The Physical Environment

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Soil Systems


Mollisols are among the most fertile soils on the Earth. Born under grassland vegetation, these soils are well-known for their dark brown to black organic rich surface layers. These soils have a granular structure and soft consistency when dry. Mollisols are rich in calcium and others nutrients, and generally posses high moisture retention.  Calcium nodules are found near the base of the soil as calcium carbonate precipitates out of soil water. Mollisols are found in the drier portions of the humid continental climate through the steppe climate.

Figure 11.27 Mollisol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)



Spodosol soil is commonly found in cool, moist environments under coniferous forest vegetation. Surface litter composed of pine needles breaks down in the presence of water to form a weak organic acid. Acidic soil water removes base ions in solution to create an acidic soil. Easily dissolved materials are leached from surface layers leaving behind the most resistant material like quartz, creating an ashy-gray near-surface layer. Layers at depth are stained with iron and aluminum oxides.


Figure 11.28 Spodosol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)



AlfisolAlfisols are soils developed under temperate forests of the humid midlatitudes. Eluviation is moderate and base status is fairly high in these soils. Common to the humid continental (like the one in Michigan on the left) and humid subtropical climates, these soils are well-developed and contain a subsurface layer of clay called an argillic horizon. Some alfisols are found in the wet/dry tropical climate of Africa, South America, Australia, and Southeast Asia. Having a favorable moisture balance and good fertility, they are very productive soils for agriculture. In fact, they have been successfully used for farming in China and Europe for thousands of years. Alfisols are abundant on older glacial deposits in the United States, and loess deposits in and near the Mississippi embayment.


Figure 11.29 Alfisol 
Click image to enlarge (Image Source:  USDA NRCS Used with permission)



Ultisols share many of the same properties as Oxisols. Highly weathered soils, they are often red/yellow in color reflecting the oxidation of iron and aluminum. Found in the moister portions of the Humid Subtropical climate, they have a illuvial clay layer which distinguishes them from Oxisols which do not.



Figure 11.30 Ultisol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)




Oxisol soil is found in warm, rainy climates under broadleaf, evergreen vegetation like that found in the rain forest. Chemical weathering  (especially oxidation) in the presence of warm temperatures combined with heavy rainfall creates a soil rich in iron and aluminum oxides called "sesquioxides". A rich diversity of decomposers, rapid uptake by vegetation, and heavy precipitation quickly removes nutrients from the soil. What is left is a nutrient poor soil, not well-suited for agriculture. Cleared of vegetation, the exposed surface is easily eroded.


Figure 11.30 Oxisol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)


Self Assessment 

Which of the following soil forming processes created this oxisol?

Choose one of the answers below by clicking a button




gelisolGelisol soil develops on permafrost and common to the tundra. These soils consist of mineral or organic material, or both and have experienced cryoturbation (frost churning) due to annual freeze-thaw cycles. 


Figure 11.31 Gelisol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)


Assess your basic understanding of the preceding material by "Looking Back: Soil Orders" or skip and continue reading.

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For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography.
2006. Date visited.  ../title_page.html

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