Soils and landscapes of Minnesota
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Soils are the basic resource upon which all terrestrial life depends. The bounty from our fields and forests, and the quality of our lakes and streams, is dependent upon the quality of our soils. Many of the environmental decisions about using a resource in Minnesota are based on the kind of soil and the ability of the soil to support that resource use. The productive agriculture of south central Minnesota is a direct result of the fertile soils and favorable climate in that area. The productivity of Minnesota's forests, too, depends on soil properties. The quality of Minnesota's environment is directly dependent on the health of the soil resource.
The Soil Suborders of Minnesota map (below) is not meant to show all soil differences, rather it is based on Soil Taxonomy and reflects the major soil-forming factors operating in Minnesota and the relationship of soils to one another. A map of this scale cannot represent all the intricate patterns of soils present, but can indicate the major soil areas and their characteristics.
Soils lie in a complex mosaic across Minnesota and may seem to lack a pattern. This mosaic is not random, however, but is the result of five major environmental elements blended together. The five elements, or soil-forming factors, that lead to the pattern of soils we observe are: 1) the parent material, the geologic material from which the soil was originally formed; 2) the climate in which the material is found; 3) the relief or landscape properties upon which that material lay, such as the slope and aspect; 4) the organisms that can potentially live on or in the material; and 5) the length of time during which the previous four elements have interacted. A unique combination of these five factors produces a soil. At any location in the state, that combination of factors may be unique, and produce a unique soil; or a given combination may recur at many locations, producing similar soils. In Soil Taxonomy, climate and vegetation determine what order, suborder, and great group a soil is in. Parent material will to a large extent determine the soil family and series (Soil Survey Staff, 1999).
One of the most important elements affecting soil formation in Minnesota is the parent material. The color, texture (number of various size particles, such as sand silt, and clay), and chemical makeup of the soil are closely related to the color, texture, and chemistry of the parent material. The texture of a soil helps determine its ability to hold and transmit water, thereby determining the wetness of the soil. The moisture and nutrient supplying properties of the soil greatly affect what kind of plants can grow on it.
Figure 1. Parent materials
Between ten and twenty thousand years ago, Minnesota was largely covered with glaciers. The materials deposited through the direct and indirect action of the glaciers provide the parent material for our soils (Figure 1). The material deposited directly by the glaciers is called till, and is an unsorted mixture of material ranging in size from very fine clay to stones and boulders. As the glaciers melted, considerable quantities of water were released. These waters washed the till, removing fine particles and sorting the remaining sands and gravels, which formed glacial outwash sediments. Outwash material is a very common parent material in Minnesota.
In some cases, melt waters collected in low areas, often behind dams of unmelted ice. Lakes were formed that lasted from a few years to thousands of years. One of the best known of these lakes is glacial Lake Agassiz, which occupied the area now known as the Red River Valley in northwestern Minnesota. This lake actually extended, in its early stages, well into northeastern Minnesota. Lacustrine deposits, materials that settled out of glacial lakes, vary from sandy and gravelly at the beaches to clayey near the middle of the lake. The topography of the land that was once covered by these lakes is often subdued, with extensive level areas.
For long periods, both during glaciation and while the ice melted, very little vegetation grew on till and outwash areas. Wind blowing over these areas picked up silt and clay-sized particles and carried them for many miles before depositing them. Areas in both southwestern and southeastern Minnesota are covered by these materials called loess. The loess may rest on older till, on weathered bedrock, or on fresh bedrock. In northeastern Minnesota the fixed bedrock is near the surface so that soil development must take place in both a thin veneer of glacially deposited material and underlying bedrock.
In addition to differences in parent material associated with differences in mode of deposition, the material carried by the glaciers differed depending on the direction from which the glaciers originated. Glaciers moved over Minnesota from both the northwest and the northeast. Glaciers originating in the northwest passed over limestone deposits in southern Canada and carried materials high in lime. Glaciers from the northeast passed over the Laurentian Shield (granite, sandstone and basalt). Depending on the kind of bedrock, these deposits have diverse chemistries but they all lack lime, which affects many other chemical properties.
Figure 2. Mean annual temperatures (From Baker & Straub, 1965)
Figure 3. Annual precipitation (From Baker & Straub, 1965)
Temperature and moisture, both as averages and as extremes, affect chemical and physical changes that occur in the original parent material. Chemical changes proceed very slowly at low temperatures, and especially during the winter in Minnesota. The mean annual temperature in the state ranges from about 35 degrees F near the Canadian border to about 45 degrees F at the Iowa border (Baker and Strub, 1965). Temperature tends to increase relatively uniformly from north to south (Figure 2).
