Soil orders and suborders in Minnesota
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Many environmental and agricultural decisions in Minnesota are based on soil and what it’s capable of supporting. For example, the fertile soils and favorable climate of south-central Minnesota support productive agriculture. The productivity of Minnesota's forests also depends on soil properties, but these soils are very different from the ones that support crops.
When producers understand the soil they’re working with, they can make better decisions about cropping systems and fertilizer and water management.
Soil orders and suborders
All soils are arranged into one of 12 major units, or soil orders, seven of which are in Minnesota. A soil order’s location mainly depends on climate and organisms, with the exception of the orders Vertisol, Andisol and Histosol, which depend on parent material.
Orders can be further broken down into suborders, great groups, subgroups, families, and series. Suborders are separated on the basis of important soil properties that influence soil development and plant growth (Figure 1). How wet the soil is throughout the year is the most important property.
Mollisols cover a considerable Minnesota land area and is the basis for the state's productive agricultural base. Suborders of this soil also have the formative syllable “oll” in their names, which is derived from the Latin word mollis, meaning “soft.”
Its most distinguishing feature is a thick, dark surface layer that’s high in nutrients. It occurs throughout Minnesota’s former prairie areas. The “soft” in its name is descriptive because most of these soils usually have a rather loose, low-density surface.
Three suborders of mollisols occur in Minnesota: Aquolls, Udolls, and Ustolls.
Aquolls are wet prairie soils. Here, “aqua” is derived from the Latin word for “water.” The “oll” part of its name shows it’s a mollisol. These mollisols occur in areas where the water table is near the surface.
The most extensive area of these soils is the Red River Valley, on the bed of an old glacial lake, although they also occur in the beds of other former glacial lakes. They’re very productive soils, especially when drainage removes excess water. They produce small grains, sunflowers, and sugarbeets in northwestern Minnesota, and corn and soybeans in the south.
Moist prairie soils are Udolls. The Latin root udus refers to “humid.” These are soils of humid climates. These soils cover much of the western half and southern third of the state and are very productive agricultural soils. The dominant crops on these soils are corn and soybeans.
Ustolls are dry prairie soils. The Latin root ust refers to “burnt.” These are soils of dry climates, that are hot in summer.
They occur in the southwest corner of Minnesota in areas where extended dry periods have occurred in the past, and which even now are subject to periodic droughts. They also produce corn and soybeans, although in some years dry conditions limit their productivity.
Alfisols cover a large land area in Minnesota, part of which is now cultivated and partially forested.
Suborders of this soil also have the formative syllable “alf” in their names, which is coined from the soil term “pedalfer.” Pedalfers were identified in the 1930s as soils of the eastern United States that had an accumulation of aluminum and iron. “Alf” refers to the chemical symbols for aluminum (Al) and iron (Fe).
Alfisols primarily are fertile soils of the forest, formed in loamy or clayey material. The surface layer of soil, usually light gray or brown, has less clay than the subsoil. These soils are usually moist during the summer but may dry during occasional droughts.
Two suborders of alfisols occur in Minnesota: Aqualfs and Udalfs.
Aqualfs are wet forest soils. It has the formative element aqua in its name, which again implies “wetness.” Because of their position on the landscape, these soils are wet during much of the growing season.
Especially in northern Minnesota, they support aspen forests with admixtures of black ash and alder. They’re most common in the basins of glacial lakes that formed in the latter part of the Ice Age. The aqualfs that extend across Minnesota’s northern border lie in the basin of glacial Lake Agassiz.
Udalfs are soils of the forests. They can be found in the southern third of Minnesota, where hardwood forests were dominant (in the northern two-thirds of the state, you’d find pine and oak forests). Udalfs are similar to the boralfs but occur in a warmer climate.
Udalfs support hardwood forests where they haven’t been cleared for cultivation. Those in south-central Minnesota support forests dominated by sugar maple and basswood, while those in the southeast part of the state support forests dominated by oak and some hickories.
