Ultisols: Soil Characteristics, Profile, Crops and Lime

An ultisol is a strongly leached, acid soil with a clay-enriched subsoil and base saturation below 35%. Base saturation is the share of the soil’s nutrient-holding sites filled by calcium, magnesium, potassium and sodium. Ultisols cover about 8.1% of the world’s land outside ice sheets and glaciers and support 18% of its people, and they occupy about 9.2% of US land, according to the University of Idaho. This page gives the field test, the profile, the US picture and the lime sequence, with a calculator for your own soil report.

Ultisols Characteristics: The Diagnostic Traits

Ultisols are one of the 12 soil orders in USDA Soil Taxonomy. You will also see the singular “ultisol” and the misspelling “utisols”; all mean the same order. Two tests define the order, and the other traits below are common companions rather than the definition.

Chart

Ultisols among the larger soil orders

Share of global land outside ice sheets, % (selected orders)

Defining Ultisol Soil Characteristics

• Clay accumulation in the subsoil: an argillic or kandic horizon, which is a layer where clay has built up. This is the first of the two USDA criteria.
• Base saturation below 35% in the subsoil: this second criterion separates ultisols from alfisols, which have at least 35%.
• Strong acidity: the Cecil series, a common Piedmont ultisol, is very strongly acid to moderately acid unless limed. There is no pH limit in the definition, so a pH reading alone does not classify a soil.
• Low native fertility: much of the calcium, magnesium and potassium has been leached out.
• Red or yellow color: iron oxides give the strong colors of the “red clay” soils of the southeastern United States.
• Low-activity clays in many soils: the official Cecil description is “kaolinitic”, meaning kaolinite dominates the clay fraction. Kaolinite holds fewer nutrients than more active clays.
• Phosphorus tie-up at low pH: NC State Extension notes that aluminum below about pH 5.4 combines with fertilizer phosphorus and makes it less soluble.

What the name ultisol means, and its five suborders

The name comes from the Latin ultimus, meaning last, a reference to how far weathering has gone. Oxisols are the order that the University of Idaho describes as very highly weathered, with few weatherable minerals left.

USDA Soil Taxonomy splits ultisols into five suborders:

• Aquults: a water table at or near the surface for much of the year.
• Humults: well drained with high organic matter content.
• Udults: soils of humid climates, common in the US Southeast.
• Ustults: soils of semiarid and subhumid climates.
• Xerults: soils with very dry summers and moist winters.

The suborder sits at the end of a soil’s taxonomic name on Web Soil Survey. A Typic Hapludult is an Udult, and a Typic Kanhapludult is too. Knowing the suborder tells you early on whether wetness or a dry season is the larger constraint.

Is It an Ultisol? A Quick Field Test

Red soil alone does not confirm an ultisol, because oxisols and alfisols can also be red or yellow. A shovel can raise or lower your suspicion, but the deciding number comes from a lab.

1. Look it up first. Search your parcel on USDA Web Soil Survey. If the taxonomic name ends in “-ult”, the mapped soil is an ultisol.
2. Dig a pit. Look for a clear rise in clay from the surface into the subsoil. Cecil, for example, moves from sandy loam at the surface to clay in the subsoil.
3. Check for clay films. Shiny coatings on the faces of soil blocks support an argillic horizon, a layer where clay has moved in from above.
4. Send a subsoil sample to a lab. Ask for base saturation. Below 35% points to an ultisol; 35% or more points to an alfisol.

What to ask the soil lab for

A routine fertility test on the topsoil will not settle the classification, because the definition looks at the subsoil and because past lime changes the surface. Sample both layers separately and keep the bags labeled by depth.

• Topsoil sample: pH and nutrient levels, used for lime and fertilizer rates.
• Subsoil sample: clay content and base saturation, used to confirm the order.
• Field history: tell the lab about any lime applied within the past 12 months. Lime may not have fully reacted, and North Carolina’s method gives a residual credit for it.

Mapped soil units contain small inclusions of other soils, so a pit on your own ground can legitimately differ from the map. If the two disagree, trust the lab result for that spot and sample again elsewhere before changing plans for the whole field.

How Ultisols Form: Weathering, Regions & Process

Ultisols are intensely weathered soils of warm, humid climates, typically on older land surfaces and in parent material that is already well weathered. Four processes matter most.

