Mollisols: Dark Grassland Soils and the Mollic Epipedon

What makes a soil a Mollisol

NRCS pairs the dark surface horizon, relatively high in organic matter, with chemistry and history: Mollisols are base rich throughout and therefore quite fertile, nearly all have a mollic epipedon, and most supported grass at some time, though many apparently were forested at times.^[3]

The University of Idaho boils the order down to two requirements: a mollic epipedon, a thick, dark surface horizon, and a base saturation of at least 50% throughout the subsoil.^[4]

The bases are chemical bases such as calcium and magnesium, which the Soil Science Society of America says Mollisols are rich in. It calls them prairie or grassland soils, often found where the climate has pronounced dry seasons; the name comes from the Latin mollis, soft, and they cover about 7% of the world’s glacier-free land.^[1]

The mollic epipedon, in the Keys’ terms

In plain words, a mollic epipedon is a dark, organic-rich, base-rich surface layer that is at least about 7 inches (18 cm) thick by default. The Keys to Soil Taxonomy set the details:^[5]

So the 10-inch (25 cm) minimum is the sandy case, not the usual one: unless one of the Keys’ special conditions applies, a surface horizon that passes the color, carbon and base-saturation tests needs 7 inches (18 cm).^[5]

Why Mollisols are dark

Plant roots darken them. The mollic epipedon results from the long-term addition of organic materials derived from plant roots, the University of Idaho explains.^[4]

The Soil Science Society of America adds that the addition comes every year, from the roots of prairie plants.^[1]

Dark doesn’t mean the most organic matter, though. In the survey the first mineral layer of Mollisols has a median 3.0% organic matter, number 5 of 11 orders, tied with Inceptisols. Alfisols, at 2.0%, and Ultisols, at 1.5%, run lower.^[2]

First mineral layer below any organic (O) layer; Histosols left out (their figure would describe a buried mineral layer).

The seven suborders NRCS describes

• Albolls have both an albic horizon and fluctuating ground water. They’re mostly on the Great Plains, in eastern Washington, and in Oregon and Idaho, mostly on gentle slopes and used as cropland.
• Aquolls are the wet Mollisols, most extensive in glaciated areas of the Midwest, mainly Iowa, Minnesota, North Dakota and Indiana. Most have been artificially drained and are cropland.
• Cryolls are the cool or cold, more or less freely drained Mollisols, mostly in the high mountains of the Western States, and are used mostly as rangeland.
• Rendolls are the Mollisols of humid regions formed in highly calcareous parent material such as limestone, chalk and drift made mainly of limestone, or on shell bars. They’re in Florida, on tropical islands and in a few mountain areas of the West.
• Udolls are the more or less freely drained Mollisols of humid climates, most extensive in Minnesota, Illinois, Iowa and North Dakota. At settlement they carried mostly tall grass prairie, and where slopes aren’t too steep nearly all are cropland.
• Ustolls are the more or less freely drained Mollisols of subhumid to semiarid climates, mostly on the western Great Plains, where drought is frequent and low moisture usually limits yields without irrigation.
• Xerolls are the more or less freely drained Mollisols of Mediterranean climates, in the Pacific Northwest, California, Idaho, Nevada and western Utah.^[3]

The survey data doesn’t record suborders, so the page gives no acres or shares for them.

The Mollisol profile in USDA’s survey

Mollisols have the thickest A horizon of any order, a median 9.8 inches (25 cm), against 6.3 inches (16 cm) for all US soils. That’s the A horizon’s own thickness, not the mollic rule, which the Keys set out above. A B horizon follows on 87.5% of described acres, starting at a median 9.8 inches (25 cm).^[2]

The most common sequences are A–B–C (191.8 million acres), A–B (83.7 million acres) and A–B–R (32.5 million acres). Where a parent material is recorded it’s most often alluvium (28.2%), loess (17.0%) and residuum (15.5%), and the first mineral layer is mostly loam (34.7%), silt loam (22.7%) and silty clay loam (12.7%).^[2]

Typical properties of the first mineral layer

Median, with the middle half of the acres in brackets.

The median pH is 7.0, neutral, and 68.7% of Mollisol acres are well drained, with 11.3% poorly or very poorly drained; the wet Mollisols have their own suborder, Aquolls.^[2][6][7]

Farming on Mollisols

Mollisols are used mainly as cropland. Grains and sorghum are grown in the drier regions, and corn and soybeans in the warmer, humid ones, NRCS notes, and the University of Idaho counts them among the most important and productive agricultural soils in the world.^[3][4]

Another 16.6% is prime if drained, protected or irrigated, and 14.9% is of statewide or local importance. On land capability, 62.8% of rated acres are in the cultivable Classes I–IV, 31.0% in Classes I–II.^[2][8][9]

Look up one address with the prime farmland checker.

Where Mollisols lead

Texas has the most Mollisol acres, 51.3 million, but as a share of a state’s own soil the order is strongest in North Dakota and Kansas, where it covers more than four-fifths. Mollisols are the leading order in 13 states:^[2]

Most widespread Mollisol series

Williams (North Dakota), Ector (Texas) and Harney (Kansas) cover the most ground; each links to its official USDA description.^[2][10]

Download the Mollisols data (CSV)

Mollisols compared with other orders

Against the forest orders the contrast is the topsoil: Alfisols have a median A horizon of 7.1 inches (18 cm) and Ultisols 5.9 inches (15 cm), against 9.8 inches (25 cm) here.^[2]

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.

Frequently asked questions

What is a Mollisol?

A soil with a dark surface horizon relatively high in organic matter, base rich and fertile; nearly all have a mollic epipedon, and most supported grass at some time.^[3]

What is a mollic epipedon?

The dark, base-rich surface horizon that nearly all Mollisols have: dark in color (in the main option, a color value of 3 or less moist), with at least 0.6% organic carbon and 50% base saturation, built up over time from plant roots.^[5][4]

How thick is a mollic epipedon?

At least about 7 inches (18 cm) by default. The Keys ask for about 10 inches (25 cm) only when the epipedon is loamy fine sand or coarser throughout, and 4 inches (10 cm) or 7 to 10 inches (18 to 25 cm) in some other cases.^[5]

Why are Mollisols dark?

Organic matter from plant roots, added year after year by prairie plants, darkens the surface horizon.^[4][1]

Where are Mollisols found in the US?

They’re the leading order in 13 states, from North Dakota (85.9% of its soil acres) to Washington (32.3%), and Texas has the most acres.^[2]

Are Mollisols good for farming?

Yes: they’re used mainly as cropland, and 109.5 million of their acres (23.2%) are prime farmland.^[3][2][8]

What are the suborders of Mollisols?

NRCS describes Albolls, Aquolls (wet), Cryolls (cold), Rendolls (on highly calcareous material), Udolls (humid), Ustolls (subhumid to semiarid) and Xerolls (Mediterranean climates).^[3]

What does ‘Mollisol’ mean?

It comes from the Latin mollis, soft.^[1]