Soil classification

Amber Anderson

Learning Objectives
  • Understand structure of both NRCS and FAO classification systems
  • Discuss major soils features given a classification at the great group or subgroup level
Keywords: classification, soil order, suborder, great group, subgroup, reference soil groups


As for classification systems of living organisms, classification for soils helps organize our knowledge and communicate important information. Different systems have been developed in different countries, so in this section we will cover the basics of the classification system commonly used in the US, as well as the younger world reference base classification.

Using the system

This system works on the first-fall out principle, accept whatever order you cannot reject first when moving down the list as ordered above.

  • Order-most broad group, 12 options (ex: Mollisol, Entisol)
  • Suborder-adds one more distinct trait, usually related to water/climate (ex: udoll, aquent)
  • Great group-adds another trait for a total of three syllables (ex: hapludoll, fluvaquent)
  • Subgroup– additional trait (ex: Typic Hapludoll, Aeric Fluvaquent)
  • Family-adds temperature, mineralogy, and textural info (Fine-loamy, mixed, superactive, mesic Typic Hapludolls)
  • Series-locally described soil with a range of properties and horizons within a described range: (ex: A Clarion series is a Fine-loamy, mixed, superactive, mesic Typic Hapludoll)

World Reference Base Classification system

View full WRB documentation and versions here

The international classification system, developed more recently than the NRCS system, to create an ‘international units’ for communicating soil properties globally. This system incorporates aspects of several national systems, including the US and Russian systems.

Instead of soil orders, 32 reference soil groups (RSG) are used instead of soil orders. Principal and supplementary qualifiers are used to communicate additional information.


Key Takeaways
  • Soils are classified by major features, generally that impact management
  • Classification helps us to communicate significant amounts of information quickly


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Introduction to Soil Science Copyright © 2023 by Amber Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.