The twelve soil orders, measured
Mollisols cover the most mapped ground, Oxisols the least. All twelve orders by area, number of series and typical profile depth.
Mollisols cover more mapped ground in the United States than any other soil order: 435.60 million acres, or 27.3% of the 1.60 billion acres that soil surveys assign to a named series. Alfisols come second with 17.3%. At the other end, Oxisols cover 0.03%, which is 490,795 acres. The twelve orders are the top level of U.S. Soil Taxonomy, and every series belongs to exactly one of them.
All twelve, ranked by mapped area
The figures below come from current SSURGO data: the acres of every map-unit component named for a series, summed by the series’ order. “Series mapped” counts the series with at least one mapped acre. The median described depth is taken from the official descriptions of the order’s established series.
| Order | Million acres | Share | Series mapped | Acres per series | Median described depth, cm | Largest series | In atlas |
|---|---|---|---|---|---|---|---|
| Mollisols | 435.6 | 27.30% | 7,593 | 57,368 | 152 | Williams | 19 |
| Alfisols | 276.5 | 17.33% | 4,001 | 69,100 | 156 | Amarillo | 16 |
| Entisols | 196.1 | 12.29% | 3,003 | 65,317 | 152 | Valentine | 9 |
| Ultisols | 189.1 | 11.85% | 1,278 | 147,989 | 178 | Gilpin | 21 |
| Inceptisols | 182.2 | 11.42% | 3,264 | 55,818 | 152 | Berks | 7 |
| Aridisols | 154.3 | 9.67% | 2,777 | 55,557 | 152 | Reagan | 6 |
| Spodosols | 59.2 | 3.71% | 717 | 82,569 | 152 | Tunbridge | 5 |
| Vertisols | 42.6 | 2.67% | 481 | 88,501 | 165 | Sharkey | 2 |
| Histosols | 30.6 | 1.92% | 358 | 85,472 | 155 | Cathro | 0 |
| Andisols | 20.5 | 1.28% | 1,093 | 18,725 | 152 | Lapine | 3 |
| Gelisols | 8.3 | 0.52% | 123 | 67,122 | 150 | Tatalina | 1 |
| Oxisols | 0.5 | 0.03% | 61 | 8,046 | 152 | Los Guineos | 1 |
The distribution is lopsided. Mollisols and Alfisols together account for 44.6% of the mapped series acres, and the four largest orders for 68.8%. The three smallest, Andisols, Gelisols, and Oxisols, add up to 1.83%.
Many series or big series
Area and number of series do not move together. Dividing each order’s acres by its number of mapped series gives a rough measure of how large a typical series in the order is. Ultisols come out on top at 147,989 acres per series, and Oxisols last at 8,046. Mollisols have the most series (7,593 mapped), while Ultisols cover 11.8% of the area with only 1,278 series. Three of the four largest series in the country are Ultisols: Gilpin, Smithdale, and Cecil.
The Soil Classification database lists 23,568 established series across the twelve orders. The “series mapped” column counts every series name in that database with mapped acres, tentative and inactive names included, so it is higher than the established count for eleven of the twelve orders.
What a typical profile looks like in each order
The medians of the official descriptions show how differently the orders are built. The median solum, meaning topsoil plus subsoil, is thickest in Vertisols and Oxisols at 152 cm and thinnest in Entisols at 69 cm. The first B horizon starts closest to the surface in Aridisols, at a median of 10 cm. Official descriptions go deepest in Ultisols, with a median of 178 cm, while seven of the twelve orders have a median of exactly 152 cm, which is 60 inches. For Histosols and Gelisols the solum medians are left out because fewer than 30 descriptions contribute; only 16 of the 327 Histosol descriptions have a solum to measure, since these organic soils seldom have a B horizon.
Each order page on this site draws that median profile to scale, for example Ultisols and Mollisols.
The orders in this atlas
The atlas profiles 90 series: every state soil and the largest series nationally. Because of that selection, its mix of orders is not the national mix. Ultisols are the most common order here with 21 series, though they rank 4th by area; Histosols have none, since no state soil is a Histosol and none of the largest series is one either. State soils are spread across 11 orders, led by Alfisols with 11. You can filter the atlas by order in the soil series finder, or read how order names are built in the guide on reading a soil series name.
One line per order
- Mollisols: soils with a thick, dark, organic-rich topsoil. 27.30% of mapped series acres; largest series Williams (4.23 million acres).
- Alfisols: moderately leached soils with a clay-enriched subsoil. 17.33% of mapped series acres; largest series Amarillo (3.04 million acres).
- Entisols: young soils with little or no horizon development. 12.29% of mapped series acres; largest series Valentine (10.80 million acres).
- Ultisols: strongly weathered, leached soils with a clay-enriched subsoil. 11.85% of mapped series acres; largest series Gilpin (5.84 million acres).
- Inceptisols: young soils with weakly developed horizons. 11.42% of mapped series acres; largest series Berks (3.24 million acres).
- Aridisols: soils of dry climates. 9.67% of mapped series acres; largest series Reagan (2.25 million acres).
- Spodosols: acid soils with a subsoil of humus and iron or aluminum. 3.71% of mapped series acres; largest series Tunbridge (1.94 million acres).
- Vertisols: clay soils that shrink and crack when dry. 2.67% of mapped series acres; largest series Sharkey (3.65 million acres).
- Histosols: organic soils: peat and muck. 1.92% of mapped series acres; largest series Cathro (1.88 million acres).
- Andisols: soils formed in volcanic ash and similar material. 1.28% of mapped series acres; largest series Lapine (689,156 acres).
- Gelisols: soils with permafrost. 0.52% of mapped series acres; largest series Tatalina (763,988 acres).
- Oxisols: very highly weathered soils of the tropics. 0.03% of mapped series acres; largest series Los Guineos (45,936 acres).
The one-line descriptions are short summaries of the order definitions in the Keys to Soil Taxonomy; the numbers are SSURGO mapped acres.
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 Survey Staff, Keys to Soil Taxonomy, USDA–NRCS (order 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