Soil drainage classes, counted
Seven classes, from excessively to very poorly drained. In this atlas 62 of 90 series are well drained; the wet ones span 7 orders.
A drainage class says how quickly water leaves a soil and, as a result, how often and for how long the soil is wet. NRCS uses seven classes, from excessively drained to very poorly drained, and every official series description names one. Among the 90 series in this atlas, well drained is by far the most common class, with 62 series (69%); the wet end of the scale has 10 series and the dry end 7.
The seven classes
The classes are defined in the NRCS Soil Survey Manual. The short readings below are our own summaries, the same ones used on each series page. The counts and the share of mapped acres refer to the atlas series only.
| Drainage class | What it means | Series | Share of atlas acres | Largest examples |
|---|---|---|---|---|
| Excessively drained | Water leaves very quickly; the soil holds little for plants. | 4 | 10.7% | Valentine, Valent, and Lakeland |
| Somewhat excessively drained | Water leaves quickly; the soil tends to be droughty. | 3 | 3.7% | Clarksville, Troup, and Kalkaska |
| Well drained | Water drains away readily but not rapidly; the root zone is seldom wet. | 62 | 68.1% | Gilpin, Smithdale, and Cecil |
| Moderately well drained | The root zone stays wet only for short periods. | 11 | 6.8% | Loring, Sacul, and Crete |
| Somewhat poorly drained | The soil is wet close to the surface for long stretches. | 2 | 1.8% | Blount and Stuttgart |
| Poorly drained | The soil is wet at or near the surface for long periods. | 6 | 6.1% | Bibb, Guyton, and Pelham |
| Very poorly drained | Water sits at or near the surface for most of the growing season. | 2 | 2.8% | Sharkey and Bohicket |
One caution on reading these numbers. The atlas holds the state soils and the most extensive series in the country; it is not a random sample of the 24,791 U.S. series that have mapped extent. 37 of the 52 state soils are well drained, and so are 25 of the 38 atlas series that are not state soils. These counts describe the atlas, not every soil in the country.
The dry end: sand
All four excessively drained series in the atlas are Entisols, and all four of them are Psamments, soils that are sand throughout: Valentine, Valent, Lakeland, and Windsor. Valentine alone, the largest series in the country, covers 10.80 million acres, so this small class holds 10.7% of the atlas acreage. The three somewhat excessively drained series are Clarksville (Ultisols), Kalkaska (Spodosols), and Troup (Ultisols).
The wet end: many orders
The wet classes are more varied. The 10 series that are somewhat poorly, poorly or very poorly drained belong to 7 different orders: Alfisols (3), Entisols (2), Mollisols (1), Spodosols (1), Ultisols (1), Vertisols (1), and Gelisols (1). Wetness is a property that cuts across the order system, which is why Soil Taxonomy marks it lower down, in suborders such as Aquolls and Aquults and in subgroups such as Aquic and Aeric. The guide to series names shows how those prefixes line up with the drainage classes.
Only two atlas series are very poorly drained. Sharkey is a Vertisol of flood plains, mapped on 3.65 million acres. Bohicket is an Entisol of tidal marshes, mapped on 673,575 acres. Both are described with a very slow water-movement class.
Drainage by soil order
The table counts atlas series by order and by drainage group. A dot means no series.
| Order | Excessively or somewhat excessively | Well drained | Moderately well drained | Somewhat poorly to very poorly |
|---|---|---|---|---|
| Mollisols | · | 17 | 1 | 1 |
| Alfisols | · | 10 | 3 | 3 |
| Entisols | 4 | 3 | · | 2 |
| Ultisols | 2 | 15 | 3 | 1 |
| Inceptisols | · | 7 | · | · |
| Aridisols | · | 6 | · | · |
| Spodosols | 1 | 2 | 1 | 1 |
| Vertisols | · | · | 1 | 1 |
| Andisols | · | 1 | 2 | · |
| Gelisols | · | · | · | 1 |
| Oxisols | · | 1 | · | · |
Inceptisols, Aridisols, and Oxisols have only well-drained series in the atlas. Entisols cover the widest range, from Lakeland (excessively drained, uplands) to Bohicket (very poorly drained, tidal marshes).
Drainage is not the same as permeability
Drainage class describes the wetness of the soil as a whole, in its landscape. Permeability and saturated hydraulic conductivity (Ksat) describe how fast water moves through the soil material when it is saturated. The two often agree, but not always. Five atlas series are well drained yet have a slow or very slow permeability class: Bayamon, Casa Grande, Pullman, Scobey, and Seitz. In the other direction, 3 of the wet series are described with water movement of moderately high or faster: Drummer (moderately high), Myakka (rapid), and Tanana (moderately high). A high water table or a position low in the landscape can keep a soil wet even when water moves through it readily.
Drainage and described depth are only loosely linked in the atlas. The median described depth is 203 cm for the excessively drained group, 165 cm for well-drained series and 203 cm for the wet group. The guide to soil depth explains what described depth measures.
Where to see it
Each series page shows its drainage class on a seven-step scale in the “In the field” band, for example Sharkey or Valentine. In the finder you can pick a drainage class and add a state or an order to narrow it down. Drainage classes describe typical soils from the official descriptions, not conditions at a 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 (drainage class definitions)
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