The 12 Soil Orders

What is a “Soil Order”?

A Soil Order is the highest and broadest classification level in the USDA soil taxonomy, grouping soils by dominant soil‑forming processes and properties.

How many Soil Orders are there in the USDA system?

There are 12 Soil Orders in the USDA soil taxonomy.

Which soil order is associated with permafrost (Polar climates & tundra)?

Gelisols

What causes Gelisols to develop the way they do?

They form due to permanently frozen subsoil (permafrost), which limits drainage, rooting, and biological activity.

What does the Gelisol soil profile look like?

A thin or weak A horizon over frozen material, with little horizon development.

Which soil order is associated with Subarctic climates & Boreal Forests?

Spodosols

What does the Spodosol soil profile look like?

O/A: organic‑rich; E: strongly leached (light); B: accumulation of iron, aluminum, organic matter (dark).

Why do Spodosols develop in Boreal Forests?

Acidic needle litter, cold temperatures, and high precipitation promote strong eluviation and illuviation.

Which soil order is associated with Humid Continental climates (deciduous forests)?

Alfisols

What does the Alfisol soil profile look like?

Well‑developed A, E, and clay‑rich B horizon, with relatively high base saturation.

Why are Alfisols mostly north of the Ohio River?

Because of cooler climate and younger soils affected by glaciation, which limits leaching.

Which soil order is associated with Humid Subtropical climates?

Ultisols

What does the Ultisol soil profile look like?

Strongly developed A, E, and thick B horizon, but with low base saturation.

Why are Ultisols mostly south of the Ohio River?

Warmer, wetter conditions and longer soil development time cause intense leaching.

Which soil‑forming factor controls Alfisol vs Ultisol patterns?

Climate (Cl)

Which soil order is associated with tropical rainforests?

Oxisols

What does the Oxisol soil profile look like?

Very thick, deeply weathered soil with weak horizon contrast and iron‑rich material throughout.

What is the dominant color in Oxisols?

Red or reddish‑yellow, due to iron oxides.

Are Oxisols fertile?

No, not without significant added inputs.

Why are Oxisols not naturally fertile?

Nutrients are rapidly leached, and most fertility is tied up in vegetation, not soil.

Which soil order is associated with semi‑arid grasslands?

Mollisols

What does the Mollisol soil profile look like?

Very thick, dark, organic‑rich A horizon, with a less developed B.

Which horizon is most pronounced in Mollisols?

The A horizon

Why isn’t there much eluviation in Mollisols?

Limited rainfall and high biocycling by grasses keep nutrients near the surface.

How does A‑horizon depth change east → west in N. American grasslands?

The A horizon becomes thinner westward as rainfall decreases.

What else may accumulate in the B horizon of soils in dry areas?

Calcium carbonate (caliche) or salts

Which soil order is associated with deserts?

Aridisols

Which soil orders are defined as relatively young?

Entisols and Inceptisols

Which is youngest and second youngest?

Youngest: Entisols; Second youngest: Inceptisols

Which soil order is associated with poorly drained environments?

Histosols

Which soil order forms from volcanic ash?

Andisols

Which soil order has shrink‑swell clays?

Vertisols

How can soils inform us about past climates and vegetation?

Soil properties preserve evidence of past temperature, precipitation, drainage, and dominant vegetation, acting as a record of paleoenvironments.