The Physical Environment

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Soil Systems

Soil Orders

The are several different ways in which soils have been classified. The brief discussion below reflects the way soils are classified in the United States. Soils are classified on the basis of diagnostic horizons, that are different from the O, A, B, C horizons. A diagnostic horizon has a unique feature that reflects the soil development processes acting at a site. For instance, a mollic epipedon (an epipedon is a  type of diagnostic horizon) is a organic-rich horizon typical of a mollisol soil. The following material is intended to give you a basic understanding of the major categories of soils called soil orders and the environments under which they form. For a comprehensive description of how soils are classified in the United States, see the Soil Taxonomy (only for those of you dying to learn more about soil classification than you ever wanted to know!)

soil orders

Figure 11.20 Global Soil Regions
Click map for high resolution map
(Courtesy NRCS)


EntisolEntisols, like this one found in southwestern Wisconsin, are soils lacking horizons because their parent material has only recently accumulated. Entisols also form where the parent material is quartz sand, in which horizons do not easily form. They have a wide geographic distribution and can be found in any climate and under any vegetation. Entisols and Inceptisols are often found on floodplains, delta deposits, or steep slopes where parent material has difficulty accumulating.

Figure 11.21 Entisol 
Click image to enlarge (Image Source:  USDA NRCS Used with permission)





Inceptisols are soils just starting to show horizon development because the soil is quite young. You can see the differentiation of layers in an inceptisol formed on colluvium in West Virginia on the right. Inceptisols, like Entisols, are found in any type of environment and are commonly found forming in alluvium on floodplains and delta deposits.

Figure 11.22 Inceptisol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)



HistosolHistosols have a very high content of organic matter in the dark upper layer of the profile. Found in many different environments from the tundra to the tropics, Histosols form in places where organic matter is slow to decompose and thus accumulates over time such as bogs and swamps. They are often "mined" for peat which is dried and burned as fuel.


Figure 11.23 Histosol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)



Aridisols are soils of arid and semiarid environments where moisture is scarce. They are typically light in color as there is little vegetation to add organic matter to the soil profile. A negative moisture balance in these soils inhibits eluviation. Calcification and salinization are important soil forming processes acting in these soils. Soil horizons are weakly developed and sodium is often high in concentration making them alkaline. The coarse texture of aridisols makes it difficult to retain much moisture. Aridisols can be quite fertile soil if irrigation is properly used. Used improperly, a salt crust can form on the soil. Most aridisols are used for grazing.

Figure 11.24 Aridisol Soil

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



Figure *.* Desert Soil & Erosion Courtesy BBC


AndisolAndisols are soils developing in parent material containing at least fifty percent volcanic ash. The layers of ash can be seen in this Andisol from Hawaii. Naturally fertile soils, they support a dense natural cover in moist climates. Andisols occur around individual volcanoes created from andesite-rich magma. They are common on the volcanic islands and mountains of "The Ring of Fire", that encircles the Pacific Ocean from North America through Japan.


Figure 11.25 Andisol
Click image to enlarge (Image Source:  USDA NRCS Used with permission)




Vertisols are dark black soils rich in expandable clay minerals. The clay readily swells upon wetting and shrinks when dried. Though found in every type of climate, they are often found in steppe and wet/dry tropical climates where the soil develops deep cracks as it dries. Surface fragments fall into the cracks and are "swallowed" when the soil swells upon wetting. The soil then develops an "inverted profile" with organic material that is typically located near the surface of the profile is now found at depth. 

Figure 11.26 Vertisol
Click image to enlarge (Image Source: Soil Science Division at the University of Idaho Used with permission)


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For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography.
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