The Twelve Soil Orders

There are 12 soil orders:


Alfisols

Alfisols are moderately leached soils that have relatively high native fertility. These soils have formed primarily under forest and have a subsurface horizon in which clays have accumulated. Alfisols are found mostly in temperate humid and subhumid regions of the world.

Alfisols occupy ~10.1% of the global ice-free land area. In the United States, they account for ~13.9% of the land area. Alfisols support about 17% of the world's population.

The combination of generally favorable climate and high native fertility allows Alfisols to be productive soils for both agricultural and silvicultural use.

Alfisols are divided into five suborders: Aqualfs, Cryalfs, Udalfs, Ustalfs, and Xeralfs.

Adapted from: The Twelve Soil Orders: Alfisols. University of Idaho, College of Agriculture and Life Sciences. http://soils.cals.uidaho.edu/soilorders/alfisols.htm


Andisols

Andisols are soils that have formed in volcanic ash or other volcanic ejecta. They differ from those of other soil orders in that they typically are dominated by glass and short-range-order colloidal weathering products such as allophane, imogolite, and ferrihydrite (minerals). As a result, andisols have andic properties - unique chemical and physical properties that include high water-holding capacity and the ability to "fix" (and make unavailable to plants) large quantities of phosphorus.

Globally, Andisols are the least extensive soil order and account for only about 1% of the ice-free land area. They occupy about 1.7% of the U.S. land area, including some productive forests in the Pacific Northwest region.

Andisols are divided into eight suborders: Aquands, Gelands, Cryands, Torrands, Xerands, Vitrands, Ustands, and Udands.

Adapted from: The Twelve Soil Orders: Andisols. University of Idaho, College of Agriculture and Life Sciences. http://soils.cals.uidaho.edu/soilorders/andisols.htm


Aridisols

Aridisols are calcium carbonate-containing soils of arid regions that exhibit at least some subsurface horizon development. They are characterized by being dry most of the year and having limited leaching. Aridisols contain subsurface horizons in which clays, calcium carbonate, silica, salts, and/or gypsum have accumulated. Materials such as soluble salts, gypsum, and calcium carbonate tend to be leached from soils of moister climates.

Aridisols occupy about 12% of the Earth's ice-free land area and about 8.3% of the United States.

Aridisols are used mainly for range, wildlife, and recreation. Because of the dry climate in which they are found, they are not used for agricultural production unless irrigation water is available.

Aridisols are divided into seven suborders: Cryids, Salids, Durids, Gypsids, Argids, Calcids, and Cambids.

Adapted from: The Twelve Soil Orders: Aridisols. University of Idaho, College of Agriculture and Life Sciences. http://soils.cals.uidaho.edu/soilorders/aridisols.htm


Entisols

Entisols are soils of recent origin. The central concept is that these soils developed in unconsolidated parent material with usually no genetic horizons except an A horizon. All soils that do not fit into one of the other 11 orders are entisols. Thus, they are characterized by great diversity, both in environmental setting and land use.

Many entisols are found in steep, rocky settings. However, entisols of large river valleys and associated shore deposits provide cropland and habitat for millions of people worldwide.

Globally, entisols are the most extensive of the soil orders, occupying about 18% of the Earth's ice-free land area. In the United States, entisols occupy about 12.3% of the land area.

Entisols are divided into six suborders: Wassents, Aquents, Arents, Psamments, Fluvents, and Orthents.

Adapted from: The Twelve Soil Orders: Entisols. University of Idaho, College of Agriculture and Life Sciences.


Gelisols

Gelisols are soils of very cold climates that contain permafrost within 2 meters of the surface. These soils are limited geographically to the high-latitude polar regions and localized areas at high mountain elevations. Because of the extreme environment in which they are found, Gelisols support only about 0.4% of the world's population — the lowest percentage of any of the soil orders.

Gelisols are estimated to occupy about 9.1% of the Earth's ice-free land area and about 8.7% of the United States. Although some Gelisols may occur on very old land surfaces, they show relatively little morphological development. Low soil temperatures cause soil-forming processes such as decomposition of organic materials to proceed very slowly. As a result, most Gelisols store large quantities of organic carbon; only soils of wetland ecosystems contain more organic matter. Gelisols of the dry valleys of Antarctica are an exception; they occur in a desert environment with no plants and consequently contain very low quantities of organic carbon.

The frozen condition of Gelisol landscapes makes them sensitive to human activities.

Gelisols are divided into three suborders: Histels, Turbels, and Orthels.

Adapted from: The Twelve Soil Orders: Gelisols. University of Idaho, College of Agriculture and Life Sciences.


Histosols

Histosols are soils that are composed mainly of organic materials. They contain at least 20 to 30% organic matter by weight and are more than 40 cm thick. Bulk densities are quite low, often less than 0.3 grams per cubic centimeter.

Most Histosols form in settings such as wetlands where restricted drainage inhibits the decomposition of plant and animal remains, allowing these organic materials to accumulate over time. As a result, Histosols are ecologically important because of the large quantities of carbon they contain. These soils occupy about 1.2% of the ice-free land area globally and about 1.6% of the United States.

Histosols are often referred to as peats and mucks and have physical properties that restrict their use for engineering purposes. These include low weight-bearing capacity and subsidence when drained. They are mined for fuel and horticultural products.

