The Five Soil Forming Factors
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Soils form through a complex interaction of many factors. However, these processes can be grouped into five general categories known as the five soil forming factors: parent material, climate, biota, topography, and time.
Classification of Parent Materials (top)
- Residual parent material or residuum:
Unconsolidated, weathered mineral material that accumulates in place as bedrock is broken down via chemical and physical weathering processes (SSSA, 1997)
- Transported Parent Material:
Colluvial parent material or colluvium
Is unconsolidated, unsorted earth material (soil and rock) that has been transported and deposited on sideslopes or at the base of a slope by mass movement (e.g., gravity) and by local, unconcentrated runoff (SSSA, 1997).
Alluvial parent material or alluvium
Sediments deposited by running water of streams and rivers. May be located in present flood plains, on stream terraces (old flood plains) above present stream level, or as a fan at the base of a slope (SSSA, 1997)
Lacustrine parent material
Sediments and chemical precipitates deposited in lakes that have been exposed at the Earth's surface through lake drainage occurring by human activities or geologic events.
Eolian parent material
Earth material transported and deposited by the wind.
Eolian sand consists of well-sorted sands transported and deposited by the wind, and these materials are the main component of sand dunes. The Sand Hills of Nebraska are an example of eolian sand deposits.
Loess consists of silt-sized particles transported and deposited by the wind. Loess deposits can be found throughout Missouri, and the origin of these silts is attributed to glacial materials that were blown across the state by wind.
- Organic parent material
Organic deposits (peat) that have accumulated in wetlands. Organic matter accumulates in these environments because plant litter decomposition is limited by oxygen availability.
Parent Materials of Missouri
Parent material is the unconsolidated weathered mineral or organic matter from which soil is developed (SSSA, 1997). In other words, it is the initial starting material that is modified through physical, chemical, and biological processes to form soil. Due to the wide variety of parent materials, a classifiecation system has been developed to categorize these materials.
- Sedimentary Rocks
- Alluvial and Colluvial Deposits
- Glacial Drift
- Eolian Deposits
Climate (precipitation and temperature) (top)
Climate is one of the most important soil forming factors due to its widespread influence both geographically and on other soil forming factors (see Biota below). Precipitation and temperature are the principle climatic variables influencing soil formation (Brady and Weil, 1999).
Precipitation provides water that drives nearly all chemical weathering reactions occurring in soils. Soil water percolating through a soil profile transports soluble and suspended materials from surface horizons to subsurface horizons. Percolating soil water exiting a soil as drainage water may also remove soluble materials (e.g., salts and nutrients) from a soil profile. Soil water, therefore, induces weathering reactions resulting in different soil horizons (Brady and Weil, 1999).
Temperature has a significant influence on soil weathering processes and soil formation. As temperature increases, biological activity is stimulated and chemical reactions can proceed at a more rapid rate.
Organic Material and Climate of the United States
When coupled together, both precipitation and temperature can have a profound impact on soil formation in a particular geographic location. For example, soil moisture content and temperature influence soil organic matter content by affecting plant growth and litter decomposition.
Humid climates encourage the growth of trees (Photo courtesy of Garry Gunder)
Grass is the dominate vegetation in subhumid and semiarid climates (Photo courtesy of Keith Goyne)
Cacti, shrubs, and brush dominate arid climates (Photo courtesy of Keith and Jennifer Goyne)
Biota (microorganisms, vegetation, animals, and humans) (top)
Bacteria, fungi, arthropods, and mammals enhance the availability of nutrients through bioturbation (digging and aeration of the soil) and decomposition of organic residue.
Topography (slope, aspect, and land position) (top)
Topography influences the movement of water in the landscape and soil temperature. Soils positioned higher in the landscape are generally more well-drained than those in lower landscape positions, due to the movement of water downslope to lower lying areas. This movement of water also influence erosional processes that result in soils on steep slopes being thin or shallow, whereas soils in valleys are often deep (Plaster, 1992). Additionally, aspect of a landscape position has a strong influence on soil temperature. South-facing slopes in the northern hemisphere are generally warmer, less moist, and contain lower amounts o f soil organic matter than north-facing slopes.
Time (period of time since parent material was exposed to soil formation) (top)
The period of time since a parent material has been exposed has a profound influence on soil formation and development. The longer a parent material is exposed the greater length of time for physical, chemical, and biological processes to act upon the soil, thereby influencing properties of a soil. For example, under favorable conditions an A horizon could form in 10-20 years, whereas it could require centuries for the development of a B horizon (Brady and Weil, 1999).