Soil Horizons: The Layers of Soil, Explained
Dig a hole or look at a road cut and you’ll see soil in layers. Those layers are soil horizons, and soil scientists label them with capital letters.[1] Below: what each letter means, how deep the layers run across USDA’s survey, which ones hold back water and roots, and what’s under your own address.
This page is about horizons in the ground, not the line where the land meets the sky.
See the layers under an address
US addresses only: street address, city and state, or ZIP.
The master horizons: O, A, E, B, C and R
Each capital letter names a master horizon. The table gives the six our survey figures count, with the share of described US soil acres whose profile has each one.[2]
| Horizon | What it is | Share of described acres |
|---|---|---|
| O | Organic layer of leaves and other plant debris, on the surface in most soils that have one; common under forests and in wetlands, and it can also be buried.[1][3][4] O horizons are dominated by organic soil material: 12% or more organic carbon by weight, about 20% organic matter.[5][6] | 28.4% |
| A | Surface soil, or topsoil: the mineral layer at the surface, or just below an O.[1][3] | 87.7% |
| E | A subsurface layer that has lost minerals (eluviation); often light-colored and leached.[1][4] | 15.5% |
| B | Subsoil: where material such as clay has collected (illuviation), or where color and structure have developed.[1][3] A noticeable clay increase gets the suffix t, as in Bt; see also the argillic horizon. | 73.9% |
| C | Substratum: material with very little horizon development.[1][3] | 65.7% |
| R | Hard bedrock. It gets a letter, but bedrock isn’t soil.[1][3] | 13.3% |
How many soil horizons are there?
NRCS’s simple model gives most soils three major horizons: the surface horizon (A), the subsoil (B) and the substratum (C). The Soil Survey Manual uses ten capital letters for master horizons and layers: O, L, V, A, E, B, C, R, M and W.[1][6]
The manual defines the four not in the table like this:
- L, limnic layers: organic or mineral material deposited in water by precipitation or by aquatic organisms such as algae and diatoms, or derived from underwater and floating plants and then modified by aquatic animals. The manual describes them only for Histosols.
- V: mineral horizons formed at the surface or below a layer of rock fragments, dominated by vesicular pores, with platy, prismatic or columnar structure.
- M: root-limiting layers below the surface made of nearly continuous, horizontally oriented, human-made material such as geotextile liners, asphalt, concrete, rubber and plastic.
- W: water layers within or beneath the soil, zones of water between soil layers rather than soil that is merely saturated.[6]
What is a soil profile?
A soil profile is the arrangement of a soil’s horizons, from the surface down. Horizons differ in properties you can see, such as color, texture, structure and thickness, and in others that need lab tests, such as chemical and mineral content and reaction (pH). Soil scientists describe profiles to classify a soil and interpret it for different uses.[1]
Not every soil has every horizon, and the horizons a soil has, and their properties, are what soil scientists use to describe and name it. The University of Georgia’s tip for reading a profile in the field: find the B horizon first; the C lies below it and the A above, with or without an E.[4]
In USDA’s survey the most common arrangement is A over B over C, on 25.7% of described acres, and 41.7% of described acres have an A, a B and a C somewhere in the profile.[2]
The solum
The solum is the set of horizons that formed together, related through the same period of soil formation. The Soil Survey Manual lists what it consists of: O, V, A, E and B horizons and their transitional horizons. A buried soil or layer isn’t part of it unless soil-forming processes active today have given it some of its properties.[6]
How thick is topsoil?
