Soil Horizons and the Soil Orders

Joseph Mason

3 Soil Horizons and the Soil Orders

In this chapter, we’ll first discuss labels used to identify different kinds of soil horizons. We’ll then look at the soil orders, the twelve major groups into which all soils can be classified. Note that both horizon labels and soil orders discussed here are specific to the US Department of Agriculture system for describing and classifying soils. This system is also used or at least understood in many other parts of the world, but there is another major system developed in Europe that is also widely used. In addition, many countries have their own national soil classification systems.

Labels for Soil Horizons

Soil horizon labels start with an upper case letter, including O, A, E, B, C, or R. Lower case letters are then added to characterize the horizon in more detail. For example, a Bk horizon is a subsoil horizon (B) in which calcite has formed (k). We will cover all of the upper case letter labels here, but only a few examples of the lower case labels. You will also often see numbers attached to the letter labels, for example Bk1 and Bk2. In that case, these are labels used to identify two subsoil horizons with calcite accumulation, one above the other, which differ in some property like soil structure. We won’t cover the use of numbers here.

Upper case labels for soil horizons are as follows:

O Organic layer, dominated by organic matter rather than mineral material. Can be a thin layer of leaf litter at the surface, or a thicker horizon, usually in the Histosol order

A Topsoil, has often accumulated organic matter but is still dominated by mineral material. Usually at the ground surface unless it is buried by sediment deposition

E Eluvial horizon, that is, one that has lost clay, iron minerals, or organic matter. The materials lost have moved downward to underlying horizons.

B Subsoil horizon, often also an illuvial horizon, which has gained clay, iron and aluminum minerals, organic matter, calcite, or other materials from overlying horizons

C Parent material, the sediment or weathered rock that the soil developed in.

R Solid rock if it occurs within the soil profile that is described

Here are two examples of soil profiles with major horizons labeled.

And here is an example of using lower case letters to be more specific about horizon characteristics

Soil Orders

In the USDA soil classification system, all soils can be placed in one of 12 soil orders. Soil orders have been added over time, and there are some additions now in progress, so in the near future there could be 13 or more soil orders.

The twelve soil orders are as follows. We will focus here on the first eight (in bold), which are widespread in the continental US. Except for Ultisols and Aridisols, these eight are also common in Wisconsin.

Mollisols
Alfisols
Ultisols
Spodosols
Histosols
Entisols
Inceptisols
Aridisols
Oxisols
Vertisols
Andisols
Gelisols

For many of these orders, we can identify the factor(s) of soil formation that are most important for them. These five factors, which influence the processes of soil formation, are the following:

climate
organisms (vegetation, animals, others)
topography
parent material
time (how long the soil has been forming)

Mollisols are soils with thick, dark-colored, organic matter-rich A horizons, formed mainly in grasslands of the midlatitudes. The key factors responsible for Mollisol formation are climate and organisms. Dry, but not desert-like climates favor grasslands. The cooler climates of the midlatitudes lead to slower organic matter decomposition, so more accumulates in the soil. Grass roots are often dense and deep, and add lots of organic matter well below the ground surface. Burrowing animals are common in grasslands and mix organic matter downward in the soil, also helping to create the thick, dark A horizon. Humans are indirectly responsible for some major areas of Mollisols, through the use of fire that favors grassland over forest.

Mollisols are the soils of the tallgrass prairies that were once widespread in southern Wisconsin and other Midwestern states. There are patches of Mollisols under forests in parts of the Midwest, probably areas of former prairie that were recently occupied by forest. Mollisols also occur across much of the Great Plains in the US and Canada, and in grasslands of the Rocky Mountains, California, Oregon, and Washington. Mollisols are the predominant soils of the grasslands of Eurasia, from Ukraine eastward to southeastern Siberia and northeastern China. They are less common in grasslands of the subtropics, though some do occur there.

Field of mostly bare dark soil. A few trees, silos, and farm buildings in the distance — A plowed field in an area of Mollisols in northern Illinois, which formed under tallgrass prairie

Profile of a Mollisol formed under tallgrass prairie in Iowa. Look closely and you can see animal burrows below the A horizon, filled with dark A horizon material that fell into the burrows below (or was pushed in by the burrowing animal).

