Histosols: Organic Soils of Peat, Mucky Peat and Muck

What makes a soil a Histosol

NRCS gives a general rule: a soil is classified as a Histosol if half or more of the upper 31 inches (80 cm) is organic. Histosols are high in organic matter, have no permafrost, and go by common names such as bogs, moors, peats and mucks. Most are saturated year round, though a few are freely drained.^[2]

What counts as organic is set by carbon: soil material is organic at 12% or more organic carbon by weight, the threshold our soil horizons page gives with the O horizon.^[3]

The same rule settles the forest-floor question. A thin forest-floor O horizon over mineral soil doesn’t make a Histosol: it falls far short of half the upper 31 inches (80 cm).^[2]

Histosols form where organic matter such as leaves, mosses or grasses decomposes more slowly than it accumulates, most often in extremely wet places or under water. The name comes from the Greek histos, tissue, and they cover about 1% of the world’s glacier-free land.^[1]

Peat, mucky peat and muck

The Soil Survey Manual grades organic soil material by how far it has decomposed:^[4]

• Peat is slightly decomposed organic material with a high content of original fibers.
• Mucky peat is intermediate between muck and peat in decomposition, fiber content, bulk density and water content.
• Muck is well decomposed, with a low content of fibers (plant tissue, not counting live roots).^[4]

These materials count as Soil Taxonomy’s diagnostic sapric, fibric and hemic soil materials only in organic soils: the Histosols and the Histels suborder of Gelisols.^[4]

The four suborders NRCS describes

• Folists are the more or less freely drained Histosols, made mainly of horizons from leaf litter, twigs and branches resting on bedrock or on fragmental material. The largest extent is in Hawaii and Alaska.
• Fibrists are the wet, slightly decomposed Histosols, most extensive in southern Alaska.
• Hemists are wet Histosols whose organic material is moderately decomposed, most extensive in Minnesota and Alaska.
• Saprists are wet Histosols whose organic material is well decomposed. The largest extent is in Michigan, Florida, Wisconsin, Minnesota and Alaska.^[2]

Some Hemists and Saprists have been cleared and drained and are used as cropland.^[2] The survey data doesn’t record suborders, so the page gives no acres for them.

What Histosols are like

In USDA’s survey, 99.9% of described Histosol acres have an O (organic) horizon, and on 33.1 of 48.8 million acres (67.8%) the first mineral layer lies under organic material. The most common sequences are O–C (18.8 million acres), O (16.6 million acres), O–W–C (3.2 million acres) and O–R (2.5 million acres); the W in O–W–C marks a water layer, one of the master layers our soil horizons page defines. An A horizon appears on only 6.5% of the acres, and where a parent material is recorded it’s organic material on 98.9%.^[5][4]

The material is light. Histosol bulk densities are quite low, often less than 0.3 g/cm³, the University of Idaho notes, and most Histosols form in wetlands, where restricted drainage holds back the decomposition of plant and animal remains so the organic material accumulates over time.^[6]

For comparison, the first mineral layers of the other orders have median bulk densities from 0.80 to 1.45 g/cm³ in the survey.^[5][7]

83.8% of the acres are in a dual hydrologic group such as A/D, where the first letter applies only once the land is drained, and 15.2% are in group D, where water soaks in slowly.^[5][9]

Why drained Histosols sink

Subsidence when drained is one of the physical properties the University of Idaho lists as restricting Histosols for engineering, beside low weight-bearing capacity.^[6]

The Soil Science Society of America gives the mechanism: once drained, Histosols can decompose rapidly and subside dramatically, even though drained Histosols can be highly productive farmland.^[1]

Building on Histosols

They make poor ground for it. Histosols aren’t stable for foundations or roadways and may be highly acidic.^[1] Their low weight-bearing capacity and subsidence when drained are the engineering limits named above.^[6]

For one address, the building site checker and the septic suitability checker show USDA’s ratings for the soil mapped there.

Farming on Histosols

Almost none of the order is prime farmland as mapped: 0.0% of Histosol acres, against 12.5% of all US soil acres, with another 0.8% prime if drained, protected or irrigated and 6.6% of statewide or local importance. 73.9% of rated acres are in capability Classes VII and VIII, and excess water (subclass w) is the hazard on 74.4%.^[5][10][11]

Drained ones are another matter: NRCS notes that some cleared and drained Hemists and Saprists are cropland, and the Soil Science Society of America calls drained Histosols potentially highly productive, at the cost of the subsidence above.^[2][1]

Where Histosols are in the US

Alaska has the most, 21.5 million acres, then Minnesota (7.0 million) and Florida (4.1 million). Histosols aren’t the leading order in any state, but as a share of a state’s own soil acres they reach 14.4% in Minnesota:^[5]

Most widespread Histosol series

Cathro (Minnesota), Greenwood (Minnesota) and Lupton (Michigan) cover the most ground; each links to its official USDA description.^[5][12]

Download the Histosols data (CSV)

Histosols compared with other orders

Histosols rank ninth of the twelve orders by US acres. In the table their surface pH and topsoil (A) columns show a dash, for the reason in the note below it.^[5]

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 Histosols?

Organic soils: as a general rule, half or more of the upper 31 inches (80 cm) is organic. They have no permafrost, and most are saturated year round.^[2]

What soil order is organic soil?

Histosols, the order dominated by organic material in its upper part: bogs, moors, fens, peats and mucks.^[1]

What is the difference between peat and muck?

Decomposition. Peat is slightly decomposed and full of original fibers; muck is well decomposed with few fibers; mucky peat sits between them.^[4]

Is a forest floor an organic soil?

Not in the Histosol sense. A thin forest-floor O horizon over mineral soil falls far short of the rule that half or more of the upper 31 inches (80 cm) be organic.^[2]

Why do drained Histosols sink?

Drained, their organic material can decompose rapidly, and the soil subsides.^[1]

Can you build on Histosols?

They’re poor ground for it: low weight-bearing capacity, subsidence when drained, and no stability for foundations or roads.^[6][1]

What does ‘Histosol’ mean?

It comes from the Greek histos, tissue.^[1]