Plant and Soil Sciences eLibrary

6.4 - Soil Orders

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

(Images 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 - Continental Areas.

Map Legend


North America	Europe/West Asia	East Asia/Japan

South America	Africa/Middle-East	Australia/Indonesia

(Images 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

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This project was supported in part by the National Research Initiative Competitive Grants CAP project 2011-68002-30029 from the USDA National Institute of Food and Agriculture, adminstered by the University of California-Davis and by the National Science Foundation (NSF), Division of Undergraduate Education, National SMETE Digital Library Program, Award #0938034, administered by the University of Nebraska. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the USDA or NSF.

Lesson Outline

Soil Genesis and Development, Lesson 6 - Global Soil Resources and Distribution

6.4 - Soil Orders

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