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USDA Soil Taxonomy

How soils are classified in the United States

Every soil report on this site uses data from the USDA's SSURGO database, which classifies soils using a system called Soil Taxonomy. Developed by the USDA Natural Resources Conservation Service over several decades and first published in 1975, it's the standard system used across the United States for naming and organizing soils. Understanding this system helps you make sense of what the soil at your property actually is — and what it means for building, gardening, septic, and land use.

The Six Levels of Classification

Soil Taxonomy organizes soils into a hierarchy from broad to specific — like biological taxonomy goes from kingdom to species. Each level adds more detail about the soil's properties and behavior.

1

Order

The broadest category. There are 12 soil orders, defined by the dominant soil-forming process. The order tells you the most fundamental thing about a soil — whether it's young or ancient, wet or dry, organic or mineral.

Example: Mollisols — grassland soils with thick, dark, fertile topsoil
2

Suborder

Subdivides orders by moisture and temperature conditions. This level tells you about the climate the soil formed in — wet, dry, cold, or warm.

Example: Ustolls — Mollisols in semi-arid climates with dry summers (ust = dry)
3

Great Group

Further divides suborders by diagnostic horizons and features — the distinct layers visible in a soil profile. This is where specific physical characteristics come in.

Example: Haplustolls — typical Ustolls with simple (haplo) profile development
4

Subgroup

Identifies the soil's position within its great group — whether it's a "typical" example or has properties that grade toward another group.

Example: Fluventic Haplustolls — Haplustolls on floodplains with stratified deposits
5

Family

Groups soils by physical and chemical properties that matter for practical use: particle size (sandy, silty, clayey), mineralogy, temperature regime, and more.

Example: Fine-silty, mixed, active, thermic — silty texture, mixed minerals, warm climate
6

Series

The most specific level — over 20,000 named soil series in the US. Each series is a specific soil with defined characteristics, named after the place it was first described.

Example: Teller series — a specific well-drained Mollisol found in Oklahoma

Reading a Taxonomic Class Name

When you see a classification like the one below on a soil report, here's how to decode it:

Fine-silty, mixed, superactive, thermic Fluventic Haplustolls
Fine-silty
Particle size — the soil has a silty texture with moderate clay content. Good water-holding capacity.
Mixed
Mineralogy — no single mineral dominates. Common in soils with a blend of clay types.
Superactive
Cation exchange activity — the clay is highly reactive, meaning it holds nutrients well for plants.
Thermic
Temperature regime — mean annual soil temperature is 15–22°C (59–72°F). Typical of the southern US.
Fluventic Haplustolls
Subgroup — a Mollisol (fertile grassland soil) on a floodplain, with simple profile development and a semi-arid moisture regime.

Common Temperature Regimes

Frigid
Cold — below 8°C (46°F). Northern states, high elevations.
Mesic
Moderate — 8–15°C (46–59°F). Mid-Atlantic, Midwest, Pacific Northwest.
Thermic
Warm — 15–22°C (59–72°F). Southeast, southern Plains.
Hyperthermic
Hot — above 22°C (72°F). Southern Florida, Hawaii, tropical areas.

Common Moisture Prefixes

Aqu- (aquic)
Wet — saturated long enough for reducing conditions. Swamps, bogs, poorly drained lowlands.
Ud- (udic)
Humid — moisture available most of the year. Eastern US, Pacific Northwest.
Ust- (ustic)
Semi-arid — limited moisture, dry for extended periods. Great Plains, parts of the Southwest.
Xer- (xeric)
Mediterranean — dry summers, wet winters. California, Oregon.

The 12 Soil Orders

Every soil in the US belongs to one of these 12 orders. The order is the most important thing to know about your soil — it tells you fundamentally what kind of ground you're dealing with.

Alfisols

~13% of US
From alf — pedalfer (aluminum/iron-rich)

Moderately weathered forest soils with a clay-enriched subsoil (argillic horizon). Formed under deciduous forests with enough moisture to leach clay downward but not strip the soil of nutrients.

