A soil profile is a vertical section through soil showing its layered structure. Soil horizons are the distinct, approximately horizontal layers within that profile, produced by long-term interactions and soil-forming processes.
How a Soil Profile Develops
In IB ESS Subtopic 5.1 Soil, soil is treated as a system with inputs, outputs, storages and flows. Inputs include dead organic matter and weathered minerals, while processes such as decomposition transform materials within the soil.
Transfers move materials between horizons. For example, water may carry dissolved minerals downward through the profile. Over time, differences in organic matter, mineral content, colour and texture create recognizable horizons.
For SL and HL, you should understand that a profile is the complete layered structure. The detailed identification of the O, A, B and C horizons is HL-only content.
| Horizon | Main characteristics |
|---|---|
| O horizon | Surface organic layer containing leaf litter and decomposing material; may be absent in some soils. |
| A horizon | Topsoil or mixed layer, containing minerals and substantial organic matter. It is usually important for plant growth and vulnerable to erosion. |
| B horizon | Mineral soil below the A horizon. Materials transferred downward may accumulate here. |
| C horizon | Partly weathered parent material from which the mineral component of soil develops. |
A named context is Surtsey, Iceland, where newly exposed volcanic material has gradually developed soil as organisms colonize, organic matter accumulates and weathering occurs.
A common misconception is that a soil profile and a soil horizon are the same thing. The profile is the whole vertical section; each horizon is one layer within it.
Exam Technique
For a systems question, draw horizons as labelled storages and use arrows to show transfers into, out of and within the soil. For an HL description, identify each horizon precisely and connect its properties to processes such as decomposition, weathering or downward transfer rather than merely listing the letters.