5.2 - Soil Orders | Soil Genesis and Development, Lesson 5 - Soil Classification and Geography

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.*

Quiz

Question

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

A. Spodosol Order

Incorrect: Check Table 5.1 for the descriptions of this soil order.

B. Aridisol Order

Incorrect: Check Table 5.1 for the descriptions of this soil order.

C. Histosol Order

Incorrect: Check Table 5.1 for the descriptions of this soil order.

D. Entisol Order

Correct: Yes, Entisol soils display a wide variety of characteristics, but have an undeveloped soil profile.

E. Mollisol Order

Incorrect: Check Table 5.1 for the descriptions of this soil order.

Looks Good! Correct: Yes, Entisol soils display a wide variety of characteristics, but have an undeveloped soil profile.

Quiz

Question

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

A. Spodosol Order

Incorrect: Check Table 5.1: Soil Orders and their General Descriptions to see which soil order fits this description.

B. Aridisol Order

Incorrect: Check Table 5.1: Soil Orders and their General Descriptions to see which soil order fits this description.

C. Histosol Order

Incorrect: Check Table 5.1: Soil Orders and their General Descriptions to see which soil order fits this description.

D. Entisol Order

Incorrect: Check Table 5.1: Soil Orders and their General Descriptions to see which soil order fits this description.

E. Mollisol Order

Correct: Yes, The soft, dark topsoil which is a common characteristic of Mollisol is a result of long periods of grassland vegetation. Grasslands are common in semi-arid climates that have fires often enough to burn out woody vegetation.

Looks Good! Correct: Yes, The soft, dark topsoil which is a common characteristic of Mollisol is a result of long periods of grassland vegetation. Grasslands are common in semi-arid climates that have fires often enough to burn out woody vegetation.

Quiz

Question

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

A. Vertisol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

B. Aridisol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

C. Inceptisol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

D. Entisol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

E. Histosol Order

Correct: Yes, Histosol soils form faster than they decay in the decomposed plant remains that accumulate in wetlands or forests of cooler climates.

Looks Good! Correct: Yes, Histosol soils form faster than they decay in the decomposed plant remains that accumulate in wetlands or forests of cooler climates.

Quiz

Question

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

A. Vertisol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

B. Aridisol Order

Correct: Yes, The defining property of Aridisols is moisture deficiency, and they exist in regions of low rainfall.

C. Inceptisol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

D. Entisol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

E. Histosol Order

Incorrect: Refer to Table 5.1 for the description of this soil order.

Looks Good! Correct: Yes, The defining property of Aridisols is moisture deficiency, and they exist in regions of low rainfall.