Grassland, very green, with almost no trees. Hills visible in the distance — Xilingol Grassland in eastern Inner Mongolia. The soils in this cool midlatitude grassland are Mollisols, just like those in the former prairies of the Midwestern US, indicating the importance of climate and vegetation as factors in where Mollisols have formed

Alfisols are distinguished by a thin A horizon, with an E horizon below it that has lost clay. The clay has moved downward to form a clay-enriched B horizon (more specifically labeled Bt). Important factors in Alfisol formation include climate, vegetation, and parent material. Most are forest soils, which helps explain the thin A horizon: Under forest, more organic matter is added at the ground surface than in grasslands, mostly through leaf fall. Forest soils are also less commonly mixed by burrowing animals. Some Alfisols do occur in grasslands, often where the production of biomass is relatively low, which limits development of a thick organic matter-rich A horizon.

The downward movement of clay requires enough rainfall to infiltrate into the soil and carry clay particles into the B horizon, although some Alfisols occur in semiarid climates. The soil pH is usually at least mildly acidic in Alfisols, which favors clay movement, and higher precipitation drives faster acidification of the soil. Finally, the parent material must contain enough clay to form a distinct clay-enriched B horizon.

Two photos of soil profiles, one a Mollisol and the other an Alfisol — Comparison between the profile of a Mollisol formed under tallgrass prairie (left) and an Alfisol formed under forest (right). Note the thin A horizon and the E horizon of the Alfisol. The Alfisol’s B horizon (specifically a Bt horizon), is clay-enriched.

Photo of a soil pit with vertical walls and a measuring tape hanging on one wall — A pit dug to study an Alfisol in northern Minnesota. Note the thin A horizon and the fact that most roots are in that horizon. Addition of organic matter mainly from leaves deposited on the surface and shallow roots prevents the development of a thick A horizon. The Bt horizon is slightly browner than the overlying light-colored E horizon

An Alfisol in a semiarid grassland in west Texas, where the vegetation is not productive enough to form a dark, organic matter rich A horizon. The setting is shown on the left and the soil profile is on the right with horizons labeled

Ultisols are similar to Alfisols and also have clay-enriched B horizons. However, they are much more weathered and acidic than Alfisols. Factors influencing their formation included climate and soil age. Ultisols are generally in warmer climates than Alfisols, which leads to greater weathering and acidification in the Ultisols. Ultisols are also often much older soils than Alfisols, so there has been more time for them to become acidified.

Ultisols are common in the southeastern US, and in the subtropics and tropics worldwide, most of them formed under forest. Because of the their acidity and degree of weathering, Ultisols are often not productive agricultural soils without addition of nutrients. Today, this addition often occurs through fertilizer applications, but an older method was to farm these soils for several years and then let forest grow back, which eventually restored some nutrients.

An Ultisol profile in South Carolina. Note the similarity to Alfisol profiles; however this Ultisol is much more acidic and weathered than the Alfisols shown above

Photo of a very light gray soil surface with trees in the background — Surface of an Ultisol that was recently cleared to make a parking area, South Carolina. The thin A horizon is completely gone here, maybe scraped off when the forest was cleared, so you mainly see the very light gray E horizon at the land surface. This soil may have been farmed in the 1700s and 1800s when forest was cleared from many areas of Ultisols to grow tobacco or cotton. After some time, the soils became unproductive and farming was abandoned.

Spodosols are all forest soils. Like Alfisols and Ultisols, they have thin (or no) A horizons over distinct E and B horizons. However, in Spodosols the B horizon has accumulated iron, aluminum, and organic matter, not clay. Because of these materials (especially the iron and organic matter), Spodosol B horizons have a distinctive reddish brown color. The important factors here are climate, vegetation, and parent material. Spodosols form in relatively wet climates. While most are at mid- to high latitudes in North America and Europe, they also occur in sandy parent materials in the tropics and subtropics. Forest vegetation is also important. Finally, Spodosols most often have sandy parent material with low clay content, reducing the amount of clay that can move to the B horizon. Instead, iron, aluminum, and organic matter become mobile under acidic conditions, and are carried downward by water flowing through the soil after rainfall or snowmelt. Spodosols are relatively unproductive soils for agriculture, and many remain under forest.

Photo of a conifer forest — A forest of conifers (jack pine) growing on a Spodosol in northern Minnesota.

Profile of a Spodosol in the Upper Peninsula of Michigan. Note the very light-colored E horizon and the reddish brown B horizon, which has a very irregular lower boundary.

