AP® Environmental Science · Unit 4 · Topic 4.2
Soil Formation and Horizons
Soil forms as rock weathers and organic matter builds up, settling into distinct layers called horizons. This takes centuries, which is why topsoil behaves as a nonrenewable resource on any human timescale.
A road cut shows stripes because soil is not one substance, it is a stack of layers built by different processes operating at different depths over centuries. At the surface, dead leaves and organisms pile up and rot into the O horizon, a dark, loose layer of organic matter. Below it, that organic matter mixes with mineral particles from weathered rock to form the A horizon, the true topsoil, dark and fertile. Rain pushes clay, minerals, and nutrients downward in a process called leaching, draining the E horizon of color and leaving it pale and mineral-poor. Those leached materials accumulate below in the B horizon, or subsoil, often rusty red or orange from iron and clay deposits. At the bottom sits the C horizon, loose weathered rock fragments, sitting on unweathered bedrock. Each horizon has a different color and texture because each is shaped by a different balance of weathering, decomposition, and leaching.
Building this full stack takes a very long time, often many centuries, because weathering bedrock into fine mineral particles and accumulating enough organic matter to darken the A horizon both happen at a crawl. That slow formation rate is why topsoil fails the test of a renewable resource on a farming timescale. Farming, wind, and water erosion can strip away centimeters of A horizon in decades, far faster than new topsoil can form to replace it. This connects directly to plowing: turning a forest soil for the first time mixes and destroys the O and A horizons, exposes bare mineral soil to erosion, and can even start cutting into the E and B horizons, erasing a profile that took centuries to build.
Key terms
- Horizon
- A distinct horizontal layer of soil with its own color, texture, and composition, formed by different processes at different depths.
- Leaching
- The downward movement of dissolved minerals and nutrients through soil carried by water.
- Weathering
- The physical and chemical breakdown of bedrock into smaller mineral particles that become part of soil.
- Topsoil (A horizon)
- The dark, fertile upper layer of mineral soil mixed with decomposed organic matter, used for most plant growth.
- Nonrenewable on a human timescale
- A resource that regenerates so slowly that depletion outpaces replacement within a human lifetime.
Watch it explained
- How it worksWalk me through the O, A, E, B, and C horizons stacking up over centuries.
- How it worksHow do weathering, leaching, and decomposition build a soil profile out of bare rock?
- Common mistakeWhy isn't topsoil a renewable resource on a farming timescale?
- Going furtherWhat happens to the horizons the first time a forest soil is plowed?
Check yourself
End of topic.