Alfisols: Fertile Forest Soils with a Clay-Rich Subsoil

What makes a soil an Alfisol

In NRCS’s central concept, an Alfisol has an argillic, kandic or natric horizon, a subsoil layer where clay has built up, and a base saturation of 35% or more. Alfisols typically have an ochric epipedon (the diagnostic surface horizon), though some have an umbric one, and some also carry a petrocalcic horizon, a fragipan or a duripan.^[3]

The University of Idaho’s soil-order guide describes them as moderately leached soils with relatively high native fertility, formed mainly under forest, with a subsurface horizon where clays have accumulated.^[1]

The bases in that figure are chemical bases such as calcium and magnesium.^[5] How much more clay the subsoil needs before it counts as argillic depends on how much the layer above it holds; the argillic horizon rule on our soil horizons page gives the three cases.^[6]

The name comes from “Pedalfer”, a soil-science term referring to aluminum and iron. The Soil Science Society of America sets Alfisols beside Ultisols: similar soils in similar climates, but less intensively weathered and less acidic, and they tend to be more fertile. Worldwide they cover about 10% of the glacier-free land.^[5]

The Alfisol profile in USDA’s survey

The clay-rich subsoil shows up in the survey’s profile descriptions: 97.2% of described Alfisol acres have a B horizon, and it starts at a median depth of 9.1 inches (23 cm), against 7.5 inches (19 cm) for all US soils with a B. An E horizon, a subsurface layer that has lost minerals (eluviation), sits above it on 36.3% of the acres.^[2][7]

The most common sequences are A–B–C (69.2 million acres), A–B (53.5 million acres), A–E–B–C (31.4 million acres) and A–E–B (29.5 million acres). The topsoil (A horizon) has a median thickness of 7.1 inches (18 cm), a little more than the 6.3 inches (16 cm) of all US soils.^[2]

Where a parent material is recorded, it’s most often loess (24.0%), alluvium (20.3%) and residuum (17.1%). The first mineral layer is mostly silt loam (43.0%), sandy loam (19.5%) or loam (17.0%).^[2]

Typical properties of the first mineral layer

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

A median pH of 6.2 is slightly acid, and organic matter runs at 2.0%, below the 3.0% of Mollisols.^[2][9]

Alfisols vs Ultisols and Mollisols

Alfisols and Ultisols both have a clay-enriched subsoil, so the deciding test is chemical. The 35% base saturation in the Alfisol definition is the criterion that separates Alfisols from Ultisols: at 35% or more the soil is an Alfisol.^[1]

The Ultisols page covers the other side of that line. In the survey, Alfisols’ median surface pH is 6.2 and Ultisols’ 5.3; on prime farmland the order is reversed, 24.0% of Ultisol acres against 20.6% of Alfisol acres.^[2][9][10]

Against Mollisols the difference is vegetation: the University of Nebraska–Lincoln’s PASSEL lessons sum up Alfisols as deciduous forest soils and Mollisols as soft grassland soils.^[4] In the survey the grassland order’s topsoil is thicker, 9.8 inches (25 cm) against 7.1 inches (18 cm) for Alfisols.^[2]

The five Alfisol suborders

Below the order, Soil Taxonomy splits Alfisols by soil moisture and temperature. NRCS describes five suborders:

• Aqualfs, the wet Alfisols, with warm, aquic conditions. They’re mainly in an area bordering the southern part of the Mississippi River, in southern Illinois, and in Indiana and Ohio. Most have some artificial drainage or other water control and are cropland for corn, soybeans and rice; nearly all are believed to have carried forest once.
• Cryalfs, the cold Alfisols, mostly at high elevations, mainly in the Rocky Mountains. Their short, cool growing season keeps most of them in forest.
• Udalfs, with a udic moisture regime, are of large extent: a belt from Minnesota through Wisconsin, Michigan, Indiana and Ohio to New York. All are believed to have supported forest at some time.
• Ustalfs, with an ustic regime, are on the southern Great Plains, mostly in Texas and Oklahoma. Many supported savanna, some grassland, and most are cropland or grazing land.
• Xeralfs, with a xeric regime, are mostly in California, with small areas in Oregon. Small grains, mostly winter wheat, are the most common crops.^[3]

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

Are Alfisols well drained?

Most are. 54.0% of Alfisol acres are well drained and another 20.7% moderately well drained. At the wet end, 15.2% are somewhat poorly drained and 8.7% poorly or very poorly drained.^[2][11] Wet Alfisols have their own suborder, Aqualfs, and most Aqualfs have some artificial drainage or other water control.^[3]

For one address, the soil drainage map shows the class USDA maps there.

Farming on Alfisols

A generally favorable climate and high native fertility make Alfisols very productive soils for both farming and forestry, in the University of Idaho’s words.^[1]

Another 11.1% is prime if drained, protected from flooding or irrigated, and 21.6% is farmland of statewide or local importance. On land capability, 71.4% of rated Alfisol acres fall in Classes I–IV, the classes suited to cultivation, and erosion (subclass e) is the main hazard on 67.7% of the rated acres.^[2][10][12]

Check an address with the prime farmland checker, or see the pattern nationally on the prime farmland map.

Where Alfisols are in the US

Texas has the most Alfisol acres, 41.5 million. Measured as a share of each state’s own soil acres, though, Ohio, Indiana and Missouri lead.^[2]

Alfisols are the most extensive order in Ohio, Indiana, Missouri, Kentucky, Wisconsin, Illinois, Louisiana and Michigan.^[2]

Most widespread Alfisol series

Amarillo (Texas), Blount (Ohio) and Fayette (Iowa) cover the most ground; each name links to its official USDA description.^[2][13]

Download the Alfisols data (CSV)

Alfisols compared with other orders

Of the 11 orders with a surface pH figure, Alfisols are number 5 from the most alkaline down. Their topsoil ranks 3 of 11 by thickness, tied with Vertisols, and their prime-farmland share ranks 5 of 12.^[2][9][10]

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 Alfisols?

Soils with an argillic, kandic or natric horizon, where clay has built up in the subsoil, and a base saturation of 35% or more. They’re moderately leached, fairly fertile and formed mainly under forest.^[3][1]

What is the difference between Alfisols and Ultisols?

Base saturation. Both have a clay-enriched subsoil; an Alfisol keeps a base saturation of 35% or more, and that criterion is what separates the two orders.^[1]

Where are Alfisols found in the US?

Texas has the most acres, 41.5 million. As a share of a state’s soil they peak in Ohio, at 64.6%, and they’re the leading order in eight states.^[2]

Are Alfisols good for farming?

Yes. 20.6% of Alfisol acres are prime farmland, against 12.5% of all US soil acres, and 71.4% of rated acres are in capability Classes I–IV.^[2][10][12]

What are the suborders of Alfisols?

Aqualfs (wet), Cryalfs (cold), Udalfs (udic moisture regime, a belt from Minnesota to New York), Ustalfs (ustic, the southern Great Plains) and Xeralfs (xeric, mostly California).^[3]

Are Alfisols poorly drained?

Mostly not: 54.0% of their acres are well drained. The wet suborder, Aqualfs, is the exception, and 8.7% of Alfisol acres are poorly or very poorly drained.^[2][11][3]

What does ‘Alfisol’ mean?

It comes from “Pedalfer”, a soil-science term referring to aluminum and iron.^[5]