Cut a vertical slice down through a soil and you get a profile: a set of layers called horizons, each one the record of a different process. Once you can name them, you can look at a photograph or a diagram and say what has been happening to that soil for the last few thousand years.
📚 What you need to know
A soil profile is a vertical section from the surface down to bedrock.
It is made of horizons: O, A, E, B, C and R.
Profiles run from organic-rich at the top to mineral-rich at the bottom.
Horizons form because of transfers (water and material moving) and transformations (decomposition and weathering).
Different biomes give different profiles: brown earths, oxisols, podzols and aridisols.
Under intensive agriculture the O and A horizons can be lost, leaving only B and C.
What a profile tells you
Soil profiles are useful because they are readable. Colour, texture, mineral content and the sharpness of the boundaries between layers all carry information about the soil’s history, its fertility and what it can be used for.
Organic material is concentrated near the surface, where leaf litter and dead animals arrive. That is why the top is dark.
Mineral content varies with depth, mostly because water carries dissolved material downwards. Some layers are stripped; others are enriched.
Colour is a clue. Dark means organic matter. Red or yellow usually means iron oxides and good drainage. Greyish-blue means the soil has been waterlogged.
Rock fragments get bigger and more common with depth, until you reach unweathered rock.
A profile is not a static picture. It is the outcome of processes still going on. When you describe one, say what made it look like that, not just what it looks like.
The six horizons
Learn them in order from the top. The letters are not random: O for organic, A for the mixed layer, E for eluvial (washed out), B for illuvial (washed in), C for the parent material and R for rock.
Not every soil has all six. Young or disturbed soils may be missing several.
The three you must be able to separate
If you only remember three things about horizons, remember these.
The E horizon is the one material has been taken out of. That is why it is pale. Eluvial means washed out.
The B horizon is the one material has been added to. Illuvial means washed in. It is higher in clay, iron and aluminium than the layers above it, and it holds water and nutrients that deep roots can reach — but it is less fertile than topsoil.
The C horizon is not really soil yet. It is the partly weathered parent material that the soil above has been made from, and its properties set limits on everything higher up.
The syllabus asks specifically about the organic layer, the mixed layer, the mineral soil and the parent rock — in other words O, A, B and C. Know the E and R too, but those four are the core.
Different biomes, different profiles
Because climate, vegetation and parent rock differ, the same set of processes produces very different-looking profiles.
Soil type
Where you find it
What the profile looks like and why
Brown earth
Temperate deciduous woodland
Fertile, nutrient-rich upper layers with well-defined horizons; moderate rainfall and steady decomposition
Oxisol
Tropical rainforest
Deep and red, rich in iron and aluminium; intense weathering plus heavy leaching strips other nutrients
Podzol
Coniferous forest
Acidic, with a very pale E horizon; heavy rainfall and acidic needle litter leach nutrients strongly, so fertility is low
Aridisol
Desert
Shallow, sandy or rocky, with almost no organic matter; little vegetation input and very little leaching
Worth remembering
Peat soils and prairie soils
Two more that show how strongly climate controls the profile. In cold, wet boreal forests such as those in Scandinavia, decomposition is so slow that partly decayed organic matter piles up as peat, giving acidic, nutrient-poor soils with a very thick organic layer. In the semi-arid Great Plains of the USA, grassland has fed organic matter into the soil for thousands of years, producing famously deep, dark topsoil.
Natural soils versus farmed soils
This is the part of the section with the most exam mileage, because it links soil science to human impact.
Tilling and bare ground expose the O and A horizons to wind and rain. Once gone, they take centuries to rebuild.
Why it matters
Fertility falls. The O and A horizons hold almost all the organic matter and most of the available nutrients. Lose them and yields drop unless fertiliser makes up the difference.
It is a long-term problem. Soil forms over centuries. Stripping the organic-rich upper layers is degradation you cannot undo in a growing season.
What farmers can do about it
Crop rotation and cover crops keep roots in the ground, which holds soil in place and returns organic matter.
Reduced tillage means less disturbance, so the structure of the O and A horizons survives and organic matter is not exposed to the air.
EXAM QUESTION
Distinguish between the E horizon and the B horizon. [3]
Name the process for each
The E horizon is the eluvial layer: water moving down removes minerals and nutrients from it.
The B horizon is the illuvial layer: that same material is deposited and accumulates here.Then give a visible differenceThe E horizon looks paler than the layers around it because it has been stripped; the B horizon is richer in clay, iron and aluminium.Out of E, into B
EXAM QUESTION
Explain why an intensively farmed soil may have a different profile from an undisturbed soil in the same area. [4]
Start with what should be there
In an undisturbed soil all horizons are usually present, from the organic O horizon down to the parent material.
Then the human impactPloughing and leaving ground bare expose the surface to wind and rain, so the O and A horizons are eroded away.Only the B and C horizons may remain intact.Then the consequenceSince the organic matter and most available nutrients were in the lost layers, fertility falls and fertiliser inputs are needed to keep yields up.Cause, change, consequence
💡 Exam tip
Eluvial = exit, illuvial = in. That one trick will keep E and B straight under exam pressure.
When you describe a profile, always give the process behind the layer, not just its colour.
You are not expected to memorise every soil type on Earth. Two or three named profiles linked to their climate is enough.
If asked about agriculture, mention both the damage and at least one management response such as reduced tillage or cover crops.
Link back to Topic 5’s bigger picture: soil degradation is a loss of natural capital.
⚠ Common mix-up
The O horizon is not the same as the A horizon. O is almost entirely organic; A is organic and mineral mixed together.
The C horizon is not bedrock. C is weathered parent material; R is the solid rock beneath it.
A pale layer does not mean an infertile soil overall. The pale E horizon has been leached, but the material has usually gone into the B horizon just below.
Profiles are not universal. Not every soil contains all six horizons, and depths vary enormously.
Up next: What Shapes a Soil — the five factors that decide why the profile you just read looks the way it does.
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