The 12 Soil Orders, Explained with USDA Data

Soil scientists sort every soil on Earth into 12 orders, from frozen Gelisols to cracking Vertisols.^[1][2] Below: what each order is, how a soil is classified down to its series, how the orders compare across the US, and which order is under your address.

What soil order is your soil?

US addresses only: street address, city and state, or ZIP.

USDA’s Soil Taxonomy sorts every soil into one of 12 orders, often by one dominant characteristic, then narrows down level by level to individual soils called series. In USDA’s survey, Mollisols cover the most US ground: about 23.6% of soil acres that have an order.

The 12 soil orders

Each order in a line, with how much of the US it covers in USDA’s survey and how much of the world’s ice-free land.^[2][3]

Order	What it is	US acres	US share	Worldwide
Alfisols	moderately weathered, fairly fertile soils, mostly under forest	298.4M	14.9%	10%
Andisols	soils formed in volcanic ash	32.5M	1.6%	1%
Aridisols	soils of dry climates	173.5M	8.7%	12%
Entisols	young soils with little development	251.3M	12.5%	16%
Gelisols	soils with permafrost	125.2M	6.3%	9%
Histosols	organic soils (peat and muck)	48.8M	2.4%	1%
Inceptisols	moderately developed soils	276.3M	13.8%	17%
Mollisols	dark, fertile grassland soils	472.0M	23.6%	7%
Oxisols	deeply weathered tropical soils	481k	0.0%	8%
Spodosols	acidic, sandy soils with an ashy gray layer	90.3M	4.5%	4%
Ultisols	intensely weathered, acidic soils of humid regions	191.3M	9.6%	8%
Vertisols	clay soils that shrink and swell	42.2M	2.1%	2%

US share is of the mapped US soil acres that have a recorded order (87.3% of them do); worldwide shares are of ice-free land, from the Soil Science Society of America.^[3][2]

The nine orders with their own pages, each drawn in one of its three most common horizon sequences in USDA’s survey, with the feature that defines it. Not to scale.^[3][2]

How Soil Taxonomy classifies a soil

Soil Taxonomy is a hierarchy. Its four highest categories, in order of decreasing rank and increasing numbers of classes, are told apart by the presence or absence, or combinations, of diagnostic horizons and characteristics.^[4] The names are built to be taken apart syllable by syllable from right to left, and doing so gives the order, suborder, great group, subgroup and family.^[5] USDA’s Keys to Soil Taxonomy provides the keys needed to classify a soil.^[6]

Order: the 12 on this page
Suborder
Great group
Subgroup
Family
Series: the individual named soil^[5][7]

Each level holds more classes than the one above it, from the 12 orders down to individual series; the green path follows one soil through all six levels.^[4][5]

The series is the lowest category of the national soil classification system, and its name is the common reference term used to name soil map units. NRCS keeps a national collection of more than 20,000 official series descriptions, which hold the soil properties that define each series.^[7]

That’s a different figure from the 27,654 distinct series names in USDA’s survey data itself, which our soil survey page tallies.^[3] The soil horizons page defines three diagnostic layers, the argillic horizon among them; texture classes, a separate system, are on the soil types page.

How the orders compare

Median surface pH runs from 4.8 in Spodosols to 7.9 in Aridisols. Prime farmland ranges from 32.2% of Vertisols acres down to 0.0% of Gelisols and Histosols, and the median topsoil (A horizon) from 9.8 inches (25 cm) in Mollisols to 3.5 inches (9 cm) in Aridisols.^[3][8][9]

Order	Median surface pH	Prime farmland	Median topsoil (A)
Alfisols	6.2	20.6%	7.1 in (18 cm)
Andisols	5.8	1.8%	8.3 in (21 cm)
Aridisols	7.9	0.1%	3.5 in (9 cm)
Entisols	7.4	3.0%	5.1 in (13 cm)
Gelisols	5.3	0.0%	3.9 in (10 cm)
Histosols	—	0.0%	—
Inceptisols	5.6	5.3%	5.1 in (13 cm)
Mollisols	7.0	23.2%	9.8 in (25 cm)
Oxisols	5.1	20.7%	5.9 in (15 cm)
Spodosols	4.8	2.2%	3.9 in (10 cm)
Ultisols	5.3	24.0%	5.9 in (15 cm)
Vertisols	7.3	32.2%	7.1 in (18 cm)

pH is the median surface pH of each order’s acres, prime farmland the share of its acres rated prime, and topsoil the median A-horizon thickness of its described profiles.

Histosols show a dash: their surface is organic material (peat or muck), so a topsoil (A) or surface pH median would describe mineral soil beneath it.

Which order is most common in each state

Each square is a state, colored by the order that covers the most of its acres.^[3]

Each state’s most common soil order by acres (USDA survey). Alaska and Hawaii are placed at the corners; some territories are omitted.

