How deep soil series go
Most atlas series are very deep (68 of 90), and the median description runs to 168 cm. Depth class and described depth measure different things.
Most soil series in this atlas are deep. Of the 90, 68 carry the depth class very deep, meaning no bedrock or root-limiting layer within 150 cm (60 in), and the median typical pedon is described to 168 cm. Two different numbers are in play, though. The depth class says where bedrock or another root-limiting layer begins. The described depth says how far down the soil scientist recorded the profile. They often differ, and this guide shows by how much.
Depth classes in the atlas
NRCS uses five depth classes. The limits come from the Soil Survey Manual; the counts and medians are for the atlas series.
| Depth class | Meaning | Series | Median described depth, cm | Largest examples |
|---|---|---|---|---|
| Very shallow | Bedrock or a root-limiting layer within 25 cm (10 in). | 3 | 76 | Ector, Tarrant, and Eckrant |
| Shallow | Bedrock or a root-limiting layer at 25–50 cm (10–20 in). | 4 | 152 | Cabbart, Cabba, and Weikert |
| Moderately deep | Bedrock or a root-limiting layer at 50–100 cm (20–40 in). | 7 | 152 | Berks, Tunbridge, and Marlow |
| Deep | Bedrock or a root-limiting layer at 100–150 cm (40–60 in). | 3 | 152 | Harney, Natchez, and Hilo |
| Very deep | No bedrock or root-limiting layer within 150 cm (60 in). | 68 | 183 | Valentine, Smithdale, and Cecil |
| Not stated | The description gives no depth class. | 5 | 165 | Gilpin, Loring, and Bohicket |
Five descriptions name no depth class at all (Bohicket, Gilpin, Loring, Monongahela, and Pamunkey), so they are listed separately and not counted in any class.
How far the descriptions go
The bands are uneven for a reason: 24 of the 90 descriptions stop at exactly 152 cm, which is 60 inches, a depth that many descriptions use as their lower limit. The deepest description in the atlas belongs to Houston Black, at 264 cm, followed by Crider (254 cm), Jory (254 cm), and Amarillo (251 cm). The shallowest is Weikert at 46 cm, where its last layer, hard rock (R), begins.
When the two numbers disagree
A shallow depth class does not mean a short description. Of the 7 series classed shallow or very shallow, 4 are described to 100 cm or more: Tanana (shallow, described to 183 cm), Cabba (shallow, described to 152 cm), Cabbart (shallow, described to 152 cm), and Ector (very shallow, described to 102 cm). Ector shows why. Its typical pedon has 4 layers: A at 0–10 cm, Bk at 10–20 cm, Rk/Bk at 20–38 cm, and R at 38–102 cm. The lower layers are mostly rock, so the description continues past the point where the soil itself ends.
Tanana, the Alaska state soil, is another case: it is classed shallow and described to 183 cm, and its lowest horizon carries the suffix f for frozen soil.
Five descriptions end with a layer that has no recorded lower depth, usually bedrock: Berks, Crider, Gilpin, Tunbridge, and Weikert. Our drawings fade those layers out instead of inventing a bottom.
The classification agrees with the depth classes where it should. Four atlas series are in a Lithic subgroup, the subgroup for soils with hard rock close to the surface: Ector (very shallow), Tarrant (very shallow), Weikert (shallow), and Eckrant (very shallow). All of them are shallow or very shallow. The other shallow series are not Lithic: Cabba (Typic Ustorthents), Cabbart (Aridic Ustorthents), and Tanana (Typic Aquiturbels).
Where the subsoil starts
In 82 of the 90 series the typical pedon has a B horizon, and the median depth to the first one is 20 cm. The range is wide: Casa Grande has its first B horizon at 3 cm and Troup at 112 cm. Eight series have no B horizon at all, and 6 of those are Entisols, the order of young soils with little horizon development.
Against the medians of their own orders, 41 of the 81 series with a B horizon and an order median have a thicker solum (topsoil plus subsoil) than the median, and 39 a thinner one. Series pages draw that comparison as a dumbbell chart under “Against other …” wherever the gap is 5 cm or more.
How many layers
A typical pedon in the atlas is split into a median of 6 horizons. Seven is the most frequent count, found in 25 series, and Menfro has the most, 10. More horizons do not mean a deeper soil; they mean the describer found more changes worth recording.
You can sort the finder by described depth, filter by depth class, and draw two profiles at the same scale, for instance Houston Black against Weikert. The rankings page lines up the fifteen deepest descriptions side by side. Depth classes here describe typical soils from the official descriptions, not any particular site.
Sources
- USDA-NRCS Official Soil Series Descriptions (OSD), soilseries.sc.egov.usda.gov
- USDA-NRCS Soil Classification (SC) database, soilseries.sc.egov.usda.gov
- USDA-NRCS Soil Survey Geographic Database (SSURGO) via Soil Data Access, sdmdataaccess.sc.egov.usda.gov
- State soil list: NCSS aqp R package dataset us.state.soils, github.com/ncss-tech/aqp/blob/master/data/us.state.soils.rda
- Soil Science Division Staff, Soil Survey Manual, USDA Handbook 18 (depth class limits)
Data as of: Official Series Descriptions and Soil Classification database snapshot 2026-10-26; SSURGO tabular data as served by Soil Data Access on 2026-10-27 (survey area save dates 8/23/2026 7:48:35 PM to 10/26/2026 10:17:40 PM). Counts, ranks, medians and charts on this page were computed by Soil Series Atlas from those files. Data & method