What Is a Soil Horizon? Layers of Soil Explained
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What is a soil horizon? A soil horizon is a layer of soil that runs roughly parallel to the ground surface and has characteristics produced by soil-forming processes. When you dig a planting hole, you often see a dark band of topsoil sitting above lighter material, or a dense band of clay further down. Those bands are horizons. This article explains what a soil horizon is, what the standard horizon letters mean, how horizons form, and where the sources stop. It relies on two university references and quotes them directly.
Quick answer
- A soil horizon is a layer of soil with characteristics produced by soil-forming processes. The Purdue manual says a horizon is a layer “approximately parallel to the soil surface.”
- The A horizon is usually the darkest layer because organic matter builds up in it. The Buffalo reference calls the A horizon topsoil.
- The B horizon is the subsoil, and it often holds material washed down from the layers above. The C horizon is the substratum, the material below the layers above it.
- Horizons form as climate, organisms, relief, parent material and time act on a soil, and the Buffalo reference says time is required for these factors to take effect.
How we chose
We chose two university references that define soil horizons directly. We opened both on 2026-10-27 with curl, and both returned HTTP 200. We quote only sentences that appear in the fetched text, and we leave out claims the two pages do not support.
What a soil horizon means in soil science
The Purdue manual gives the most direct definition. It states that “A soil horizon is a layer of soil, approximately parallel to the soil surface, with characteristics produced by soil-forming processes.” The same manual adds that “Horizons produced by soil-forming processes are called genetic horizons.” Those are the layers gardeners care about most, because they reflect how the soil has developed in place.
The manual also explains that not every layer in a profile is a genetic horizon. It notes that “Besides genetic horizons, many soils have layers inherited from stratified (layered) parent material.” In other words, a layer can look like a horizon but come from deposited sediment rather than from soil formation. Soil scientists handle this by examining every layer of a profile, and the Purdue text says that “In studying soils, all layers or horizons of a soil profile are examined regardless of how they formed.”
The distinction matters when reading a profile. The sources do not provide a method for telling the two apart in a garden, so this article does not offer one.
The master horizons from top to bottom
The Buffalo reference describes the basic scheme. It states that “Soil Horizons are the distinct layers of a soil profile.” It lists the main layers as “Master Horizons” from top to bottom: O Horizon, A Horizon, E Horizon, B Horizon, C Horizon, and R Horizon. The same reference notes that “the most well-developed soils might have all of these layers, and the least-developed soils might only have an A and a D horizon.”
O horizon
The O horizon sits at the very top. The Buffalo reference says it “is composed of organic material that has accumulated and been modified (physically and chemically) over time, typically from the remains of plant and animals.” It is most visible “in soils that are rarely, if ever, disturbed and with plenty of foliage and/or organisms nearby to contribute to its development, such as forests.” The same reference adds that “In more barren locations such as grasslands, an O Horizon is rarer.” The sources do not describe garden beds directly.
A horizon
The A horizon is the layer most gardeners mean when they say topsoil. The Buffalo reference calls it “a well-weathered and fertile layer dominated by mineral particles but still rich in organic matter,” and states that “The A Horizon is considered topsoil.” The Purdue manual explains the color: “The A usually has the darkest color of any horizon in the soil,” and “Accumulation of organic matter in surface A horizons makes them dark.” The Buffalo reference also says that “Subterranean life (including microfauna, mesofauna , and macrofauna ) tends to be the most abundant in this layer due to the rich, soft, and well-weathered environment of the soil.”
For practical steps that work on the surface layers, see how to keep garden soil healthy.
E horizon
The E horizon is a lighter layer found in some soils. The Buffalo reference explains that “The E in “E Horizon” stands for eluviation, another word for leaching.” It also says that “Like the O Horizon, this layer is not always present, but when it is, it’s usually in forested areas and rarely in grasslands.” The Purdue manual notes that “Forest soils commonly contain E horizons a few inches below the surface.” Not every garden soil has an E horizon, and many do not.
B horizon
The B horizon is the subsoil. The Buffalo reference says that “The B Horizon is also known as the subsoil.” It adds that “B Horizons are often greatly composed of material illuviated (washed in from) layers above it, mostly clay, iron, aluminum oxides (deposited by elluviated water), and minerals that formed in the layer.” The Purdue manual gives a simple working definition: “For example, a B horizon is a subsoil horizon in which something has accumulated.” It gives the clay example: “A Bt horizon is one in which clay has accumulated.”
C horizon and R horizon
The C horizon is the layer just above bedrock. The Buffalo reference describes it as “The C Horizon, also known as the substratum is unconsolidated material above bedrock.” It warns that “Excavating this horizon generally requires specialized equipment, and roots are usually unable to take advantage of what cracks may be in this layer.” The Purdue manual describes the same layer as “the C horizon, often called the substratum,” and notes that “In most soils, the C horizon is like the material from which the overlying horizons formed.”
