Soil Layers and Formation

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Soil is a dynamic natural system that forms at Earth's surface from weathered rock, decayed organic matter, water, air, and living organisms. It matters because it supports plant growth, stores water, cycles nutrients, and helps shape ecosystems. A soil profile shows how soil is arranged in layers called horizons from the surface down to bedrock. Each horizon has different materials, colors, and textures that reveal how the soil formed over time.

Soil forms through physical and chemical weathering of rock, along with the action of roots, microbes, insects, and climate. Organic material builds up near the surface, while water moves minerals downward in a process called leaching and can deposit them lower in the profile. This creates distinct horizons such as the O, A, E, B, C, and R layers. The thickness and composition of these layers depend on five major soil-forming factors: parent material, climate, organisms, topography, and time.

Key Facts

O horizon contains leaf litter, decomposing material, and humus.
A horizon is topsoil and usually has the most roots, organisms, and organic matter mixed with minerals.
E horizon is a zone of eluviation where clay, iron, and other materials are washed out.
B horizon is subsoil and often gains clay, iron, or minerals from upper layers by illuviation.
C horizon is partly weathered parent material, and R horizon is solid bedrock.
Soil formation rate can be summarized as Soil = f(climate, organisms, relief, parent material, time).

Vocabulary

Horizon: A horizon is a distinct soil layer with its own color, texture, and composition.
Humus: Humus is dark organic material formed by the decomposition of plants and animals.
Weathering: Weathering is the breakdown of rock into smaller pieces or new minerals at Earth's surface.
Leaching: Leaching is the downward movement of dissolved minerals and fine particles through soil by water.
Parent material: Parent material is the original rock or sediment from which a soil develops.

Common Mistakes to Avoid

Confusing weathered parent material with bedrock, because the C horizon is broken and altered material while the R horizon is solid unweathered rock.
Assuming every soil profile has all horizons, because many real soils are missing layers such as the O or E horizon depending on climate and environment.
Thinking topsoil is only dead plant matter, because the A horizon is a mixture of mineral particles, organic matter, water, air, and living organisms.
Believing soil forms quickly, because most mature soil profiles take hundreds to thousands of years to develop distinct horizons.

Practice Questions

1 A soil profile has an O horizon that is 3 cm thick, an A horizon that is 18 cm thick, an E horizon that is 6 cm thick, and a B horizon that begins below them. At what depth below the surface does the B horizon start?
2 A farmer examines a soil pit where the A horizon is 22 cm thick, the B horizon is 35 cm thick, and the C horizon extends from 57 cm to 95 cm depth. How thick is the C horizon?
3 A region has heavy rainfall and strong downward water movement through the soil. Explain which horizon is most likely to lose minerals and which horizon is most likely to gain them, and why.