Histosols are divided into five suborders: Folists, Wassists, Fibrists, Saprists, and Hemists.

Adapted from: The Twelve Soil Orders: Histosols. University of Idaho, College of Agriculture and Life Sciences.

Inceptisols are soils that exhibit minimal horizon development. They are more developed than Entisols but still lack the features that are characteristic of other soil orders.

Inceptisols are widely distributed and occur under a wide range of ecological settings. They are often found on fairly steep slopes, young geomorphic surfaces, and resistant parent materials. Land use varies considerably with Inceptisols. A sizable percentage of Inceptisols are found in mountainous areas and are used for forestry, recreation, and watershed.

Inceptisols occupy an estimated 15% of the global ice-free land area. Only the Entisols are more extensive. In the United States, they occupy about 9.7% of the land area. Inceptisols support about 20% of the world's population — the largest percentage of any of the soil orders.

Inceptisols are divided into seven suborders: Aquepts, Anthrepts, Gelepts, Cryepts, Ustepts, Xerepts, and Udepts.

Adapted from: The Twelve Soil Orders: Inceptisols. University of Idaho, College of Agriculture and Life Sciences.

Mollisols are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon. This fertile surface horizon, known as a mollic epipedon, results from the long-term addition of organic materials derived from plant roots.

Mollisols primarily occur in the middle latitudes and are extensive in prairie regions such as the Great Plains of the United States. Globally, they occupy about 7.0% of the ice-free land area. In the United States, they are the most extensive soil order, accounting for about 21.5% of the land area.

Mollisols are among some of the most important and productive agricultural soils in the world and are extensively used for this purpose.

Mollisols are divided into eight suborders: Albolls, Aquolls, Rendolls, Gelolls, Cryolls, Xerolls, Ustolls, and Udolls.

Adapted from: The Twelve Soil Orders: Mollisols. University of Idaho, College of Agriculture and Life Sciences.


Oxisols

Oxisols  are very highly weathered soils that are found primarily in the intertropical regions of the world. These soils contain few weatherable minerals and are often rich in iron (Fe) and aluminum (Al) oxide minerals.

Oxisols occupy about 7.5% of the global ice-free land area. In the United States, they only occupy about 0.02% of the land area and are restricted to Hawaii.

Most of these soils are characterized by extremely low native fertility, resulting from very low nutrient reserves, high phosphorus retention by oxide minerals, and low cation exchange capacity (CEC). Most nutrients in Oxisol ecosystems are contained in the standing vegetation and decomposing plant material. Despite low fertility, Oxisols can be quite productive with inputs of lime and fertilizers.

Oxisols are divided into five suborders: Aquox, Torrox, Ustox, Perox, and Udox.

Adapted from: The Twelve Soil Orders: Oxisols. University of Idaho, College of Agriculture and Life Sciences.


Spodosols

Spodosols are acid soils characterized by a subsurface accumulation of humus that is complexed with Al and Fe. These photogenic soils typically form in coarse-textured parent material and have a light-colored E horizon overlying a reddish-brown spodic horizon. The process that forms these horizons is known as podzolization.

Spodosols often occur under coniferous forest in cool, moist climates. Globally, they occupy ~4% of the ice-free land area. In the US, they occupy ~3.5% of the land area.

Many Spodosols support forest. Because they are naturally infertile, Spodosols require additions of lime in order to be productive agriculturally.

Spodosols are divided into 5 suborders: Aquods, Gelods, Cryods, Humods, and Orthods.

Adapted from: The Twelve Soil Orders: Spodosols. University of Idaho, College of Agriculture and Life Sciences.


Ultisols

Ultisols are strongly leached and acidic forest soils with relatively low fertility. They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes. Intense weathering of primary minerals has occurred, and much calcium (Ca), magnesium (Mg), and potassium (K) has been leached from these soils. Ultisols have a subsurface horizon in which clays have accumulated, often with strong yellowish or reddish colors resulting from the presence of iron (Fe) oxides. The red clay soils of the southeastern United States are examples of Ultisols.

Ultisols occupy about 8.1% of the global ice-free land area and support 18% of the world's population. They are the dominant soils of much of the southeastern United States and occupy about 9.2% of the total U.S. land area.

Because of the favorable climate regimes in which they are typically found, Ultisols often support productive forests. The high acidity and relatively low quantities of plant-available Ca, Mg, and K associated with most Ultisols make them poorly suited for continuous agriculture without the use of fertilizer and lime. With these inputs, however, Ultisols can be very productive.

Ultisols are divided into five suborders: Aquults, Humults, Udults, Ustults, and Xerults.

Adapted from: The Twelve Soil Orders: Ultisols. University of Idaho, College of Agriculture and Life Sciences.


Vertisols

Vertisols are clay-rich soils that shrink and swell with changes in moisture content. During dry periods, the soil volume shrinks and deep wide cracks form. The soil volume then expands as it wets up. This shrink/swell action creates serious engineering problems and generally prevents formation of distinct, well-developed horizons in these soils.

Globally, Vertisols occupy about 2.4% of the ice-free land area. In the United States, they occupy about 2.0% of the land area and occur primarily in Texas.

Vertisols are divided into six suborders: Aquerts, Cryerts, Xererts, Torrerts, Usterts, and Uderts.

Adapted from: The Twelve Soil Orders: Vertisols. University of Idaho, College of Agriculture and Life Sciences.