In USDA’s survey, the A horizon has a median thickness of 6.3 in (16 cm); half of described soils fall between 3.9 and 9.8 inches (10 and 25 cm).[2]
The thickest typical topsoil is in Iowa (15.0 in); the thinnest is a three-way tie at 3.1 in: Alaska, Arizona and West Virginia.[2]
Thickest topsoil
| Iowa | 15.0 in (38 cm) |
| Nebraska | 11.8 in (30 cm) |
| Delaware | 9.8 in (25 cm) |
| Kansas | 9.8 in (25 cm) |
| Illinois | 9.1 in (23 cm) |
| Indiana | 9.1 in (23 cm) |
| Maryland | 9.1 in (23 cm) |
| Minnesota | 9.1 in (23 cm) |
| Idaho | 7.9 in (20 cm) |
| Michigan | 7.9 in (20 cm) |
Thinnest topsoil
| West Virginia | 3.1 in (8 cm) |
| Arizona | 3.1 in (8 cm) |
| Alaska | 3.1 in (8 cm) |
| Wyoming | 3.9 in (10 cm) |
| Nevada | 3.9 in (10 cm) |
| New Mexico | 3.9 in (10 cm) |
| New Hampshire | 3.9 in (10 cm) |
| Vermont | 4.7 in (12 cm) |
| Maine | 4.7 in (12 cm) |
| California | 4.7 in (12 cm) |
All states (n ≥ 500)
| State | Median A | Components |
|---|---|---|
| Iowa | 15.0 in (38 cm) | 24,509 |
| Nebraska | 11.8 in (30 cm) | 20,592 |
| Delaware | 9.8 in (25 cm) | 1,245 |
| Kansas | 9.8 in (25 cm) | 20,491 |
| Illinois | 9.1 in (23 cm) | 12,038 |
| Indiana | 9.1 in (23 cm) | 15,106 |
| Maryland | 9.1 in (23 cm) | 6,871 |
| Minnesota | 9.1 in (23 cm) | 32,942 |
| Idaho | 7.9 in (20 cm) | 24,871 |
| Michigan | 7.9 in (20 cm) | 18,942 |
| North Dakota | 7.9 in (20 cm) | 57,847 |
| New Jersey | 7.9 in (20 cm) | 6,760 |
| New York | 7.9 in (20 cm) | 10,301 |
| Ohio | 7.9 in (20 cm) | 13,572 |
| Oklahoma | 7.9 in (20 cm) | 12,028 |
| Texas | 7.9 in (20 cm) | 13,189 |
| Oregon | 7.5 in (19 cm) | 23,549 |
| Georgia | 7.1 in (18 cm) | 3,431 |
| Hawaii | 7.1 in (18 cm) | 1,362 |
| Missouri | 7.1 in (18 cm) | 26,224 |
| North Carolina | 7.1 in (18 cm) | 14,250 |
| Pennsylvania | 7.1 in (18 cm) | 13,986 |
| South Carolina | 7.1 in (18 cm) | 4,823 |
| South Dakota | 7.1 in (18 cm) | 25,302 |
| Wisconsin | 7.1 in (18 cm) | 21,480 |
| Alabama | 5.9 in (15 cm) | 4,428 |
| Colorado | 5.9 in (15 cm) | 11,773 |
| Florida | 5.9 in (15 cm) | 16,253 |
| Kentucky | 5.9 in (15 cm) | 7,013 |
| Louisiana | 5.9 in (15 cm) | 3,035 |
| Rhode Island | 5.9 in (15 cm) | 690 |
| Tennessee | 5.9 in (15 cm) | 7,039 |
| Virginia | 5.9 in (15 cm) | 1,805 |
| Washington | 5.9 in (15 cm) | 7,688 |
| Arkansas | 5.1 in (13 cm) | 5,763 |
| Connecticut | 5.1 in (13 cm) | 3,062 |
| Massachusetts | 5.1 in (13 cm) | 4,959 |
| Mississippi | 5.1 in (13 cm) | 5,171 |
| Montana | 5.1 in (13 cm) | 59,114 |
| Utah | 5.1 in (13 cm) | 9,723 |
| California | 4.7 in (12 cm) | 24,469 |
| Maine | 4.7 in (12 cm) | 2,746 |
| Vermont | 4.7 in (12 cm) | 2,067 |
| New Hampshire | 3.9 in (10 cm) | 2,443 |
| New Mexico | 3.9 in (10 cm) | 7,178 |
| Nevada | 3.9 in (10 cm) | 9,513 |
| Wyoming | 3.9 in (10 cm) | 25,269 |
| Alaska | 3.1 in (8 cm) | 12,319 |
| Arizona | 3.1 in (8 cm) | 4,592 |
| West Virginia | 3.1 in (8 cm) | 3,361 |
Topsoil thickness by soil order
The middle half shows how much the A horizon varies within an order: a wide range means the order spans thin and thick topsoils alike.
