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Soil Genesis and Development, Lesson 5 - Soil Classification and Geography

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5.2 - Soil Orders

Soil Orders

The most general level of classification in the USDA system of Soil Taxonomy is the Soil Order. All of the soils in the world can be assigned to one of just 12 orders (Table 5.1). Soil orders are frequently defined by a single dominant characteristic affecting soils in that location, e.g., the prevalent vegetation (Alfisols, Mollisols), the type of parent material (Andisols, Vertisols), or the climate variables such as lack of precipitation (Aridisols) or the presence of permafrost (Gelisols). Also significant in several soil orders is the amount of physical and chemical weathering present (Oxisols, Ultisols), and/or the relative amount of Soil Profile Development that has taken place (Entisols).

This lesson will examine each of these 12 soil orders in turn: Entisols, Inceptisols, Andisols, Mollisols, Alfisols, Spodosols, Ultisols, Oxisols, Gelisols, Histosols, Aridisols, and Vertisols. To get the most out of this lesson, the student should carefully study each soil order, including the supplementary material provided by the embedded links.

 

 

Table 5.1 - Soil Orders and General Descriptions
 Type Description  Type Description
Entisols Little, if any horizon development Inceptisols Beginning of horizon development
Aridisols Soils located in arid climates Mollisols Soft, grassland soils
Alfisols Deciduous forest soils Spodosols Acidic, coniferous forest soils
Ultisols Extensively weathered soils Oxisols Extremely weathered, tropical soils
Gelisols Soils containing permafrost Histosols Soils formed in organic material
Andisols Soil formed in volcanic material Vertisols Shrinking and swelling clay soils

 

 

Figure 5.1  Dominant Soil Orders of the United States.  Image courtesy of USDA-NRCS.
 

 

Question 4: The soil order which includes only the least developed soils is the...

A. Spodosol Order
B. Aridisol Order
C. Histosol Order
D. Entisol Order
E. Mollisol Order


Question 5: The soil order that results from the interaction of grassland vegetation and climate is the...

A. Spodosol Order
B. Aridisol Order
C. Histosol Order
D. Entisol Order
E. Mollisol Order



 
Question 6: Soils of the __________ are formed mostly from organic materials and are often found in cool, wet climates.

A. Vertisol Order
B. Aridisol Order
C. Inceptisol Order
D. Entisol Order
E. Histosol Order

 
Question 7: The soil order which is found mostly in very dry climates is the __________.


A. Vertisol Order
B. Aridisol Order
C. Inceptisol Order
D. Entisol Order
E. Histosol Order

 

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