The R horizon is the bedrock itself. The Buffalo reference states that “This layer is simply bedrock with minimal to no weathering visible. It is composed of the parent material that would eventually be transformed into soil.”
How soil horizons form
The Purdue manual lists the processes in one place. It states that “Soil horizons form through several processes:” and names “organic matter accumulates in surface horizons” and “clay moves down” among them. It then concludes that “These processes work together to form soil horizons.” The manual also observes that “This leaching has contributed to the development of horizons.”
The Buffalo reference puts the process into a broader framework. It describes soil as “a function of climate, organisms, relief/slope, parent material, time, and any other potential factors.” It then explains each part. “Climate affects the rates of both physical and chemical weathering, Organisms affect the rate of soil formation and contribute organic matter to it, Relief affects the amount of water and erosion in a soil, Parent Material affects the initial properties of developing and mature soils, and time is required for these factors to go into effect and eventually form a soil and its Soil Horizons.”
Time is part of that list, and the Purdue manual shows that young deposits can lack distinct horizons. It says that on “deposits, such as alluvial material laid down on flood plains, recently deposited sand dunes and steep eroded hillslopes, genetic horizons may not be distinguished at all.” As soil formation proceeds, the same processes can build those horizons over time.
Transitional horizons and lowercase letters
Real soils do not always follow the textbook sequence. The Buffalo reference states that “Soil Horizons do not always form distinct bands with unique and easily identified properties.” It explains that “Often Soil Horizons form Transitional Horizons.” When one layer has dominant properties of one horizon and some properties of another, “these Transitional Layers are designated by putting the dominant horizon properties letter first, followed by the subordinate horizon; an example would be a BC horizon, with properties more like a B Horizon but still demonstrating sufficient similarities to a C Horizon.”
The Buffalo reference also describes lowercase letters added to the master letters. It says that “In order to more accurately describe the characteristics of the master horizons, lowercase letters from the Latin Alphabet are added.” The Purdue manual gives an example profile: “the horizons in a profile, from the top down, may be Ap, E, 2Bt, 2C.” It explains the example this way: “The upper two horizons formed in one kind of parent material, such as loess, and the lower two formed in a different parent material.”
Where the term solum fits
Soil scientists also group the upper layers into a single unit. The Purdue manual states that “Together, A, E, and B horizons are called the solum.” It adds that “If a soil lacks B and E horizons, as in recent alluvial materials, the A horizon alone is the solum.” The C horizon sits below the solum, and the manual notes that it “may be the weathered rock material immediately below the solum, or it may be material that was moved by ice, water, or wind.”
The Buffalo reference describes the A horizon as “a well-weathered and fertile layer,” so the solum’s upper part is the layer the sources describe as most biologically active. For a common garden problem that involves the subsoil, see how to improve clay soil over time.
Frequently asked questions
What is the difference between topsoil and a soil horizon?
In the Buffalo reference, topsoil and the A horizon are the same layer. It states that “The A Horizon is considered topsoil.”
Is subsoil the same as the B horizon?
In the Buffalo reference, yes. It states that “The B Horizon is also known as the subsoil.” The Purdue manual uses the same idea when it describes the B horizon as a layer where something has accumulated.
Why is the A horizon darker than the layers below it?
The Purdue manual gives the reason: “Accumulation of organic matter in surface A horizons makes them dark.”
Do all soils have every horizon?
No. The Buffalo reference says that “the most well-developed soils might have all of these layers, and the least-developed soils might only have an A and a D horizon.”
Do soil horizons always have sharp boundaries?
No. The Buffalo reference states that “Soil Horizons do not always form distinct bands with unique and easily identified properties.”
What we could not verify
The two sources explain the horizon system, but they do not settle every practical question. We could not verify numeric depth ranges for each horizon from the fetched text, so this article gives none. We also could not verify how long a given horizon takes to form, since the sources describe time as a factor without giving a timescale.
What the sources do not say
The sources do not give a field key for identifying horizons in a home garden, and they do not say how to measure horizon depth with simple tools. They also do not give a rule for deciding when a layer counts as a new horizon rather than a transitional one. For those details, a soil test or local extension office is the next step, and a practical guide such as how to assess soil drainage may help with the water side of the question.
Sources
- Soil Horizons. University at Buffalo soil reference page, https://soil.evs.buffalo.edu/index.php/Soil_Horizons. HTTP 200. Opened 2026-10-27.
- Indiana Soils: Evaluation and Conservation Online Manual, Chapter I, Processes of Soil Formation, Purdue University Agronomy, https://www.agry.purdue.edu/soils_judging/new_manual/ch1-processes.html. HTTP 200. Opened 2026-10-27.
