Five factors of soil formation
Scientists attribute soil formation to the following factors: Parent material, climate, biota (organisms), topography and time.
These factors interact to form more than 1,108 different soil series in Minnesota. The physical, chemical and biological properties of the different soils can have a big effect on how to best manage them.
The five factors
Minnesota is a land of geologically young soils with many different parent materials (Figure 1). The common factor among Minnesota soils is that they were formed by the last glacier in the northern United States, 11,000 to 14,000 years ago.
This may seem like a long time but is considered recent in the context of soil formation and geology. Figure 1 lists five major parent materials: Till, loess, lacustrine, outwash and till over bedrock.
Till is predominant in the south-central, west-central and southwestern parts of the state. As the last glacier was melting, these materials were deposited.
Soils formed in this material generally have silty clay loam to silty clay textures, many different rock sizes and poor internal drainage. The poor drainage has a large influence on nitrogen management and cultural practices.
Loess is windblown, silt-sized material deposited after the glacier melted. These silt deposits can range in depth from a few inches to many feet. Soils formed in loess generally have silt loam textures and no rocks.
Most soils formed in loess occur in southeastern Minnesota where the loess deposits are on top of limestone or sandstone. Because of the porous state of the underlying materials in southeastern Minnesota, the soils are generally well-drained.
Loess in southwestern Minnesota is deposited over glacial till. Soils formed in this material are generally poorly drained and behave similarly to soils formed in glacial till. Erosion is a major concern for these soils because of the silt loam texture. Residue management becomes an important factor in maintaining high productivity.
Lacustrine parent materials result from sediment deposited in lakes formed by glacial meltwater. The lakes existed long enough that the large particles, such as rocks and sand, were deposited immediately after the lake was formed, while the smaller clay-sized particles were deposited later.
An example is the soil formed under Glacial Lake Agassiz in northwestern Minnesota and eastern North Dakota (Red River Valley of the North). Soils formed in lacustrine deposits have clay, clay loam and silty clay loam textures, poor internal drainage and no rocks. Many soils in northwestern Minnesota were formed in lacustrine material.
Outwash is material deposited on the edges of fast-running rivers from the melting ice of receding glaciers. This includes rocks, gravel, sand and other materials large enough to drop out of the water flow, as the river current continued transporting smaller particles.
Soils formed in outwash are excessively well-drained and have sand and sandy loam textures. Examples of Minnesota areas with soils formed in outwash include the Anoka Sand Plain, North Central Sands and Bonanza Valley regions in east-central, north-central and central Minnesota, respectively.
Till over bedrock
Till bedrock deposits occur in northeastern Minnesota. Materials from the glacier were deposited over bedrock, similar to south-central Minnesota but with material from different glacial ice.
There are also significant areas of soils formed directly from bedrock. These soils tend to be shallow and aren’t extensively used for crop production.
Temperature and precipitation
Temperature and precipitation influence how fast parent materials weather and, thus, soil properties such as mineral composition and organic matter content.
Temperature directly influences the speed of chemical reactions. The warmer the temperature, the faster reactions occur. Temperature fluctuations increase physical weathering of rocks.
Precipitation governs water movement in the soil. The amount of water the soil receives and the amount of evapotranspiration that occurs influence water movement. Normal annual precipitation in Minnesota is the least in the northwest corner at 16 inches, and increases as you go toward the southeast corner, where 34 inches is the normal annual precipitation (Figure 2).
Evapotranspiration is the combination of water evaporated from the soil surface and water transpired by growing plants. As air temperatures increase, evapotranspiration increases. High evapotranspiration relative to precipitation means less water is available to move through the soil.
In Minnesota, the greatest evapotranspiration occurs in the southwestern part of the state and decreases as you go toward the northeastern corner.
A leaching index or moisture index (Figure 3) is calculated by subtracting evapotranspiration from precipitation. This index is an indicator of average soil moisture conditions.
The greater the index, the more soil moisture is present. Higher soil moisture increases chemical weathering and moves minerals, such as bases, deeper into the soil profile. This affects management practices such as drainage and inputs of mobile nutrients.
Biotic agents have greatly affected the soil formation process. These include organisms that live in the soil, such as bacteria and gophers, and vegetation growing on the surface.
Organisms in the soil can speed up or slow down soil formation. For example, microorganisms can facilitate chemical reactions or excrete organic substances to improve water infiltration in the soil. Other organisms such as gophers slow soil formation by digging and mixing soil materials, and destroying soil horizons that have formed.
Minnesota soils have been formed under two major types of vegetation: Forest and prairie.
Soils formed under forests tend to be more weathered (older in soil terms) because forests grow in higher rainfall areas. There’s more water movement in the root zone, and a smaller amount of organic matter forms.
Soils formed in prairie tend to be in areas with less precipitation. Grasses tend to use the provided moisture, reducing the water movement through the soil profile. Organic matter forms in large quantities and to a deeper depth in the soil surface than forest soils.
Regional differences in vegetation
Figure 4 shows the different vegetations soils were formed in. The soils in the southwestern, south-central and western parts of the state were formed in prairie. The soils in the northeastern part of the state were formed under forest vegetation.
The savannah between the forest and prairie is a transitional area known as an ecotone. Prairie and forest vegetation existed in this area, changing between forest and prairie as climate changed over time. Forest vegetation would creep into the prairie in wetter climates, while events such as fires changed forested areas to prairie.
Slope and aspect are two topography features that affect soil formation.
Slope refers to steepness (in degrees or percent) from horizontal, which affects how much soil material is deposited or eroded. Level soil is the most developed, as it doesn’t lose or gain material. It’s the change in material that slows the soil-forming process.
