The layer of soil beneath topsoil, lighter in color with less organic matter and fewer organisms, less fertile but often containing clay that affects drainage.
In plain terms
Soil has layers. Topsoil (top 6-12 inches) is dark, rich, alive with organisms. Below it is subsoil—lighter, less alive, less nutrient-rich. Subsoil often contains more clay, which can create a hard layer (hardpan) that restricts root penetration. Gardening happens in the top 12-18 inches (topsoil plus upper subsoil); deeper digging doesn't help most plants.
Why this matters
Understanding soil layers helps you understand drainage problems. Dense, impenetrable subsoil or hardpan restricts roots and water movement.
In practice
Examples
New construction: topsoil removed, subsoil exposed; gardens need serious amendment before productivity.
Clay subsoil underlying sandy topsoil: water pools at clay layer; compacted drainage.
Hardpan (compacted subsoil layer): blocks root penetration and water; requires deep ripping or raised beds to bypass.
Well-drained site: subsoil is permeable; water moves through; good drainage.
Practical applications
Amend topsoil thoroughly; don't worry about subsoil.
If subsoil is exposed (new construction), add 4-6 inches of quality topsoil before gardening.
Raised beds bypass poor subsoil entirely.
Deep ripping can break up hardpan but is labor-intensive; raised beds are easier.
Understand your subsoil composition to plan raised bed height and amendments.