ages and climatic histories.
Why the B Horizon Matters
Understanding the B horizon is practical, not just academic. Because this layer accumulates clay and becomes dense, it directly affects how water drains through soil. A thick, clay-rich B horizon can slow drainage significantly, creating waterlogged conditions above it or causing water to flow sideways along its surface. This influences everything from where basements flood to which crops grow well in a given field. Builders, farmers, and engineers all pay attention to the B horizon when making decisions about land use, septic systems, and foundations.