How much of the US each order covers

Acres of US soil USDA classifies in each soil order.

Download the data (CSV)

Three more orders, mapped but less common here

Andisols

Andisols form in volcanic ash and other material thrown out by volcanoes. Their minerals have a poorly ordered crystal structure that holds unusually large amounts of water and nutrients, which makes them very productive, though they erode easily on slopes. They're typical of cool areas with moderate to high rainfall. The name comes from the Japanese ando, black soil.^[2]

USDA maps about 32.5 million acres, 1.6% of ordered US soil, most of it in Alaska, Oregon and Washington. Worldwide, Andisols cover about 1% of ice-free land. The most common surface textures are silt loam, sandy loam and loam. Median surface pH is 5.8, and 1.8% of the acres are prime farmland.^[3][2]

Inceptisols

Inceptisols show a moderate amount of development but haven't built up much clay in their subsoil. Because they form on many parent materials and in many climates, their properties vary widely. The name comes from the Latin inceptum, beginning.^[2]

USDA maps about 276.3 million acres, 13.8% of ordered US soil, most of it in Alaska, Montana and Texas. Worldwide, Inceptisols cover about 17% of ice-free land. The most common surface textures are silt loam, loam and sandy loam. Median surface pH is 5.6, and 5.3% of the acres are prime farmland.^[3][2]

Oxisols

Oxisols are deeply weathered soils of tropical and subtropical regions, dominated by iron oxides, quartz and kaolinite clay. They usually sit on old, stable, gently sloping land and show few distinct layers. Their natural fertility is low, but lime and fertilizer can make them productive. The name comes from 'oxide'.^[2]

USDA maps about 0.5 million acres, 0.0% of ordered US soil, most of it in Hawaii. Worldwide, Oxisols cover about 8% of ice-free land. The most common surface textures are clay, silty clay and silty clay loam. Median surface pH is 5.1, and 20.7% of the acres are prime farmland.^[3][2]

Keys to Soil Taxonomy and its editions

The current Keys to Soil Taxonomy is the 13th edition, from 2022; the 12th edition dates to 2014. Besides the keys themselves, it tells users about recent changes to the classification system.^[5][6]

The book Soil Taxonomy is in its second edition, published in 1999 as Agriculture Handbook 436. That edition added two new orders, Andisols and Gelisols, and gives each of the twelve orders its own chapter. The original edition was published in 1975, and since then eight international committees have made proposals that were approved and incorporated.^[4] NRCS lists the same twelve orders today.^[10]

Frequently asked questions

How many soil orders are there?: Twelve: Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols and Vertisols.^[10]
What is soil taxonomy?: USDA's system for classifying soils. Its levels run order, suborder, great group, subgroup, family and series, and the four highest categories are told apart by diagnostic horizons and characteristics.^[4][5][7]
What are Inceptisols?: Moderately developed soils that haven't built up much clay in their subsoil. They form on many parent materials and in many climates, so their properties vary widely. USDA maps about 276.3 million acres of them in the US, 13.8% of soil acres with an order.^[2][3]
What is the most common soil order in the US?: Mollisols, about 23.6% of the US soil acres that have a recorded order, in our aggregation of USDA's survey.^[3]
Which soil order is most common worldwide?: Inceptisols, at about 17% of the world's ice-free land, followed by Entisols at about 16%.^[2]
How many soil series are there?: NRCS's national collection holds more than 20,000 official soil series descriptions. The series is the lowest category of the national soil classification system.^[7]
What is the latest edition of Keys to Soil Taxonomy?: The 13th edition, published in 2022. The 12th edition dates to 2014.^[5][6]
Which soil orders were added most recently?: Andisols and Gelisols. Soil Taxonomy's second edition, published in 1999, added them as two new orders, and its twelve orders are the twelve NRCS lists today.^[4][10]
What's the difference between soil orders and soil types?: A soil order is often defined by one dominant characteristic, such as the vegetation, the parent material, the climate or the amount of weathering. Texture classes sort soils by sand, silt and clay alone; the soil types page covers those twelve.^[1]
What soil order is my soil?: Enter your address above. USDA records an order for 87.3% of mapped US soil acres.^[3]

How we calculate this

Acres come from USDA SSURGO via Soil Data Access, weighted by each soil component's share of its map unit (USDA's representative percent) and summed across 3,386 detailed USDA soil surveys; shares are of the US acres that have a recorded order. Surface medians (pH, organic matter, water held, Ksat and bulk density) are of each component's first mineral layer below any organic (O) layer. Topsoil thickness, horizon sequences, farmland class and parent material come from a matching read of one profile per soil component. Data generated 2026-10-07.^[3]

More in our methodology.

Sources

Marko Visic · MPharm, BSc Physics · Last reviewed November 13, 2026