6.4 - Soil Orders | Soil Genesis and Development, Lesson 6 - Global Soil Resources and Distribution

Soil properties can vary greatly from one location to the next, even within distances of a few meters. These same soil properties can also exhibit similar characteristics over broad regional areas of like climate and vegetation.

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 12 orders. By surveying soil properties of color, texture, and structure; thickness of horizons; parent materials; drainage characteristics; and landscape position, soil scientists have mapped and classified nearly the entire contiguous United States and much of the rest of the world.

Use Table 6.1 (below) for a brief overview of Soil Orders, or go to Lesson 5, where Soil Orders are described in detail.

Table 6.1 - 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

Global Soil Regions Key and Map. *Image courtesy of the USDA-NRCS, edited by UNL.*

The nature and properties of soils can vary widely from one location to the next, even within distances of a few meters. These same soil properties can also be found to exhibit similar characteristics over broad regional areas of like climate and vegetation. The soil forming factors of parent material, climate, vegetation (biota), topography, and time (Lesson 3.2) tend to produce a soil that describes the environment in which it is formed. By surveying properties of soil color, texture, and structure; thickness of horizons; parent materials; drainage characteristics; and landscape position, soil scientists have mapped and classified nearly the entire contiguous United States and much of the rest of the world.

Global Soil Regions-Orders Key and Map. *Image courtesy of the USDA-NRCS, edited by UNL.*

For descriptions of the 12 Soil orders, see Lesson 5.2

Table 6.2 Percent of World Ice-free Land Area of Each Soil Order.

Soil Order	Estimated % of World Land Area	Approximate Area (million km²)	Soil Order	Estimated % of World Land Area	Approximate Area (million km²)
Entisols	16	21.0	Inceptisols	17	22.3
Aridisols	12	15.7	Mollisols	7	9.2
Alfisols	10	13.1	Spodosols	4	5.2
Ultisols	8	10.5	Oxisols	8	10.5
Gelisols	9	11.8	Histosols	1	1.3
Andisols	1	1.3	Vertisols	2	2.6
Other	5	6.6	Total	100	131

Quiz

Question

Although considered some of the most naturally fertile and productive soils, Mollisols constitute what percent of total world ice-free land area?

A. less than 5%

Incorrect: Check Table 6.2 above for estimated percentages of world land area for the individual soil orders

B. 5 – 10%

Correct: Mollisol soils occur on less than 10% of the total ice-free land area.

C. 10 – 20%

Incorrect: Check Table 6.2 above for estimated percentages of world land area for the individual soil orders

D. 20 – 40%

Incorrect: Check Table 6.2 above for estimated percentages of world land area for the individual soil orders

E. greater than 40%

Incorrect: Check Table 6.2 above for estimated percentages of world land area for the individual soil orders

Looks Good! Correct: Mollisol soils occur on less than 10% of the total ice-free land area.

Question

Entisol and Aridisol soils are commonly found together in arid desert regions. How much of the global ice-free land area is covered by these two soil orders combined?

A. less than 5%

Incorrect: Check Table 6.X above for estimated percentages of world land area for the individual soil orders

B. 5 – 10%

Incorrect: Check Table 6.X above for estimated percentages of world land area for the individual soil orders

C. 10 – 20%

Incorrect: Check Table 6.X above for estimated percentages of world land area for the individual soil orders

D. 20 – 40%

Correct: Correct! Over one-third of the global ice-free land area is classified in these two commonly associated soil orders.

E. greater than 40%

Incorrect: Check Table 6.X above for estimated percentages of world land area for the individual soil orders

Looks Good! Correct: Correct! Over one-third of the global ice-free land area is classified in these two commonly associated soil orders.

Question

Looking at the soil orders maps, which continental area appears to have the greatest single expanse of combined Entisol-Aridisol soils?

A. North America

Incorrect: Check the colors of the Entisol and Aridisol orders on the map legend and look for large areas of their colors combined on the map.

B. South America

Incorrect: Check the colors of the Entisol and Aridisol orders on the map legend and look for large areas of their colors combined on the map.

C. Europe

Correct: The Sahara Desert is a land area equal to the size of the continental United States and is classified as mainly Entisol and Aridisol soils.

D. Asia

Incorrect: Check the colors of the Entisol and Aridisol orders on the map legend and look for large areas of their colors combined on the map.

E. Africa

F. Australia

Incorrect: Check the colors of the Entisol and Aridisol orders on the map legend and look for large areas of their colors combined on the map.

Looks Good! Correct: The Sahara Desert is a land area equal to the size of the continental United States and is classified as mainly Entisol and Aridisol soils.

Question

Use the soil orders maps above to locate regions of Mollisol soils. At what latitudes in the world do Mollisol soils tend to exist to the greatest extent?

A. equatorial regions, 0 – 15 degrees

Incorrect: Check the Global Soil Regions map above to locate where the greatest extent of Mollisol occurs.

B. subtropical regions, 20 – 35 degrees

Incorrect: Check the Global Soil Regions map above to locate where the greatest extent of Mollisol occurs.

C. mid-latitude regions, 35 – 50 degrees

Correct: Mollisol is found frequently in semi-arid, mountain steppe regions of the middle latitudes.

D. subpolar regions, 50 – 70 degrees

Incorrect: Check the Global Soil Regions map above to locate where the greatest extent of Mollisol occurs.

E. polar regions, 70 – 90 degrees

Incorrect: Check the Global Soil Regions map above to locate where the greatest extent of Mollisol occurs.

Looks Good! Correct: Mollisol is found frequently in semi-arid, mountain steppe regions of the middle latitudes.