Aspect is the direction the slope faces relative to the sun (compass direction), which affects the amount of water that moves through the soil.
The north side tends to have more water because there’s less evaporation and, as a result, potentially more vegetation. In addition, the north aspect’s colder soil temperatures slow soil chemical processes. A soil with a southern aspect tends to have grass vegetation, warmer soil temperatures and more evaporation.
The net effect is more soil aging with a northern aspect compared to soil with a southern aspect, even with the cooler soil temperatures.
Soils across a landscape
In a landscape, a sequence of soils with different horizons caused by differences in their depth to the water table is called a catena.
A catena normally consists of four soil series, with soils located on the summit, shoulder, backslope and footslope as shown in Figure 5.
Drainage and water table depth
For each soil series, here’s how drainage is characterized and how deep the water table is:
Summit: Well-drained, with the water table more than 4 feet below surface.
Shoulder: Moderately well-drained, with the water table between 3 and 4 feet below surface.
Backslope: Somewhat poorly drained, with the water table between 2 and 3 feet below surface.
Footslope: Poorly drained, with the water table less than 2 feet below surface.
In this group of soils, the summit and backslope are the most developed. If the backslope has a slope greater than 20 percent, it’ll erode and be less developed than the summit. The summit is level so there’s no erosion to slow soil development.
The shoulder is eroded, slowing development. Development also slows with the footslope because it’s subject to a considerable amount of soil deposition. Poor drainage further slows development, as water doesn’t move through the soil and soil temperatures tend to be cooler.
The footslope soil in a catena generally is the least developed or youngest in the group. An example of a catena in Minnesota consists of the Clarion, Nicollet, Webster and Glencoe soil series.
Time is the fifth factor in soil formation. Over time, vegetation and climate act on parent material and topography. Development, not chronological age, determines a soil’s age.
The degree of aging depends on the intensity of the other four soil-forming factors. Factors that slow soil formation include:
High lime content in parent material.
High quartz content in parent material.
High clay content in parent material.
Hard rock parent material (resistant to weathering).
High water table.
Constant deposition, accumulations and mixing by animals or man.
Soil horizons and series
These five soil-forming factors have different influences, causing different soil horizons to form.
Scientists use the differences or similarities of soil horizons to categorize similar soils into soil series. The properties of each soil series influence soil management decisions.
Soil horizons are horizontal bands or layers in the soil profile. The main horizons, called master horizons, are O, A, E, B, C and R.
Horizons and characteristics
The O horizon is an organic horizon with little mineral material. It can be found in forest soils, when leaves or needles that fall on the ground form a thin organic layer. In old sedge areas and peat bogs, the organic horizon can be 30 to 60 inches thick. The rest of the horizons are predominantly composed of mineral materials.
The A horizon is normally found at the surface. It’s a zone of organic matter accumulation, with up to 10 percent organic matter. Because of the organic matter, it’s darker in color. In a good soil, the soil structure is granular.
The E horizon is normally found in forest landscapes. It’s found in the horizon just below the A horizon, where the organic matter, clay particles and other chemicals have been moved into. E horizons tend to be light-colored (gray to white) and have a platy structure.
The B horizon is a subsoil horizon that’s a zone of accumulation. It accumulates material including clay, organic matter and other chemicals. The B horizon usually has a blocky structure.
The C horizon is a zone in the subsoil that has little structure or little development. In many Minnesota soils, the C horizon is similar to the parent material.
The final master horizon is the R horizon, which is made of rock.
The number of horizons in a soil is indicative of its developmental age. Minnesota soils are young compared to the rest of the world—only 10,000 to 14,000 years old. Soils formed under forest vegetation in Minnesota tend to be more developed than soils developed under prairie.
Forest soils typically have A, E, B and C horizons, and you’ll usually see them in the northeastern and southeastern parts of the state. If the soils have been farmed, the E horizon may be destroyed, but the organic matter content will be lower.
Prairie soils generally have a thick, dark A horizon (greater than 10 inches), as well as B and C horizons. These soils are found in the southern and western parts of Minnesota. Soils formed on the state’s sand plains have an A and C horizon, and sometimes a weakly formed B horizon.
A soil profile is a vertical exposure of the soil that reveals the combination and types of horizons. The combination of master horizons, thickness of the horizons, and sequence in which they occur in the profile can cause different chemical, biological and physical properties in each soil.
Soils with similar profile characteristics are grouped together into named soil series. Knowing the different soil series allows you to group or separate them for management purposes.
Example: Management differences
The master horizons for the two soils in Figure 6 differ in thickness. The soil on the left was formed in a footslope position of the landscape. It has a very thick A horizon, a thin B horizon and a water-saturated C horizon.
The soil on the right was formed on the slope’s shoulder. Even though it’s only 400 feet from the soil on the left, it has much different soil horizons. The soil on the right has a thinner A horizon and a thicker B horizon than the soil on the left. The water table is much deeper in the profile, indicating a better-drained soil on the right than on the left.
Because these soils formed differently, you should manage them differently. An example of management differences could be that the soil on the left should be tile-drained for optimum crop production, while the soil on the right may not need tile drainage.
Anderson, J.L., Bell, J.C., Cooper, T.H., & Grigal, D.F. (2018). Soil orders and suborders in Minnesota.
Minnesota Department of Natural Resources. (2005). Field guide to the native plant communities of Minnesota: The eastern broadleaf forest province. St. Paul, MN: Minnesota Department of Natural Resources.
Reviewed in 2018