Histosols are soils dominated by organic matter, although they usually also contain some mineral material as well. Common names applied to Histosols include peat and muck. The major factor in Histosol formation is topography, specifically topography that creates wetlands. In Wisconsin, for example, depressions left by melting glacier ice often contain wetlands with Histosols. In wetlands, the water table is usually at or near the ground surface, so the soil is saturated. The microorganisms that usually break down organic matter are not able to function very effectively in saturated soil, so organic matter accumulates instead of decomposing. Eventually, much of the soil profile is made up of organic matter, which has often accumulated over thousands of years. Histosols can occur in almost any climate, and there are even a few in deserts, where water comes to the surface at springs for example. These soils can form under a wide range of vegetation.

Organic matter in Histosols worldwide contains much of the total amount of carbon stored in soils. Disturbance of Histosols can lead to release of that carbon into the atmosphere, potentially driving faster global warming. In many parts of the world, including Wisconsin, Histosols have been drained and are used to grow specialized crops like carrots and onions, or sod for landscaping. When these soils are drained, by digging ditches or using drain tiles (tubes underground that carry away water), the organic matter in them can break down rapidly. Eventually the soil surface will sink as the organic matter is lost. Drained peats are susceptible to fires, which destroys the organic matter and releases carbon as well. Histosols are also destroyed by mining peat for use in growing and shipping landscaping plants. Finally, climate change itself can lead to loss of Histosols, as the organic matter in them decomposes faster at higher temperatures.

A pond in a valley next to a hill. Some people are working in the lower left corner, where dark colored organic soil is visible — A common setting for Histosols in southern Wisconsin. This is a low area between two hills, which are glacially formed drumlins. Histosols have formed here because of the high water table. You can see the dark organic matter of a Histosol exposed around where the people are standing. Many years ago, a landowner had a pond dug here and fossil bones of a prehistoric giant beaver were found. The people are excavating in the peat to find more beaver remains.

Dark-colored soil exposed in a pit — A close-up of the Histosol soil exposed in the excavation shown in the previous image. When the excavation is finished, pumping will be shut off and the water table will rise again, preserving the organic material.

Entisols are very weakly developed soils. They typically have only A and C horizons. B horizons have not developed because of the parent material or the age of the soil. Soil development is very slow in some parent materials, such as pure sand deposits. Young soils in many kinds of parent material often have not had time to develop B horizons. Entisols occur in all climates and vegetation types and in all land areas of the world. The largest areas of Entisols in Wisconsin are on the Central Sand Plain.

Rolling, grass covered topography, with no trees visible — A landscape of Entisols in the Nebraska Sand Hills. These are dunes that were bare sand moving in the wind as recently as 500 years ago. They are now stabilized by grassland vegetation, but there has not been time for much soil development. That, plus the sandy material, means that the soils here are virtually all Entisols.

An Entisol of the Nebraska Sand Hills, with only A and C horizons. A B horizon has not developed yet.

Inceptisols are also weakly developed soils, but they have B horizons. Those B horizons are not very well developed, and have not accumulated much clay or other material; they often only differ from the C horizon because of color or structure. Like Entisols, Inceptisols are mainly in places where there has not been enough time for much soil development, or soil formation is limited by the parent material. Steep slopes where erosion removes soil almost as fast as it can form are good places to find Inceptisols. They can occur in all climates and all types of vegetation.

An Inceptisol on a steep slope in southeastern Minnesota. The parent material is sandstone, which weathers slowly and limits soil formation. The weakly developed B horizon is labeled.

Aridisols are well-developed soils of the world’s deserts. The major factors involved are climate, soil age, and topography. Entisols and Inceptisols are also common in deserts, but Aridisols are located where the landscape is relative flat and erodes very slowly, and therefore soils have had a long time to form. Under these conditions, Aridisols with well-developed B horizons can form, over tens to hundreds of thousands of years. These B horizons often have accumulated calcite, sometimes enough to cement the soil so it is similar to a rock. Some Aridisol B horizons can be clay-enriched. The clay may have been moved downward during past periods when the climate was wetter.

An Aridisol in southern New Mexico. A substantial amount of calcite has formed in the B horizon, cementing it so it is almost like a rock. Note the lack of dark color from organic matter in the A horizon

Photo of a desert landscape, with mountains in the distance — Setting of the same Aridisol, in the desert near Las Cruces, New Mexico.

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