Udalfs dominate the ridges of extreme southeast Minnesota, which also contains fertile udolls in the valley floor. In the north, large aspen forests now cover udalfs. Some of the largest white and red pine were found on these soils.
Inceptisols are also common in Minnesota. Suborders of this soil also have the formative syllable “ept” in their names, which comes from the Latin word inceptum, meaning “beginning.” These soils’ properties change with depth, but the soil formation process has been subdued due to lack of intensity in at least one of the five soil-forming factors.
Aquepts and Udepts are the major suborders of inceptisols in Minnesota.
Aquepts are wet soils of the mixed conifer-deciduous forest. The formative element “aqua” in its name implies that seasonal wetness is one of the soil’s distinguishing properties.
They’re scattered over the northern two-thirds of the state, and support forests of aspen, black ash, willows, and alder. They often occur in association with peatlands.
Udepts are soils of the mixed conifer-deciduous forest. This name’s formative element is ochros, Greek for “pale.”
These soils primarily occur under forest vegetation in the northern two-thirds of the state. The parent material is primarily glacial till from the state’s rocky northeastern part, so these soils are low in lime and contain many large boulders.
Now they’re primarily covered by aspen forests, although they once were dominated by red and white pine. Aspen on these soils doesn’t grow quite as well as aspen on the alfisols.
Entisols occur throughout Minnesota. Suborders of this soil also have the formative syllable “ent” in their names, which refers to recent soil.
Soils developed in recent river bottom alluvium and sandy soils where the parent materials typically consist of weather-resistant quartz. Soil properties change very little with depth, due to insufficient time or weather-resistant material.
Aquents, Orthents, and Psamments are the major suborders of entisols in Minnesota.
Aquents are wet, poorly developed soils. These are predominantly sandy soils supporting forest vegetation. They occur in north-central and northwestern Minnesota.
Orthents are shallow or poorly developed soils. Orthos means “true” in Greek, and orthent soils are the true or common entisols.
These soils primarily occur in two areas. In northeastern Minnesota, they occupy the tops of ridges where rock outcrops are common. Any present trees are usually pine. These soils and associated vegetation are picturesque reminders of wilderness.
Orthents are also scattered in other areas, especially west-central and southwest Minnesota, where glacial deposits have steep slopes and the material isn’t easily weathered.
Psamments are sandy soils. The formative element, the Greek word psammos, means “sand,” and refers to their dominant property. These soils, occurring both in the state’s forested and prairie areas, are predominantly formed from quartz sand.
These sand grains weather very slowly, so the result of soil formation is simply a sandy soil with little sign of development. In forested areas, these soils support jack pine and oak forests. Toward the south and west, grasses are common beneath the trees, while in the east and north, blueberries are.
Many of these soils are now irrigated and are very productive for a variety of agricultural crops.
This soil order has become increasingly important in Minnesota. Suborders of this soil also have the formative syllable “ist” in their names, which comes from the Greek word histos, meaning “tissue.”
This association is appropriate because soils form from plant remains in wet environments, such as marshes and bogs. Although they occur throughout most of Minnesota, soils are found most extensively in the north, in the beds of former glacial lakes.
Histosols, or organic soils, have been termed peat and muck. In soil taxonomy, they’re Saprists, Hemists, and Fibrists.
Soil scientists recognize three suborders of histosols, based on degree of change of plant material. However, it’s difficult to separate the three suborders in a statewide map like in Figure 1.
In the north, histosols support open peatland vegetation or spruce and tamarack forests. In the south, some peatlands are drained for specialty crops such as vegetables and sod.
Soils of this order aren’t common in uniform, extensive areas but they do occur in many northeastern Minnesota locations. The formative element is “od” and refers to the accumulation of organic matter, iron, and aluminum.
Sandy parent materials and a humid climate with intense leaching promote the development of Spodosols. Trees are the common vegetation, which were mostly coniferous in native stands. These soils’ most distinguishing feature is the bright red horizon immediately underlying the white, leached horizon.