| Soil order | Median A | Middle half |
|---|---|---|
| Mollisols | 9.8 in (25 cm) | 6.7–14.2 in (17–36 cm) |
| Andisols | 8.3 in (21 cm) | 3.9–18.5 in (10–47 cm) |
| Alfisols | 7.1 in (18 cm) | 5.1–9.1 in (13–23 cm) |
| Vertisols | 7.1 in (18 cm) | 5.1–11.8 in (13–30 cm) |
| Ultisols | 5.9 in (15 cm) | 3.9–7.9 in (10–20 cm) |
| Oxisols | 5.9 in (15 cm) | 3.9–11.0 in (10–28 cm) |
| Inceptisols | 5.1 in (13 cm) | 2.4–7.9 in (6–20 cm) |
| Entisols | 5.1 in (13 cm) | 3.1–9.1 in (8–23 cm) |
| Spodosols | 3.9 in (10 cm) | 2.0–5.9 in (5–15 cm) |
| Gelisols | 3.9 in (10 cm) | 2.8–6.3 in (7–16 cm) |
| Aridisols | 3.5 in (9 cm) | 2.0–5.9 in (5–15 cm) |
| Histosols | — | — |
Histosols show a dash: their surface is organic material (peat or muck), so a topsoil (A) or surface pH median would describe mineral soil beneath it.
Adding topsoil? The topsoil calculator works out how much you need.
Which soils have an O or an E horizon
E horizons are on 80.6% of described Spodosols acres, then 38.6% of Ultisols and 36.3% of Alfisols; every other order is at 7.9% or less.[2] That fits the Spodosols’ ashy gray layer, which forms as organic acids carry iron, aluminum and organic matter down out of it.[7]
O horizons cover 100.0% of described Gelisols acres and 99.9% of Histosols, the two highest shares, and 0.0% of Aridisols, the lowest.[2]
| Soil order | Described acres | With an O | With an E |
|---|---|---|---|
| Mollisols | 389.1M | 5.4% | 2.8% |
| Alfisols | 238.6M | 8.2% | 36.3% |
| Inceptisols | 234.0M | 52.8% | 7.9% |
| Entisols | 200.5M | 12.3% | 4.8% |
| Ultisols | 144.4M | 10.8% | 38.6% |
| Gelisols | 124.9M | 100.0% | 0.0% |
| Aridisols | 110.8M | 0.0% | 2.5% |
| Spodosols | 82.6M | 75.4% | 80.6% |
| Histosols | 48.2M | 99.9% | 0.2% |
| Vertisols | 33.2M | 0.2% | 0.6% |
| Andisols | 27.1M | 89.5% | 5.7% |
| Oxisols | 0.3M | 21.3% | 0.0% |
Orders sorted by described acres, largest first.
The most common horizon sequences
A-B-C is the most common, on 25.7% of described acres. Of the ten most common sequences, four include an O (18.5% of described acres together) and two include an E (7.9%).[2]
| Sequence | Share of described acres | O | E |
|---|---|---|---|
| A-B-C | 25.7% | No | No |
| A-B | 15.7% | No | No |
| A-C | 8.1% | No | No |
| O-A-B-C | 6.9% | Yes | No |
| O-A-C | 6.8% | Yes | No |
| A-B-R | 4.8% | No | No |
| A-E-B-C | 4.1% | No | Yes |
| A-E-B | 3.8% | No | Yes |
| O-C | 2.7% | Yes | No |
| O | 2.1% | Yes | No |
Depth to subsoil and bedrock
Where a profile has a B horizon (73.9% of described acres), the subsoil starts at a median depth of 7.5 in (19 cm); half of them start between 5.1 and 11.0 inches (13 and 28 cm). Hard bedrock (R) turns up within the described profile on 13.3% of described acres; where it does, the median depth to it is 24.0 in (61 cm), and a tenth of those profiles reach it within 10.6 in (27 cm).[2]
| Percentile | Depth to the first B | Depth to hard bedrock |
|---|---|---|
| 10th | 3.1 in (8 cm) | 10.6 in (27 cm) |
| 25th | 5.1 in (13 cm) | 15.0 in (38 cm) |
| Median | 7.5 in (19 cm) | 24.0 in (61 cm) |
| 75th | 11.0 in (28 cm) | 35.8 in (91 cm) |
| 90th | 16.9 in (43 cm) | 52.0 in (132 cm) |
Each column covers only the profiles that have that layer.