1. Intense, Long-Term Weathering in Warm, Humid Regions

Large areas occur in the southeastern US, China, Indonesia, South America and equatorial Africa. Rain carries calcium, magnesium and potassium down through the profile, and warm temperatures speed the breakdown of primary minerals.

2. Parent Material and the Role of Time

Older, stable landscapes give weathering the time it needs. The international equivalent in many cases, the Acrisol, is most extensive on weathered acid rocks, according to the FAO’s World Reference Base, though it forms on many parent materials.

3. Clay Illuviation: Kaolinite Dominates

Illuviation is the downward movement of fine clay with percolating water. The clay collects in the subsoil, where it shapes rooting, drainage and nutrient retention. In many mature profiles, including Cecil, kaolinite is the leading clay mineral.

4. Iron and Aluminum Oxide Accumulation

Iron oxides stay behind as other minerals weather, and they produce the red and yellow colors. Dissolved aluminum becomes an agronomic problem at low pH, which is why lime sits at the center of management.

Ultisols Soil Profile and Horizon Dynamics

A typical ultisol profile has up to four principal horizons, each with a direct management consequence.

Principal Soil Horizons in Ultisols

• A horizon (surface): the zone with most of the organic matter. FAO stresses that keeping this layer in place is a precondition for farming Acrisols.
• E horizon (where present): a pale layer that has lost clay and organic matter. It does not appear in every profile.
• Bt horizon (the key diagnostic layer): clay-enriched, often blocky, with clay films and red or yellow colors. The “t” marks the clay accumulation.
• C horizon: weathered parent material moving toward bedrock.

The official Cecil description shows how this looks in a real pit: a plow layer of sandy loam, usually 5 to 20 cm thick, above a clayey subsoil. Eroded phases have a surface of sandy clay loam or clay loam.

Ultisols in the United States: Where and How Much

Ultisols occupy about 9.2% of US land and dominate much of the Southeast, according to the University of Idaho. The Coastal Plain has Aquults and Udults, and the Piedmont has Kanhapludults. Cecil itself is mapped in Alabama, Georgia, North Carolina, South Carolina and Virginia, with a large extent. Idaho’s examples also include Humults in western Oregon.

Chart

Share of world land versus US land

% of global land outside ice sheets and % of US land

The official description lists corn, tobacco, soybeans, cotton and small grains where Cecil is cultivated, and loblolly pine, Virginia pine and oaks where it is wooded. To benchmark a farm, compare Web Soil Survey’s map for your county with USDA NASS Quick Stats county yields for your crop.

Agriculture & Forestry on Ultisols

The University of Idaho sums up the position: ultisols are poorly suited to continuous agriculture without fertilizer and lime, but with those inputs they can be very productive. PASSEL, the University of Nebraska plant and soil lessons, adds that their clay gives them more nutrient retention than oxisols, and less than alfisols or mollisols.

Challenges and Opportunities

• Low native fertility: calcium, magnesium and potassium have been leached, so plan on regular nutrient additions.
• Acidity: low pH brings aluminum and, on most Piedmont and mountain soils, manganese toxicity that limits roots.
• Erosion: FAO names preserving the surface soil and preventing erosion as preconditions for farming Acrisols.
• Phosphorus tie-up: correct acidity first so that less fertilizer phosphorus is held by aluminum.

FAO lists pineapple, cashew, tea and rubber as acid-tolerant cash crops on Acrisols, and notes growing oil palm plantings in Malaysia and on Sumatra. It adds that low-input farming on these soils is not very rewarding.

Forestry on Ultisols

Favorable climates mean ultisols often support productive forests. FAO notes that most tree roots on Acrisols concentrate in the humus-rich surface horizon, with few reaching far into the subsoil. Land clearing therefore matters: FAO warns that mechanical clearing that pulls out root balls can leave largely sterile patches where subsoil aluminum reaches toxic levels.

Ultisols in Agroforestry

FAO recommends agroforestry as a soil-protecting alternative to shifting cultivation on Acrisols, aiming at higher yields without expensive inputs. It also says that rotating annual crops with improved pasture maintains organic matter.

Ultisols Management & Soil Fertility Enhancement

Four levers cover the advice in the sources above: lime, balanced fertilizer, organic matter and an integrated yearly plan. For more on the wider approach, see integrated soil fertility management and these soil quality restoration ideas.