In contrast to temperature, the normal annual precipitation (Figure 3) in the state increases from west to east (Baker and Kuehnast,1978). Areas along the Dakota border receive about 20 inches rain per year, and those along the Wisconsin border receive about 30 inches per year. Warmer temperatures usually lead to more evaporation of precipitation to the atmosphere so that less is available to cause changes in the soil. Soils in the warmer and dryer areas of the state, for example, retain the high lime concentrations near the surface that they inherited from the parent material. In cooler and moister areas, lime has been removed by water passing through the soil.
Figure 4. General relief
Relief, or the lay of the land, is another element of soil formation (Figure 4). It influences both the movement of materials and the soil climate. Local relief affects drainage, runoff, and erosion, and in turn affects the formation and distribution of soils. Steeper slopes contribute to greater runoff, as well as to greater translocation of surficial materials downslope through surface erosion and movement of the soil mass. Soils in depressions and swales collect both water and sediment from higher lying terrain. The loss or addition of water affects the kinds of plants that grow there.
The direction a slope faces also affects soil formation by affecting the soil's climate. A south-facing slope is warmer and dryer than is a north-facing slope in the same locale. As the steepness of the slope increases, this effect becomes more important. In some areas in Minnesota, the borders between forest and prairie vegetation often have forest vegetation and forest soil on the northfacing slopes and prairie soils on the south-facing slopes.
Figure 5. Original vegetation
Plants, animals, bacteria, and other organisms are active in soil formation. They aid in the breakdown of parent materials and in the formation and decomposition of organic matter in the soil. Vegetation affects soil formation by depositing organic materials in the soil and by transferring plant nutrients from the subsurface to the surface (Figure 5).
In the prairie areas, fibrous plant roots die and decay each year and supply the surface soil with organic matter. This imparts a dark color to the surface. In forested areas, plant roots are not concentrated in the mineral soil and so the surface is usually a lighter color. Earthworms and other organisms can be very important in mixing soils and incorporating organic matter. Gophers and other small soil-dwelling mammals continually mix the soil.
Soil properties are also determined by the length of time during which all the other soil-forming elements have been acting. Although soil properties differ appreciably if we compare very old soils to very young soils, most soils in Minnesota are about the same age. They have all been developing since the glaciers left the state approximately 12,000 years ago, a relatively short time in geological terms. Although the other elements of formation have affected the soils, many of their properties were inherited from the parent material.
To understand and communicate about soils, a standard system of classes or categories was developed. These classes are based on the presence or absence of certain soil properties. Soils can also be categorized by their location (northern versus southern soils), the kind of vegetation growing on them (forest soils versus prairie soils), their topographic position (hilltop soils versus valley soils), or other distinguishing features. The system used to classify soils based on their properties is called Soil Taxonomy (Soil Survey Staff, 1999). It was developed by the U.S. Department of Agriculture, with the help of soil scientists in universities throughout the country.
The classification of soils: 1) aids in remembering the basic characteristics of individual soils, 2) defines relationships between soils, 3) aids in discovering new facts, 4) clarifies relationships between soils and the environment, and 5) aids in the ability to predict properties and uses of an unknown soil.
In Soil Taxonomy, all soils are arranged into one of twelve major units, or soil orders. The twelve orders are defined largely on the basis of having certain kinds of diagnostic horizons or diagnostic materials. The locations the soil orders depends mainly on climate and organisms with the exception of the orders Vertisol, Andisol and Histosol, which depend on parent material. Minnesota has seven of the eleven orders.
These orders are further broken down into suborders, great groups, subgroups, families, and series.
Suborders within a soil order are separated on the basis of important soil properties that influence soil development and plant growth. The most important property is how wet the soil is throughout the year.
At the great group level, the most significant features of the soil profile are considered. Soil great groups are subdivisions of suborders.
Each great group is divided into three kinds of subgroups. These are: 1) the central concept of the great group (typic); 2) soil properties which intergrade to or are transitional toward other orders, suborders, or great groups; and 3) subgroups with properties that are not representative.
The next lower category is the soil family. Soil families are separated within a subgroup on the basis of physical and chemical properties that influence plant growth, land management decisions, and engineering purposes. Soil properties include texture, mineralogy, pH, soil temperature regime, and precipitation pattern.
The lowest level recognized is the soil series. Soils comprising an individual series are nearly homogeneous and their range of properties is limited. Soil series are separated on the basis of observable and mappable properties such as color, structure, texture, and horizon arrangement. A soil series is named for the location where the soil was first identified. The series is the level that is generally used to name mapping units of detailed soil surveys completed at scales between 1:24,000 and 1:15,840. There are 20,000 soil series recognized in the United States and over 1,000 in Minnesota (Buol, Hole, and McCracken, 1981).
The legend section presents the classification at the suborder level. This level includes a broad climatic grouping of soils, with further separation based primarily on soil wetness.
