Spodosols: Acidic Soils with a Spodic Horizon

What makes a soil a Spodosol

NRCS describes the process first: Spodosols formed from weathering that strips organic matter combined with aluminum, with or without iron, from the surface layer and deposits it in the subsoil. The central concept is a soil where amorphous mixtures of organic matter and aluminum, with or without iron, have accumulated. Most Spodosols have little silicate clay.^[2]

The University of Idaho calls them acid soils with a subsurface accumulation of humus complexed with aluminum and iron. They typically form in coarse-textured parent material, often under coniferous forest in cool, moist climates, and the name comes from the Greek spodos, wood ash.^[3]

Worldwide they cover about 4% of the glacier-free land surface.^[1]

The gray E horizon and the spodic B

The Soil Science Society of America describes the look: often a dark surface over an ashy gray layer, over a reddish, rusty, coffee-colored or black subsoil. Rain passing through acidic litter, such as conifer needles, forms organic acids that dissolve iron, aluminum and organic matter in the topsoil and the ashy gray (eluvial) horizon, and the dissolved material moves down (illuviates) into the colorful subsoil.^[1]

In horizon terms, the ashy gray layer is an E horizon, light-colored over a reddish-brown spodic horizon, and the process that forms them is called podzolization. NRCS describes the overlying eluvial horizon of undisturbed Spodosols as generally gray to light gray, and the Keys’ albic horizon is an eluvial horizon about 0.4 inch (1 cm) or more thick.^[3][2][4]

The spodic horizon is a B horizon, and its name carries a suffix: h marks an illuvial accumulation of amorphous, dispersible complexes of organic matter and sesquioxides, and s the same when both the organic matter and the sesquioxides are significant, as in Bh, Bs or Bhs.^[5]

A B horizon is in 94.4% of the profiles, starting at a median 5.5 inches (14 cm), and the most common sequences are O–E–B–C (26.5 million acres), O–A–E–B–C (10.1 million acres), A–E–B–C (7.4 million acres) and O–A–B–C (7.3 million acres). An O horizon is present in 75.4%. Our soil horizons page compares the E horizon across all orders.^[6]

Aquods and the other suborders

• Aquods are the wet Spodosols, with a shallow, fluctuating water table. They’re in Florida and along the Atlantic coast. Most are forest or wildlife habitat, and some, mostly in Florida and New Jersey, have been cleared for cropland or pasture. They’re naturally infertile but can be highly responsive to good management.
• Cryods are the cold Spodosols of high latitudes or high elevations, mostly under coniferous forest. Some are in the high mountains of New York and northern New England, and most are forest or wildlife habitat.
• Humods are relatively freely drained Spodosols with a large accumulation of organic carbon. Most formed under coniferous forest and are used mainly as forest.
• Orthods are relatively freely drained Spodosols with a moderate accumulation of organic carbon, most extensive in the Northeast and the Great Lakes States. Most are forest, or were cleared for cropland or pasture.^[2]

In the drainage data, 22.6% of Spodosol acres are somewhat poorly to very poorly drained and 63.7% are well or moderately well drained. The survey doesn’t record suborders, so those acres aren’t split by suborder here.^[6][7]

What Spodosols are like

Spodosols are acid above all: the first mineral layer has a median pH of 4.8, the lowest of any order, and 73.3% of Spodosol acres are very strongly acid or more acid. Median organic matter is 5.0%.^[6][8]

The first mineral layer is most often silt loam (43.9%), sandy loam (23.9%) and sand (18.9%); sand, loamy sand and sandy loam together make up 49.4%. Where a parent material is recorded it’s most often till (27.6%), eolian (wind-blown) sand (22.5%) and glaciofluvial outwash (12.2%).^[6]

Farming on Spodosols: lime, with a warning

Spodosols are naturally infertile. The University of Idaho says they need additions of lime to be productive for farming, and NRCS says they can be highly responsive to good management, but with a warning: the spodic horizon can be destroyed under cultivation, particularly if lime and nitrogen are applied.^[3][2]

Only 2.2% of Spodosol acres are prime farmland, against 12.5% of all US soil acres; 12.1% are farmland of statewide or local importance, and 32.8% of rated acres are in the cultivable capability Classes I–IV.^[6][9][10]

Our guide to raising or lowering soil pH covers soil tests and lime, and the soil pH map shows the pH USDA maps at an address.

Where Spodosols are in the US

Alaska has the most Spodosol acres, 36.6 million, then Maine and Michigan; Alaska alone holds 40.6% of the order. As a share of a state’s own soil the order peaks in Maine, New Hampshire and Vermont, and it leads in four states: Maine, New Hampshire, Vermont and Florida.^[6]

Most widespread Spodosol series

Tunbridge (New York), Peru (Maine) and Lyman (Maine) cover the most ground; Myakka and Leon are the Florida entries. Each name links to its official USDA description.^[6][11]

Download the Spodosols data (CSV)

Spodosols compared with other orders

No other order in the table has a lower median surface pH, and among the twelve only four (Andisols, Aridisols, Gelisols and Histosols) have a smaller prime-farmland share.^[6][8][9]

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 Spodosols?

Acid soils in which organic matter and aluminum, with or without iron, have been stripped from the surface layer and deposited in the subsoil, leaving a light-colored E horizon over a reddish-brown spodic horizon.^[2][3]

What is a spodic horizon?

The subsoil layer where amorphous organic matter and aluminum, with or without iron, have accumulated: reddish-brown, and named as a B horizon with the suffix h or s (Bh, Bs, Bhs).^[2][3][5]

What are Aquods?

The wet Spodosols, with a shallow, fluctuating water table, found in Florida and along the Atlantic coast. Most are forest or wildlife habitat; some in Florida and New Jersey are cropland or pasture.^[2]

Why is the E horizon gray?

Organic acids from acidic litter dissolve the iron, aluminum and organic matter in it, and the dissolved material moves down into the colorful subsoil, leaving an ashy gray layer behind.^[1]

What is podzolization?

The process that forms a Spodosol’s light-colored E horizon over its reddish-brown spodic horizon.^[3]

Are Spodosols good for farming?

Not naturally: they’re infertile and need lime to be productive, but liming and nitrogen under cultivation can destroy the spodic horizon.^[3][2]

Where are Spodosols found in the US?

Alaska has the most acres, and they lead in Maine, New Hampshire, Vermont and Florida, reaching 67.9% of Maine’s soil acres.^[6]

What does ‘Spodosol’ mean?

It comes from the Greek spodos, wood ash.^[1]