Where in the US
Widespread across the eastern US, Upper Midwest, and Pacific Northwest. Common in Ohio, Indiana, Michigan, Wisconsin, Minnesota, and the mid-Atlantic.
Practical Implications
Good agricultural soils — among the most productive in the world. Generally good for building and septic. The clay subsoil can slow drainage in some areas, so check hydrologic group ratings.

Andisols

~2% of US
From and — ando (Japanese: dark soil)

Soils formed in volcanic ash and cinders. Extremely light, porous, and high in organic matter. They hold enormous amounts of water and nutrients despite their low density.

Where in the US
Pacific Northwest (Washington, Oregon), Hawaii, Alaska, and parts of the Northern Rockies. Anywhere with volcanic parent material.
Practical Implications
Excellent for agriculture — naturally fertile and well-drained. Can be tricky for foundations due to low density and high compressibility. Septic systems generally work well due to good permeability.

Aridisols

~12% of US
From id — aridus (Latin: dry)

Soils of dry climates. Low organic matter, often with accumulations of calcium carbonate (caliche), gypsum, or salts at depth. Limited soil development because there isn't enough water to drive weathering.

Where in the US
Dominant across the western US — Nevada, Utah, Arizona, New Mexico, and parts of California, Wyoming, Colorado, and west Texas.
Practical Implications
Poor for agriculture without irrigation. Caliche layers can make digging extremely difficult — relevant for foundations, basements, and septic systems. Salinity can be a problem for gardening. Low shrink-swell potential is actually good for foundations.

Entisols

~16% of US
From ent — recent (no Latin root)

The youngest soils — little or no profile development. Found on recent deposits (floodplains, sand dunes, steep slopes) where erosion or deposition outpaces soil formation.

Where in the US
Everywhere — along rivers, on beaches, in mountains, on recent volcanic flows. Especially common in the Great Plains (sandy soils), river valleys, and steep terrain.
Practical Implications
Highly variable. Floodplain Entisols can be very fertile but flood-prone. Sandy Entisols drain fast (poor for septic, tricky for gardens). Steep-slope Entisols have erosion problems. Always check the specific series.

Gelisols

~9% of US
From el — gelare (Latin: freeze)

Soils with permafrost within 100 cm of the surface, or with permafrost within 200 cm and evidence of cryoturbation (frost churning). Organic matter decomposes very slowly in frozen conditions.

Where in the US
Alaska, almost exclusively. Some in the highest alpine areas of the lower 48. About 9% of all US land (mostly Alaska).
Practical Implications
Extremely challenging for all construction. Building on permafrost requires special engineering — conventional foundations can melt the permafrost and sink. Septic systems require engineered alternatives. Climate change is making these soils increasingly unstable.

Histosols

~1.5% of US
From ist — histos (Greek: tissue)

Organic soils — at least 40 cm of organic material (peat, muck, or bog). Formed in wetlands where waterlogged conditions prevent decomposition of plant material.

Where in the US
Great Lakes region, Everglades, coastal plains of the Carolinas, and scattered bogs across New England and the Pacific Northwest. Anywhere with persistent wetland conditions.
Practical Implications
Very poor for building — extremely compressible and unstable. Septic systems essentially don't work. Can be excellent for certain crops (blueberries, cranberries, specialty vegetables) if drained. Often in regulated wetlands.

Inceptisols

~10% of US
From ept — inceptum (Latin: beginning)

Young but developing soils — more profile development than Entisols but less than mature soils. They have a weak subsurface horizon forming but haven't had time (or the right conditions) for strong development.

Where in the US
Mountains of the West, Appalachians, Pacific Northwest, and river terraces. Common across a wide range of climates and landscapes.
Practical Implications
Highly variable — depends entirely on the specific setting. Mountain Inceptisols may have shallow bedrock (bad for building, septic). Lowland versions can be quite productive for farming. Check drainage class and bedrock depth.

