Five factors of soil formation
On this page
Factors of soil formation
Soil scientists attribute the formation of soil to five factors: parent material, climate, biota (vegetation), topography, and time (1). In Minnesota, these factors combine to form over 1108 different soil series. The physical, chemical, and biological properties of the different soils can have a big effect on how to best manage them.
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 as a product of the last glacier occurrence in the Northern United States 11 to 14 thousand years ago. While to humans this length of time seems long, in the context of soil formation and geology this is considered recent. 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. These materials were deposited as the last glacier was melting. Soils formed in this material generally have silty clay loam to silty clay textures, many different sizes of rocks, and poor internal drainage. The poor drainage has a large influence on nitrogen management and cultural practices.
Loess is wind blown silt-sized material that was deposited after the glacier melted. These silt deposits can range in depth from a few inches in thickness to many feet deep. The soils formed in loess generally have silt loam textures and no rocks. The majority of 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 are a result of 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 size 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. A large number of 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 glaciers as they recede. These materials are large in size; rocks, gravel, and sand. The materials were large enough to drop out of the water flow while smaller particles continued to be transported in the current of the river. Soils formed in outwash are excessively well drained and have sand and sandy loam textures. Examples of areas in Minnesota with soils formed in outwash include the Anoka Sand Plain, North Central Sands, and the 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 material came from different glacial ice. There are also significant areas of soils formed directly from bedrock. These soils tend to be shallow and are not used extensively for crop production.
Climate is an important factor in soil formation. Temperature and precipitation influence the speed of weathering of parent materials 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. Fluctuations in temperature increase physical weathering of rocks. Precipitation governs water movement in the soil. Water movement is influenced by the amount of water the soil receives and the amount of evapotranspiration that occurs. Normal annual precipitation in Minnesota is the least in the Northwest corner at 16 inches and increases as you go to the southeast corner where the normal annual precipitation is 34 inches (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 to 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.
The soil formation process has been greatly affected by biotic agents. Biotic agents include organisms such as bacteria and gophers that live in the soil and vegetation growing on the surface. Organisms in the soil can speed up or slow down soil formation. Microorganisms can facilitate chemical reactions or excrete organic substances to improve infiltration of water in the soil. Other organisms, such as gophers, slow soil formation by digging and mixing soil materials, destroying soil horizons that have formed.
Vegetation has a large effect on soil formation. Minnesota soils have been formed under two major types of vegetation; forest or prairie. Soils formed under forests tend to be more weathered (older in soil terms) because forests grow in higher rainfall areas. [More water movement in the root zone and the amount of organic matter formed in forest systems is less in quantity.] Soils formed in prairie, tend to be located in areas of less precipitation, grasses tend to use the moisture provided reducing the water movement through the soil profile and form organic matter in large quantities and to a deeper depth in the soil surface than forest soils.
Figure 4 shows the different vegetations soils were formed in. The soils in the southwestern, south central, 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. In this area prairie and forest vegetation existed and changed between forest and prairie. This occurred as climate changed over time. In wetter climates the forest vegetation would creep into the prairie while events such as fires changed forested areas to prairie.
Slope and aspect are two features of topography that affect soil formation. Slope refers to steepness (in degrees or percent) from horizontal and aspect is the direction the slope faces relative to the sun (compass direction). The steepness of the slope affects the amount of deposition or erosion of soil material. A soil that is level is the most developed as there is no loss or gain of material to slow the soil forming process. The aspect of the slope affects the amount of water that moves through the soil. The north side tends to have more water because of less evaporation and therefore may have more vegetation. The soil chemical processes are slowed on the north aspect because of the colder soil temperatures. 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 when compared to soil with a southern aspect, even with cooler soil temperatures.
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. The soils are located on the summit, shoulder, backslope, and footslope as shown in Figure 5. The drainage or depth to water table is well drained (water table greater 4 foot below surface) for the summit, moderately well drained (water table between 3 and 4 feet below surface) on the shoulder, somewhat poorly drained (water table between 2 and 3 feet below surface) on the backslope, and poorly drained (water table less than 2 feet below surface) on the footslope. The summit and backslope are the most developed soils in this group of soils. If the backslope has a slope greater than 20%, it will erode and be less developed than the summit. The summit is level so there is no erosion to slow soil development. The shoulder is eroded and development is slowed. The footslope is subject to a considerable amount of soil deposition slowing development. The poor drainage also slows development as water does not 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. Vegetation and climate act on parent material and topography over time. The age of a soil is determined by development and not chronological age. Degree of aging depends on 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), low rainfall, low humidity, cold temperature, steep slopes, high water table, severe erosion, constant deposition, accumulations, and mixing by animals or man.
Soil master horizons
Soil horizons are horizontal bands or layers in the soil profile. The main horizons are called Master horizons: O, A, E, B, C, and R.
The O horizon is an organic horizon with little mineral material. The O horizon can be found as thin layers in forest situations. The leaves or needles that fall on the ground form a thin organic layer. In old sedge areas, peat bogs, the organic horizon can be 30 to 60 inches thick. The rest of the horizons are composed predominantly of mineral materials.
The A horizon is normally found at the surface. It is a zone of organic matter accumulation. It has up to 10 percent organic matter and because of the organic matter, is darker in color. The soil structure in a good soil is granular.
The E horizon is normally found in forest landscapes. It is found just below the A horizon and is a horizon where the organic matter, clay particles, and other chemicals have been moved into the horizon below it. E horizons tend to be light colored (gray to white) and have a platy structure.
The B horizon is a subsoil horizon that is a zone of accumulation. The materials accumulated include 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 a large number of soils in Minnesota, the C horizon is similar to the parent material.
The final Master horizon is the R horizon. This is a horizon that is made up 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 – 10,000 to 14,000 years. 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. You will see these 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), B, and C horizons. These soils are found in the southern and western parts of Minnesota. Soils formed on the sand plains of Minnesota have an A and C horizon and sometimes a weakly formed B horizon.
Figure 6. Two soil series: The left soil was formed at the footslope (Webster soil series) while the soil on the right was formed on the shoulder (Clarion soil series).
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. Knowledge of the different soil series allows them to be grouped together or separated for management purposes.
The master horizons for the two soils in Figure 6 are different 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 shoulder of the slope and even though it is only 400 feet from the soil on the left, it has much different soil horizons. The A horizon is thinner in the soil on the right than the soil on the left, while the B horizon is thicker on the right than the B horizon of 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. These two soils should be managed differently because of the difference in formation. 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.
Soils are formed by the interaction of five soil forming factors. They are parent material, climate, biota (organisms), topography, and time. The different influences of these factors cause different soil horizons to form. Differences or similarities of soil horizons are used to categorize similar soils into soil series. Soil management decisions are influenced by the properties of each soil.
- Anderson, J.L., J.C. Bell, T.H. Cooper, and D.F. Grigal. 2001. Soils and landscapes of Minnesota. University of Minnesota publication. Available at www.extension.umn.edu/soils/soil-properties/soils-and-landscapes-of-minnesota.
- Minnesota Department of Natural Resources. 2005. Field Guide to the Native Plant Communities of Minnesota: The eastern broadleaf forest province. MNDNR, St. Paul, MN. See http://www.dnr.state.mn.us/ecs/index.html Verified October 31, 2016