USDA also records the material each soil formed in, its parent material: alluvium leads at 24.0% of US soil acres, then residuum (15.1%) and eolian (wind-blown) deposits (11.0%).[2] Soils that formed in alluvium, sediment laid down by running water, are alluvial soils.[8] The alluvial soil page lists every parent material.
What the small letters mean
Lowercase letters after a master horizon describe it further, and a horizon can carry more than one, as in Btg or Btx.[3]
| Suffix | What it means |
|---|---|
| Oi | slightly decomposed organic material; plant pieces still recognizable |
| Oe | moderately decomposed; plant pieces barely recognizable |
| Oa | highly decomposed; very small fibers, greasy feel |
| Ap | altered by human activity; usually called the plow layer |
| Bt | a noticeable increase in clay (see the argillic horizon) |
| Bw | weak color or structure development |
| Bg | mostly gray from saturation with water |
| Bx | fragipan characteristics (see fragipan) |
| Cr | weathered or soft bedrock that machinery can dig |
Transitional layers such as AB have properties of both; the first letter is the one they resemble more.[9]
Restrictive and diagnostic layers
Some layers hold back water and roots; others decide how a soil is classified. The Keys to Soil Taxonomy defines all three below, and Soil Taxonomy tells its higher categories apart by diagnostic horizons and characteristics; the soil orders page explains how a soil is placed.[5][10]
Fragipan
A fragipan is a subsurface horizon that restricts the entry of water and roots into the soil. It is firm to extremely firm and strongly brittle when it breaks, and one of its required characteristics is that air-dry fragments of the natural soil, about 2 to 4 inches (5 to 10 cm) across, from more than half of the layer slake (crumble). In horizon names the suffix x marks a firm, brittle layer that is root-restrictive in at least part of it, as in Bx.[5][6]
Duripan
A duripan is a subsurface horizon cemented by silica, with or without other cementing agents. In horizon names the suffix m marks continuous or nearly continuous cementation; the Soil Survey Manual uses it only for horizons more than 90 percent cemented, though they may be fractured, and qm means cementation by silica.[5][6]
The argillic horizon
An argillic horizon is an illuvial horizon that holds more total clay than the eluvial horizon above it, within a vertical distance of about 12 inches (30 cm) or less. It must be at least about 3 inches (7.5 cm) thick, or at least a tenth as thick as all the horizons above it combined; if it meets the sandy or sandy-skeletal particle-size criteria, at least about 6 inches (15 cm). How much more clay it needs depends on the clay in the eluvial horizon:[5]
| Clay in the eluvial horizon | The argillic horizon needs |
|---|---|
| Less than 15% | At least 3 percentage points more clay |
| 15% to 40% | At least 1.2 times as much clay |
| 40% or more | At least 8 percentage points more clay |
Planning a septic field or a basement? For an address, the septic suitability checker and the building site checker show USDA’s ratings and the soil features behind them.
Make an edible soil profile
Georgia 4-H’s classroom activity builds a soil profile in a clear cup. You need clear cups (one per child), spoons (one each, plus extras for serving), paper towels, small sticky tabs and fine-point markers. The sheet lists four layers and their foods:[11]
- Parent material: a whole sandwich cookie
- Subsoil: chocolate chips and butterscotch chips, mixed
- Topsoil: pudding
- Organic matter: shredded coconut dyed green, and gummy worms
The sheet defines bedrock but gives it no food, and it has no E layer.[11]
Lined up with the table above, the coconut layer stands in for the O horizon, the pudding for the A (topsoil) and the chips for the B (subsoil). The cookie keeps the sheet’s name, parent material. A soil jar test makes layers too, but they’re sand, silt and clay settling out of water, a measure of texture rather than horizons.[12]
Frequently asked questions
- What are the soil horizons in order?