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Target soil pH differs by crop

Target pH on North Carolina mineral soils

1. Liming: Correcting Soil Acidity

Lime neutralizes acidity and supplies calcium, and dolomitic lime also supplies magnesium. NC State Extension reports that its benefits include healthier roots, better use of fertilizer phosphorus, higher soil CEC (cation exchange capacity, the soil’s ability to hold nutrients) and better legume nodulation.

Targets depend on the crop. In North Carolina’s table for mineral soils, corn, soybeans and tobacco target pH 6.0, cotton 6.2, tomato 6.5 and blueberry 4.5. Do not lime to one fixed number across every field.

Lime moves little in the soil over one to three years, and neutralizes acidity mainly where it is placed, so spread it evenly and incorporate it where tillage allows. Its nutrients release over about three to four years.

2. Fertilization: Strategic, Balanced Inputs

• Base nitrogen, phosphorus and potassium rates on a current soil test.
• Soils with deep sandy surfaces are especially prone to potassium leaching, according to NC State, and proper liming reduces this without stopping it.
• Test for micronutrients rather than adding them by habit, and follow state Extension rates and local rules.

3. Soil Organic Matter: The Sustainability Multiplier

Crop residues, cover crops, compost and manure feed the thin layer that holds most organic matter. Legumes can help too: NC State notes that nitrogen fixation works better where pH is not too low, and may leave the next crop substantial residual nitrogen. Credit that nitrogen using your state’s Extension figure for the legume and stand you grew, because fixation varies.

4. Integrated Nutrient and Erosion Control

Combine lime, fertilizer, rotation and ground cover into a yearly plan rather than one-off corrections. Retest on the schedule your lab or Extension office suggests. Satellite crop monitoring, such as the kind explained in this piece on AI in agriculture, can show where a field is underperforming, but a soil test still identifies the cause.

▶ The real advantages and limits of treating a field by zone, shown on an actual field. · 2:39

Lime & Nitrogen Input Calculator

This tool uses the published North Carolina soil-test equation, so you need the “Ac” value (exchangeable acidity) from a lab report. Labs elsewhere use other methods: the University of Georgia, for instance, calculates lime need from lime buffer capacity. Use your own lab’s recommendation where it differs. The nitrogen line takes the credit figure you get from your Extension office.

A worked lime example

NC State’s example starts at pH 5.0, with an Ac value of 1.2 and no recent lime. For a crop with a target of pH 6.0, the equation gives 1.2 x (1.0 / 1.6), or about 0.8 ton per acre. For a more acid-tolerant crop with a target of pH 5.5, the same soil needs about 0.4 ton per acre. Halving the pH gap therefore halves the lime in this example, which is why choosing the crop target comes before ordering lime.

Rates assume standard agricultural lime. NC State notes that a material with a different neutralizing value needs an adjusted rate: in its example, 1.19 tons of a weaker product replaces each ton recommended on the soil test. Check the label’s neutralizing value before you compare prices per ton.

Erosion Control & Water Management: Keeping Ultisols Functional

Drainage varies by suborder: Cecil is well drained, while Aquults have a water table near the surface. What most ultisols share is a valuable surface layer that is thin compared with the clay beneath.

Why Erosion Is a Major Concern in Ultisols

Most organic matter sits near the surface, so losing topsoil removes a large share of fertility and exposes clayey subsoil at the plow layer. The Cecil description records eroded phases with clay loam surfaces, a sign of how far the surface has been lost.

• Terraces and contour farming: break up slopes and slow runoff.
• Agroforestry: gives lasting cover and litter, as FAO recommends.
• Mulch and reduced tillage: keep the surface covered during heavy rain.
• Grassed waterways and drainage channels: move excess water without cutting into the soil.

Mining, Exploration & Infrastructure on Ultisols: Challenges and Emerging Solutions

This page sits with our mining guides because deep weathering also concentrates some ores, and because mining and construction projects have to handle these soils.

Resource Exploration: The Role of Ultisols in Minerals

• Bauxite (aluminum ore): forms through leaching common in humid tropical regions. A USGS bulletin on the Southeastern bauxite deposits says surficial weathering produced them by that process. USGS reports limited bauxite and bauxitic clay production in Alabama, Arkansas and Georgia in 2025.
• Nickel laterite: a residual ore over ultramafic rock, catalogued in the USGS world laterite deposit database. These deposits are a matter of deep weathering and bedrock, not of the soil order on the map. See types of ore deposits for how they compare.
• Kaolin: USGS estimates about 4.8 million tons of US kaolin sold or used in 2025, mainly for fillers, extenders and binders. A kaolinitic soil does not itself indicate a commercial clay deposit.
• Gold in the same region: the Southeast’s red soils also overlie old gold districts, covered in our guide to Piedmont gold in Alabama. For other weathering-related commodities, see bauxite and iron ore exploration.
• Infrastructure: clayey subsoils call for a site-specific geotechnical assessment of bearing capacity, compaction and drainage before building.
• Reclamation: FAO’s warning about exposed subsoil aluminum applies to any disturbance. Store topsoil separately and replace it last, in line with your permit.

