5 Factors that Influence Soil Formation
Classification of Parent Materials (geologic and organic material)
1. Residual parent material or residuum
2. Transported Parent Material
Are 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)
Are 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.
Earth material transported and deposited by the wind.
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.
3. Organic parent material
Parent Materials of Missouri
(Click here to view Missouri map of dominant soil orders.)
1. Sedimentary Rocks
2. Alluvial and Colluvial Deposits
3. Glacial Drift
4. Eolian Deposits
Climate (precipitation and temperature)
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 (see weathering slideshow on page one) 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 as 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.
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.
Organic Material and Climate of the United States
Click the icons to the right to view images.
Photos courtesy of Garry Gunder
2. Grass is the dominate vegetation in subhumid and semiaried climates
Photo courtesy of Garry Gunder
3. Shrubs and brush dominate arid climates
Photos courtesy of Keith and Jennifer Goyne
Biota (micro organisms, vegetation, animals, and humans)
Bacteria, fungi, arthropods, and mammals enhance the availability of nutrients
through bioturbation (digging and aeration of the soil) and decomposing organic residue.
Photos courtesy of Dr. Robert Kremer, University of Missouri- Columbia
Topography (slope, aspect, and land position)
Properties of south facing slopes in the northern hemisphere:
1. Warmer temperature
2. Less moist than the north facing slope
3. Lower amounts of organic matter
Topography changes the water movement and soil temperature.
Due to erosion soil is thin on the slopes and the valleys often
have deep soils that are rich and drain slowly (Plaster 1992).
Time (period of time since parent material was exposed to soil formation)
1. Formation of B Horizon could require centuries to form (Brady and Weil, 1999).
2. When favorable, an A Horizon could develop in 10-20 years (Brady and Weil, 1999).
3. Bulldozers mix soil horizons and reset the soil formation process to the beginning.