Mollisols

~22% of US
From oll — mollis (Latin: soft)

The great grassland soils. Defined by a thick, dark, organic-rich topsoil (mollic epipedon) at least 25 cm thick. Formed under prairie grasses whose deep roots build organic matter year after year.

Where in the US
The Great Plains — from Texas to the Dakotas, east into Iowa, Illinois, Indiana. Also in intermountain valleys of the West. The breadbasket of America.
Practical Implications
The best agricultural soils on Earth. Generally excellent for building — stable, well-drained, good bearing capacity. Septic systems usually work well. Gardens thrive in the naturally fertile topsoil. If your soil is a Mollisol, you're in good shape.

Oxisols

~<0.1% of continental US
From ox — oxide (iron/aluminum oxides)

The most weathered soils on Earth. Millions of years of tropical weathering have stripped away almost everything except iron and aluminum oxides. Deep, well-drained, but nutrient-poor.

Where in the US
Very rare in the continental US. Found mainly in Hawaii and Puerto Rico.
Practical Implications
Naturally infertile despite appearing lush (tropical vegetation recycles nutrients rapidly). Need heavy fertilization for agriculture. Good structure for building. Excellent drainage means septic usually works, but nutrient leaching is a concern.

Spodosols

~4% of US
From od — spodos (Greek: wood ash)

Acidic forest soils with a distinctive bleached (E) horizon over a dark, reddish-brown accumulation of iron, aluminum, and organic matter. Formed under coniferous forests in cool, humid climates.

Where in the US
New England, the Adirondacks, Upper Michigan, northern Wisconsin and Minnesota, the Pacific Northwest, and Florida (yes — the sandy Florida Spodosols form differently but are classified here).
Practical Implications
Naturally acidic and infertile — need heavy liming for most crops. Blueberries, potatoes, and Christmas trees love them. The compacted spodic horizon can cause perched water tables, creating drainage problems for septic and basements. Florida Spodosols often have high water tables.

Ultisols

~10% of US
From ult — ultimus (Latin: last)

Strongly weathered soils with a clay-enriched subsoil, like Alfisols but more leached and less fertile. Formed in warm, humid climates over long time periods.

Where in the US
Dominant across the Southeast — Virginia, Carolinas, Georgia, Alabama, Mississippi, and east Texas. Also common in the southern Appalachians.
Practical Implications
Acidic and naturally low in nutrients — need lime and fertilizer for productive farming. The clay subsoil can be a barrier for septic effluent and cause slow drainage. Shrink-swell potential varies. Old red clay that stains everything is often an Ultisol.

Vertisols

~2% of US
From ert — verto (Latin: turn)

Clay-rich soils that shrink and swell dramatically with moisture changes. Deep cracks form in dry weather; the soil swells shut when wet. This constant movement churns the soil (self-mulching).

Where in the US
Texas (especially the Blackland Prairie and Gulf Coast), parts of Alabama, Mississippi, California's Central Valley, and scattered areas across the southern Plains.
Practical Implications
Excellent natural fertility but a nightmare for construction. The shrink-swell action can crack foundations, buckle roads, and break pipes. Septic systems struggle in the dense clay. Gardening can be productive but the soil is extremely sticky when wet and rock-hard when dry. Foundation engineering is critical.

How This Connects to Your Soil Report

When you look up an address on SoilLookup.com, the data comes from the USDA's SSURGO (Soil Survey Geographic) database — the same data that powers the Web Soil Survey. Every soil component in SSURGO carries its full taxonomic classification.

On your soil report, you'll see the taxonomic class under each soil series in the Soil Series Details section. The soil order (the first level) is highlighted because it tells you the most about what to expect — but the full classification name contains information about particle size, mineralogy, temperature, moisture, and more.

The interpretations we show — drainage, septic suitability, building ratings — are derived from measured soil properties, not directly from taxonomy. But taxonomy and interpretations are closely related: a Mollisol will almost always have better drainage and building ratings than a Histosol, because the underlying properties that define each order also determine how the soil behaves.

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