Overview
A soil profile is the full vertical sequence of soil layers, or horizons, from the surface down to the bedrock. The master horizons are O (organic litter), A (humus-rich topsoil), E (a leached, lighter zone), B (the subsoil where material accumulates), C (weathered parent material) and R (bedrock). The layering is produced mainly by eluviation (downward removal) and illuviation (deposition below), and the A, E and B horizons together form the solum.
Soil profiles and horizons
What a soil horizon and a soil profile are
A soil horizon is a layer of soil lying parallel to the surface whose physical, chemical and biological characteristics differ from the layers above and beneath it. Soil is not a uniform mass but a set of these distinct layers.
A vertical section cut through all of these layers, from the surface down to the bedrock, is called the soil profile. Reading the profile, layer by layer, is the basic method by which soils are described and classified.
Why the soil profile matters
Why it matters is that the profile records how a soil formed and what it can do. Its layers reveal fertility, drainage and the parent rock, which together decide what crops will grow and how land should be used.
For physical geography and agriculture alike, the profile is the key tool. The same simple set of horizons describes soils all over the world, so learning them once unlocks the study of soils everywhere.
The master soil horizons
The O, A and E horizons
The horizons are named with letters, from the surface downward. The O horizon is the organic surface layer of plant litter, and below it the A horizon, or topsoil, is the mineral layer with the greatest accumulation of organic matter and humus.
Beneath the topsoil lies the E horizon, a mineral layer that has lost iron, aluminium, clay or organic matter by downward movement of water. This loss leaves it noticeably lighter in colour, which is how it is recognised in the field.
The B, C and R horizons
The B horizon, or subsoil, normally holds less organic matter than the A horizon but is the zone where iron oxides and clay minerals washed from above accumulate. It is often denser and more strongly coloured.
Below it, the C horizon is the substratum of poorly weathered or unweathered rock, the raw material from which the soil above has formed. The R horizon at the base is the solid, largely unweathered bedrock.
Soil-forming processes
Eluviation and illuviation
The layering of the profile is produced largely by water moving through the soil. Eluviation is the washing out of fine material, iron, aluminium, clay and organic matter, from the upper soil; where it is strong, the lighter E horizon appears.
The material removed from above does not vanish. It is carried down and deposited lower in the profile, a process called illuviation, the accumulation of clay minerals, iron, aluminium and organic compounds in the B horizon.
How a profile develops over time
A full profile is not formed overnight. It begins as bare parent rock, which weathering slowly breaks down, allowing a thin layer of soil with a simple A horizon to form on top.
As weathering and leaching continue over long periods, the layers become more distinct, until a mature profile with clear O, A, E, B and C horizons emerges. A well-developed profile is therefore a record of soil formation over time.
Reading and using the soil profile
Fertility, drainage and parent material in the profile
A trained eye can read a great deal from a profile. A deep, dark A horizon signals a fertile, humus-rich soil, while the colour and texture of the layers show how freely water drains through the soil.
The C horizon points to the parent material beneath, and the profile as a whole guides decisions on which crops to grow and where it is safe to build. The profile is thus a practical map of the soil's capability.
The solum and the standard horizon system
The upper, biologically active layers, the A, E and B horizons together, are known as the solum, the true zone of soil formation. The C and R horizons below are the parent material and bedrock rather than soil proper.
This lettered scheme of master horizons is the standard system, used worldwide and in the United States Department of Agriculture classification, so that a profile described in one country can be understood in another.
How the soil profile appears in the UPSC exam
Soil profile and horizons in GS and Geography Optional
The soil profile is a dependable Prelims topic, tested on the horizons and the processes that form them. The high-yield points are precise.
- The soil profile is the full vertical sequence of soil horizons.
- The master horizons, top to bottom, are O, A, E, B, C and R.
- Eluviation removes material from upper layers; illuviation deposits it below.
- The A, E and B horizons together form the solum.
| Horizon | What it is |
|---|---|
| O | organic surface litter |
| A | topsoil, rich in humus |
| E | leached, lighter zone of eluviation |
| B | subsoil of accumulation (illuviation) |
| C | weathered parent material |
| R | solid bedrock |
A precise answer names the horizons in order and links them to eluviation and illuviation, the discipline this article builds across its sections.
Prelims MCQ practice
Each question below tests one specific concept on the topic. Click to reveal the answer and a full option-wise explanation.
Q1. A vertical section through the soil showing all its layers from the surface to the bedrock is called the:
- Soil horizon
- Soil profile
- Solum
- Regolith
Show answer and explanation
Answer: Soil profile
Explanation.
The soil profile is the full vertical sequence of soil horizons from surface to bedrock. A single layer is a horizon. Hence (b).
Q2. Which soil horizon is the organic surface layer of plant litter?
- A horizon
- O horizon
- B horizon
- C horizon
Show answer and explanation
Answer: O horizon
Explanation.
The O horizon is the organic surface layer of plant litter. The A horizon below it is the humus-rich topsoil. Hence (b).
Q3. With reference to soil-forming processes, consider the following statements:
- Eluviation is the downward removal of fine material from the upper soil.
- Illuviation is the accumulation of clay, iron and organic compounds in a lower horizon.
- The E horizon is a zone of accumulation.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statements 1 and 2 are correct. Statement 3 is wrong: the E horizon is a zone of eluviation (loss), while the B horizon is the zone of accumulation. Hence 1 and 2 only.
Q4. The B horizon of a soil profile is best described as the:
- Organic litter layer
- Zone of accumulation (illuviation)
- Unweathered bedrock
- Leached, lighter layer
Show answer and explanation
Answer: Zone of accumulation (illuviation)
Explanation.
The B horizon, or subsoil, is the zone of illuviation, where iron oxides and clay washed from above accumulate. Hence (b).
Q5. The A, E and B horizons of a soil are collectively known as the:
- Regolith
- Solum
- Bedrock
- Substratum
Show answer and explanation
Answer: Solum
Explanation.
The A, E and B horizons together form the solum, the true zone of soil formation. Hence (b).
Q6. Arrange the soil horizons in order from the surface downward:
- O, A, E, B, C, R
- A, O, B, E, C, R
- R, C, B, E, A, O
- O, E, A, B, C, R
Show answer and explanation
Answer: O, A, E, B, C, R
Explanation.
From the surface down, the master horizons are O (litter), A (topsoil), E (leached), B (subsoil), C (parent material) and R (bedrock). Hence (a).
Sources and Further Reading
Editorial Disclaimer
This article explains the soil profile and its horizons for UPSC preparation, drawing on standard soil-science sources. Definitions reflect the cited authorities.
