Ultisols: Deeply Weathered Soils of the Humid Southeast

What makes a soil an Ultisol

NRCS traces Ultisols to intense weathering and leaching that leave a clay-enriched subsoil. The central concept is a soil with a horizon holding an appreciable amount of translocated silicate clay, an argillic or kandic horizon, and few bases: a base saturation of less than 35%. In most Ultisols base saturation decreases with depth, and the clay horizon is commonly calcium deficient.^[3]

Translocated clay is clay that has moved within the profile and collected in the subsoil. The exact test for an argillic horizon, how much more clay than the layer above it must hold, is spelled out in the argillic horizon section of our soil horizons page.^[4]

The Soil Science Society of America adds the climate and the chemistry: Ultisols formed in humid areas, are relatively acidic, and hold most of their nutrients in the upper centimeters. The name comes from the Latin ultimus, last, and they cover about 8% of the world’s glacier-free land.^[1]

Ultisols vs Alfisols: the 35% line

Ultisols and Alfisols both have a clay-enriched subsoil, so the orders split on bases. An Ultisol’s base saturation is under 35%; at 35% or more the soil is an Alfisol, and that criterion is what distinguishes the two orders.^[3][5]

The Alfisols page covers the other side. In the survey, the first mineral layer of Ultisols has a median pH of 5.3, against 6.2 for Alfisols, yet Ultisols carry the larger prime-farmland share: 24.0% of their acres against 20.6%.^[2][6][7]

The five Ultisol suborders

• Aquults are the Ultisols of wet areas, where ground water is very close to the surface for part of each year. They’re on the coastal plains, particularly along the Atlantic Ocean and the Gulf of Mexico.
• Humults are the more or less freely drained, humus-rich Ultisols, in Oregon and Washington and also in California and Puerto Rico.
• Udults are the more or less freely drained, relatively humus-poor Ultisols with a udic moisture regime, in the southern and eastern parts of the country. Many have been cleared and are used as cropland, mostly with soil amendments.
• Ustults occur in the extreme western part of the Ultisol area, in Texas.
• Xerults are the more or less freely drained Ultisols of Mediterranean climates, in California and Oregon.^[3]

The survey data doesn’t record suborders, so there are no acres for them here.

The Ultisol profile in USDA’s survey

96.2% of described Ultisol acres have a B horizon, starting at a median 7.9 inches (20 cm), and an E horizon sits above it on 38.6%. The most common sequences are A–B–C (36.4 million acres), A–B (30.1 million acres), A–E–B (27.8 million acres) and A–E–B–C (17.5 million acres). The A horizon has a median thickness of 5.9 inches (15 cm), against 6.3 inches (16 cm) for all US soils.^[2]

Where a parent material is recorded it’s most often marine (40.3%), residuum (37.4%) and alluvium (10.2%), and the first mineral layer is mostly sandy loam (36.5%), silt loam (24.7%) and loamy sand (13.6%).^[2]

Typical properties of the first mineral layer

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

The acidity shows in the classes: 82.6% of Ultisol acres are strongly or very strongly acid at the first mineral layer, and median organic matter is 1.5%. Most acres drain freely: 82.9% are well or moderately well drained.^[2][6][9]

Farming on Ultisols

Most Ultisols carried mixed coniferous and hardwood forest at the time of settlement, and some are now cropland or pasture. Many Udults have been cleared for cropland, mostly with the use of soil amendments.^[3]

The Soil Science Society of America sums up the trade-off: Ultisols are generally of low fertility, but they can become productive with fertilizer and lime.^[1]

65.5% of rated Ultisol acres are in the cultivable capability Classes I–IV, and erosion (subclass e) is the main hazard on 66.4% of the rated acres.^[2][10]

For one field, the prime farmland checker gives the class, and the soil pH map the mapped pH.

Where Ultisols lead

Georgia has the most Ultisol acres, 25.9 million, then Alabama and North Carolina. Ultisols are the leading order in twelve states and the District of Columbia, and as a share of each state’s soil acres the top six are all above 60%:^[2]

The full list of states where Ultisols lead: Georgia, Delaware, Alabama, South Carolina, North Carolina, Virginia, Maryland, District of Columbia, Arkansas, Tennessee, New Jersey, Mississippi and West Virginia.^[2]

Most widespread Ultisol series

Gilpin (West Virginia), Smithdale (Mississippi) and Cecil (Georgia) lead the list; each links to its official USDA description.^[2][11]

Download the Ultisols data (CSV)

Ultisols compared with other orders

Ultisols are one of the twelve orders of USDA Soil Taxonomy and rank fifth by US acres; the table sets their topsoil, surface pH and prime-farmland share beside the other eleven.^[12][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 are Ultisols?

Soils formed by intense weathering and leaching, with an argillic or kandic horizon of translocated clay and a base saturation below 35%.^[3]

What is the difference between Ultisols and Alfisols?

Base saturation: below 35% makes an Ultisol, 35% or more an Alfisol. Both have a clay-enriched subsoil.^[3][5]

Is Ultisol a soil order?

Yes, one of the twelve orders in USDA Soil Taxonomy. By US acres it ranks fifth, at 9.6% of soil acres with an order.^[12][2]

Where are Ultisols found in the US?

They lead in twelve states and DC, and they make up 76.0% of Georgia’s soil acres, the highest share anywhere.^[2]

Are Ultisols good for farming?

With inputs. They’re generally low in fertility but can be made productive with fertilizer and lime, and 24.0% of their acres are prime farmland.^[1][2][7]

What are the suborders of Ultisols?

Aquults (wet), Humults (humus-rich), Udults (humus-poor, udic), Ustults (the western edge of the Ultisol area, in Texas) and Xerults (Mediterranean climates).^[3]

What does ‘Ultisol’ mean?

It comes from the Latin ultimus, last.^[1]