The soil names throughout the system are made up of syllables called formative elements. Although each of these formative elements may initially seem to have little meaning, they all convey considerable information about the soil. Many of the formative elements come from Greek or Latin roots. The names of all soil orders end in the syllable sol, with the first part or prefix of the name giving some indication of the soil properties associated with this broad category of soil. The name of each suborder within an order links another formative element with the prefix from the order name to provide a ready relationship to the order. This system is best understood by examining the descriptions and legend below.
Soil orders and suborders in Minnesota
Mollisols -- This order covers a considerable land area of Minnesota and is the basis for the state's productive agricultural base. The formative syllable, oll, is derived from the Latin word mollis or soft. Its most distinguishing feature is a thick, dark-colored surface layer that is high in nutrients. It occurs throughout the former prairie areas of Minnesota. The Latin term for soft in its name is descriptive in that most of these soils usually have a rather loose, low density surface. Three suborders of mollisols occur in Minnesota: Aquolls, Udolls, and Ustolls.
Alfisols -- This order covers a large land area in Minnesota, part of which is now cultivated and part forested. Alf is the formative element and is coined from a soil term pedalfer. Pedalfers were identified in the 1930s as soils of the eastern part of the United States which, had an accumulation of aluminum and iron. The alf refers to the chemical symbols for aluminum (Al) and iron (Fe). Alfisols are primarily fertile soils of the forest, formed in loamy or clayey material. The surface layer of soil, usually light gray or brown, has less clay in it than does the subsoil. These soils are usually moist during the summer, although they may dry during occasional droughts. Two suborders of alfisols occur in Minnesota: Aqualfs, and Udalfs.
Inceptisols -- Soils of this order are also common in Minnesota. Here the formative element is ept, which comes from the Latin word incepium or beginning. These soils have changes in properties with depth, but the process of soil formation has been subdued due to lack of intensity of one or more of the five soil-forming factors. The major suborders of inceptisols in Minnesota are Aquepts and Udepts.
Entisols -- Soils of this order occur throughout Minnesota. The formative element here is ent which refers to recent soil. Soils developed in recent river bottom alluvium and sandy soils where the parent materials consist of weather-resistant quartz are typical of this soil order. Because of insufficient time or material resistant to weathering, soil properties change very little with depth. The major suborders of entisols that occur in Minnesota are Aquents, Orthents, and Psamments.
Histosols -- This is a soil order that has become increasingly important in Minnesota. The formative element in the name is ist and comes from the Greek word histos, which means tissue. This is an appropriate association because these soils are formed from plant remains in wet environments, such as marshes and bogs. Although they occur throughout most of Minnesota, these 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 are Saprists, Hemists and Fibrists.
Spodosols -- Soils of this order are not common in uniform extensive areas but they do occur in many locations of northeastern Minnesota. The formative element is od and refers to the accumulation of organic matter, iron and aluminum. Sandy parent materials and humid climate with intense leaching promote the development of Spodosols. Trees are the common vegetation and were mostly coniferous in native stands. The most distinguishing feature of these soils is the bright red horizon immediately underlying the white colored leached horizon. There is one suborder in Minnesota: Orthods.
Vertisols -- Vertisols are minerals soils that are more than 30 percent clay and have cracks at least 1 centimeter wide and 50 centimeters deep. The root is ert and 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 colored subsoils and both are of clay texture. The main suborder in Minnesota is the Aquerts.
Soil orders not in Minnesota
Andisols -- are formed on volcanic ash and cinders. They are found near the volcano source or in areas downwind from volcanos where thick ash has accumulated. They have not had time to become very weathered. They have a low bulk density. Because of the high amount of organic matter and high content of volcanic glass, they are light and fluffy soils. Usually they are very fertile soils but may be low in phosphorus. These soils are found in eastern Washington and Idaho.
Aridisols -- are soils of the desert. Water deficiency is a major characteristic for this soil. The native vegetation consists of desert shrubs and short bunchgrasses. Soluble materials like carbonates, gypsum, and salts frequently accumulate in the soil profile. They are common in the southwestern United States.
Gelisols -- are soils of the tundra where some part of the subsoil is permanently frozen or permafrost. They are young soils with little profile development. They frequently will have cryoturbation or frost churning which creates convoluted horizons and patterns on the land surface of hummocks and rocks forming rngs. They are found in Alaska and northern Canada.
Oxisols -- are the most highly weathered soils in the world. They form in hot climates with year-round moist conditions. The native vegetation is usually tropical rain forest. 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 size particles. Hawaii and Puerto Rico have Oxisols.
Ultisols -- are the most highly weathered soils in the world. They will have mostly 1:1 clay minerals, red soil colors, and a base status less than 35%. They are found in the southeastern United States. They have a clay accumulation in the subsoil and have developed under a moist condition, usually a forest.