Scale matters in the nickel case. USGS estimated in 2026 that global nickel resources contain more than 350 million tons of nickel, with 54% in laterites. That is why tropical weathering profiles draw exploration interest, although value depends on grade, thickness, processing and permits. A practical first screen is to compare geological maps of bedrock with soil and land-cover maps, then sample the ground that fits.

Licence owners who want a satellite view ahead of field work can read about satellite-based mineral detection and send a project query or contact us. Results are exploration targets to test in the field, not a resource estimate.

▶ What remote sensing for mining measures, explained without jargon. · 2:01

Comparison Table: Ultisols vs. Other Soil Orders

This table uses the University of Idaho and Rangelands Gateway descriptions, so you can compare the test that matters and the extent of each order.

Climate Trends and Ultisol Management

Rain and warmth built these soils, and heavy rain remains the main short-term threat on bare ground, because it can move the surface layer that holds most organic matter. Rather than plan around any single season’s forecast, keep the basics in place: cover through the wettest months, soil-test based lime and fertilizer, and water control on slopes. Where dry seasons matter, as with Ustults, plan for water supply as well.

A simple yearly review keeps this practical. After the wettest period, walk the slopes for rills and sediment fans, compare field edges with the center, and note any patches of exposed subsoil color. Red or yellow streaks in a formerly darker surface can mean topsoil has moved. Mark those patches and sample them separately next time, since they may need a different lime rate from the rest of the field.

Frequently Asked Questions about Ultisols

What are the core ultisols characteristics?

A subsoil clay accumulation (argillic or kandic horizon) and base saturation below 35% are the defining tests. Most are also acid, low in native fertility and red or yellow from iron oxides.

What is the difference between “ultisol” and “ultisols”?

Ultisol is the singular and ultisols the plural. “Ultisoil” and “utisols” are common misspellings of the same soil order.

What makes ultisols highly weathered?

Long exposure to warm, humid conditions breaks down primary minerals, leaches calcium, magnesium and potassium, and moves clay into the subsoil.

Where are ultisols found?

Large areas occur in the southeastern US, China, Indonesia, South America and equatorial Africa, according to the University of Nebraska’s PASSEL lessons.

What is the ultisol soil profile?

Usually a thin A horizon, sometimes a pale E horizon, then a clay-enriched Bt horizon over weathered C material.

Which crops are best suited to ultisols?

With lime and fertilizer, many. The US Cecil series lists corn, tobacco, soybeans, cotton and small grains, and FAO names pineapple, cashew, tea, rubber and oil palm on Acrisols.

Why is phosphorus availability limited in ultisols?

At low pH, chemically active aluminum combines with fertilizer phosphorus and makes it less soluble. Liming to the crop’s target pH reduces this.

Are ultisols suitable for infrastructure development?

Often yes, with a geotechnical assessment. Clay-rich subsoils can affect compaction, drainage and bearing capacity, so test the full profile at the site.

How much lime do ultisols need?

It depends on the soil test and crop. In NC State’s worked example, a soil at pH 5.0 with an Ac value of 1.2 and a target of 6.0 needs about 0.8 ton per acre. Your own soil may differ widely.

Are ultisols the same as red clay or Acrisols?

Not exactly. Southeastern US “red clay” soils are examples of ultisols, but not all red soils qualify. FAO says many Acrisols correlate with ultisols that have low-activity clays.

Do ultisols indicate mineral deposits?

No. Deeply weathered ground can host residual bauxite and nickel laterite, but a soil order is not an ore indicator. Geology, sampling and drilling decide that.

Conclusion & Next Steps

Confirm the classification on Web Soil Survey and with a subsoil base saturation test. Then lime to the target for your crop, fertilize from the soil test, keep the surface covered and control water on slopes. Those steps give ultisols, which the University of Idaho calls poorly suited to continuous farming without inputs, their chance to be very productive.

▶ The environmental impacts of mining, including what happens to the land. · 2:31