There’s one Spodosol suborder in Minnesota: Orthods.
Orthods are forested soils with a sandy texture. They’re frequently found in small areas downslope from a summit. Orthods are covered by aspen or pine forests and are rarely used for agriculture because of their acid, droughty nature.
Vertisols are minerals soils that are more than 30 percent clay, with cracks at least 1 centimeter wide and 50 centimeters deep. The root “ert” is from “inverted,” which happens when dry soil falls into the deep cracks.
Vertisols in Minnesota only occur in the lake deposits of the Red River Valley. These soils have thick, dark surface horizons overlying gray subsoils. Both are of clay texture.
Aquerts are the main suborder in Minnesota.
Aquerts are wet, clay-textured soils formed in lake sediments. The water table is near the surface. These soils are very productive because of their high water-holding capacity and inherent fertility.
Building on these soils is difficult because of the soil’s shrink-swell capacity. Internal water movement is also very slow. Soils are used to grow sugarbeets, sunflowers and small grains in northwestern Minnesota.
Andisols form on volcanic ash and cinders. They’re found near the volcano source or in areas downwind from volcanos where thick ash has accumulated.
The soils haven’t had time to become very weathered. Because of the high amount of organic matter and high content of volcanic glass, they’re light, fluffy soils with a low bulk density. They’re usually very fertile, but may be low in phosphorus.
These soils are found in eastern Washington and Idaho.
Aridisols are soils of the desert, and water deficiency is a major characteristic. The native vegetation consists of desert shrubs and short bunchgrasses.
Soluble materials like carbonates, gypsum and salts frequently accumulate in the soil profile. They’re common in the southwestern United States.
Gelisols are soils of the tundra where some part of the subsoil is permanently frozen or permafrost. They’re young soils with little profile development.
They’ll frequently have cryoturbation or frost churning, which creates convoluted horizons and land surface patterns of hummocks and non-sorted circles (flat, bare soil patches also known as frost boils). They’re found in Alaska and northern Canada.
Oxisols are the world’s most highly weathered soils. They form in hot climates with year-round moist conditions. The native vegetation is usually tropical rainforest.
Weathering and intense leaching have removed most of the bases and much of the silica from the clay minerals. Some 1:1 clays remain, but the hydrous oxides are the dominant clay-sized particles. Hawaii and Puerto Rico have Oxisols.
Ultisols are also highly weathered soils.
They’ll mostly have 1:1 clay minerals, red soil colors and a base status less than 35 percent. They have a clay accumulation in the subsoil and have developed under a moist condition, usually a forest.
They’re found in the southeastern United States.
Table 1 shows estimated area and the percent of the state it occupies.
Table 1: How much of Minnesota each soil order and suborder occupies
|Order||Suborder||Minnesota area in acres||Percent of state|
The lowest level and the one most recognized by producers is the soil series. Soils in an individual series are nearly homogeneous and their range of properties is limited.
Soil series are separated by observable and mappable properties such as color, structure, texture, and horizon arrangement. They’re named for the location where the soil was first identified.
The series is the level that’s generally used to name mapping units of detailed soil surveys completed at scales between 1:24,000 and 1:15,840 (1 inch on the map is equal to between 24,000 and 15,840 inches, or 2,000 to 1,320 feet, in the real world). There are 20,000 soil series recognized in the United States and more than 1,000 in Minnesota.
Additional information about the classification of Minnesota soils can be found in the following references:
Buol, S.W., Hole, F.D., & McCracken, R.I. (1981). Soil genesis and classification (2nd ed., pp. 404). Ames, IA: Iowa State University Press.
Department of Agriculture Natural Resources Conservation Service. (1999). Soil taxonomy (2nd ed., U.S. Department of Agriculture Handbook 436). Washington, D.C.: U.S. Government Printing Office.
Reviewed in 2018