AQUOLLS are wet prairie soils. Here the formative element is aqua, from the Latin word for water. The oll ending shows that this is a mollisol. These are mollisols that occur in areas where the water table is near the surface. The most extensive area of these soils is the Red River Valley, or the bed of an old glacial lake, although they also occur in the beds of other former glacial lakes. They are very productive soils, especially when excess water is removed by drainage. They produce small grains, sunflowers, and sugar beets in northwestern Minnesota, and corn and soybeans in the south.
UDOLLS are moist prairie soils. The Latin root udus refers to humid. These are soils of humid climates. These soils cover much of the western one-half and southern one-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, usually 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.
AQUALFS are wet forest soils. The aqua formative element 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 are most common in the basins of glacial lakes that formed in the latter part of the Ice Age. The aqualfs that extend across the northern border of Minnesota lie in the basin of glacial Lake Agassiz.
UDALFS are soils of the forests. In the southern one-third of Minnesota hardwood forests were dominant, while in the northern two-thirds pine and oak forests were found. These are alfisols that occur in the southern one-third of Minnesota. They are similar to the boralfs, but they occur in a warmer climate. Where not cleared for cultivation, they support hardwood forests. Those in the south-central part of the state support forests dominated by sugar maple and basswood, while those in the southeast support forests dominated by oak and some hickories. The area of extreme southeast Minnesota that is dominated by udalfs on ridges contains fertile udolls in the valley floor. In the north udalfs are now covered by large aspen forests. Some of the largest white and red pine were found on these soils. The map colors are based on the dominant udalfs.
AQUEPTS are wet soils of the mixed conifer-deciduous forest. The formative element aqua implies that one of the distinguishing properties of these soils is their seasonal wetness. They are 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. The formative element in this name is ochros, Greek for pale. These soils primarily occur under forest vegetation in the northern two-thirds of the state. The parent material of these soils is primarily glacial till from the rocky northeastern part of the state, and so these soils are low in lime and contain many large boulders. Now they are primarily covered by aspen forests, although they once were dominated by red and white pine. Aspen on these soils does not grow quite as well as aspen on the alfisols.
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. These are the true or common entisols. These soils primarily occur in two areas. In northeastern Minnesota, they occupy tops of ridges where outcrops of rock are common. The trees that are present are usually pine. These soils and associated vegetation are picturesque reminders of wilderness. Orthents are also scattered in other areas of the state, especially the west-central and southwest, where glacial deposits have steep slopes and the material is not 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 forested and prairie areas of the state, are predominantly formed from quartz sand. These sand grains weather very slowly, so that 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 and in the east and north, blueberries. Many of these soils are now irrigated and are very productive for a variety of agricultural crops.
Hemists--Soil scientists recognize three suborders of histosols, based on degree of change of plant material. With a map of this size, however, it is difficult to separate the three suborders. 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.
ORTHODS are forested soils with a sandy texture. They are frequently found in small areas that are downslope from a summit. Orthods are covered by forests of aspen or pine and are rarely used for agriculture because of their acid, droughty nature.
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 shrink-swell capacity of the soil. Internal water movement is also very slow. They are used to grow sugarbeets, sunflowers and small grains in northwestern Minnesota.
Estimated area of each order and suborder of soil in Minnesota, and the percent of Minnesota it occupies.
Selected references and readings
Additional information about the formation, and classification of soils and the vegetation of Minnesota can be found in the following references:
Baker, D.G. and E.L. Kuehnast. 1978. Climate of Minnesota, Tech. Bulletin 314, Ag. Expt. Stat., Univ. of Minn., St. Paul, MN.
Baker, D.G, and J.H. Strub,Jr.. 1965. Climate of Minnesota, Tech. Bulletin 248, Ag. Expt. Stat., Univ. of Minn., St. Paul, MN.
Buol, S.W., F.D. Hole, and R.I. McCracken. 1981. Soil Genesis and Classification. 2nd ed. Iowa State University Press, Ames. 404 pp.
Heinselman, M.L. 1974. Interpretation of Francis J. Marschner's map of the original vegetation of Minnesota. 1:500,000 map and text. USDA Forest Service, North Central Forest Experiment Station, St. Paul, MN.
Sims,P.K. and G.B. Morey, eds. 1972. Geology of Minnesota: A Centennial Volume. University of Minnesota, Minnesota Geological Survey, St. Paul. 632 pp.
Staff, USDA Natural Resources Conservation Service. 1999. Soil Taxonomy. Second Edition. U.S. Department of Agriculture Handbook 436. U.S. Government Printing Office, Washington, D.C.
This publication is based on the observations and ideas of soil scientists from Minnesota both past and present, including scientists from the Natural Resources Conservation Service and the University of Minnesota. Without their contributions this publication would not be possible. Special thanks to J.F. Cummins for the initial collection and summarization of the data.