- From the top down: O, A, E, B, C and R. Not every soil has all six; the most common sequence in USDA's survey is A over B over C, on 25.7% of described acres.[4][2]
- What is subsoil?
- Subsoil is the B horizon, where material such as clay has collected (illuviation) or where color and structure have developed. Where a profile has one, it starts at a median depth of 7.5 in (19 cm).[1][3][2]
- What is the O horizon?
- An O horizon is a layer dominated by organic soil material, which USDA's Keys to Soil Taxonomy defines as 12% or more organic carbon by weight, about 20% organic matter. In USDA's survey, 28.4% of described acres have one.[5][6][2]
- What is the A horizon?
- The A horizon is the surface soil, or topsoil: the mineral layer at the surface, or just below an O. Its median thickness in USDA's survey is 6.3 in (16 cm).[1][3][2]
- What is the E horizon?
- A subsurface layer that has lost minerals (eluviation); it's often light-colored and leached. It shows up on 15.5% of described US soil acres, most often in Spodosols (80.6%).[1][4][2]
- What is the C horizon?
- The substratum: the layer below the subsoil, with very little horizon development.[3][4]
- What is a soil profile?
- The arrangement of a soil's horizons: the layers you see from the surface down in a soil pit or a road cut.[1]
- What are the five major soil horizons?
- USDA's own counts are three and ten, not five. NRCS describes three major horizons for most soils (A, B and C), and the Soil Survey Manual lists ten master symbols: O, L, V, A, E, B, C, R, M and W.[1][6]
- What are the six horizons of soil?
- O, A, E, B, C and R: the organic layer, topsoil, the leached layer, subsoil, substratum and bedrock. They're the six letters our survey figures report; the Soil Survey Manual adds four more, L, V, M and W.[4][6]
- Why is topsoil called a horizon?
- Because it's one of the soil's layers, and soil scientists call each layer a horizon. Topsoil is the A horizon, the mineral layer at the surface.[1][3]
- Is bedrock a soil horizon?
- Hard bedrock gets the letter R in a profile, but it isn't soil.[1]
- What is a fragipan?
- A firm to extremely firm, brittle subsurface horizon that restricts the entry of water and roots into the soil. Horizon names mark it with the suffix x, as in Bx.[5][6]
- What is the solum?
- The set of horizons that formed together, related through the same period of soil formation. The Soil Survey Manual lists O, V, A, E and B horizons and their transitional horizons.[6]
How we calculate this
We read one profile per soil component from 3,386 detailed USDA soil surveys and weight everything by each component’s share of its map unit. A profile counts as having a master horizon when any of its layers carries that letter first; the shares cover O, A, E, B, C and R.[2]
Described acres exclude 344.1 million acres in older surveys that list layers without horizon letters (H1, H2…), and all miscellaneous areas. Described acres total 1,633.8 million. Depths and thicknesses are acre-weighted percentiles over the profiles that have the layer. Data generated 2026-10-08.[2]
Sources
- USDA NRCS, A Soil Profile
- USDA NRCS, Soil Survey Geographic Database (SSURGO), via Soil Data Access
- West Virginia Conservation Agency, Common Soil Horizon and Soil Horizon Suffixes
- University of Georgia, Soils at UGA, Soil Profiles
- USDA NRCS, Keys to Soil Taxonomy, 13th ed. (Soil Survey Staff; 2022)
- USDA NRCS, Soil Survey Manual (Agriculture Handbook 18), Ch. 3 “Examination and Description of Soil Profiles” (Soil Science Division Staff; eds. Ditzler, Scheffe, Monger; 2017)
- Soil Science Society of America (soils4teachers), The Soil Orders Simplified (D. Lindbo)
- USDA National Agricultural Library Thesaurus, alluvium
- Rhode Island DEM, Soil Evaluation Guidance
- USDA NRCS, Soil Taxonomy, 2nd ed. (Agriculture Handbook 436; Soil Survey Staff; 1999)
- Georgia 4-H, Edible Soil Profile (activity sheet)
- Colorado State University Extension, Estimating Soil Texture (GardenNotes #214)
Marko Visic · MPharm, BSc Physics · Last reviewed November 13, 2026