Soil profiles and horizons
Big idea: If you dig a pit, you see layers called horizons. Together, these layers form a soil profile. Different biomes have different characteristic profiles.
The main soil horizons
- O horizon (organic) — surface layer of leaf litter, decomposing matter
- A horizon (topsoil) — dark, rich in humus, most biological activity, plant roots
- B horizon (subsoil) — lighter colour, accumulates minerals washed down from above
- C horizon (parent material) — weathered rock fragments, little organic matter
- R horizon (bedrock) — solid rock underneath
Soil processes
- Leaching — water washes nutrients down through the profile
- Eluviation — fine particles washed out of A horizon
- Illuviation — materials deposited in B horizon
- Humification — organic matter converted to humus
In tropical rainforests, heavy rainfall causes rapid leaching — nutrients are quickly washed from soil. Most nutrients are stored in the BIOMASS, not the soil!
IB-style question — reading a soil profile
Describe the main horizons seen in a typical soil profile, from the surface downwards. [3]
How the marks are earned
- Top layers (O / A)
• dark, rich in humus — most soil life and plant nutrients here
• B horizon below: lighter, minerals leached from above accumulate - Bottom (C horizon)
• weathered fragments of parent rock
• grades into solid bedrock
Final answer
Give the correct top-to-bottom order (O/A → B → C) with one feature per horizon.
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