How many soil orders are there?

USDA Soil Taxonomy recognizes twelve soil orders, including Mollisols, Alfisols, Ultisols, Oxisols, Aridisols, Entisols, Inceptisols, Histosols, Vertisols, Andisols, Spodosols, and Gelisols, each defined by characteristic diagnostic horizons and properties.

Why are there competing soil classification systems?

Different countries developed systems suited to their own soils and traditions; USDA Soil Taxonomy and the international World Reference Base are the two most widely used, and the World Reference Base exists specifically to correlate soils described under different national systems.

Soil Taxonomy and Classification

Soil classification organizes the diversity of soils into named groups based on measurable diagnostic properties, allowing soils to be identified, mapped, and compared consistently across the world.

Definition

Soil taxonomy is a hierarchical classification of soils into categories, from broad orders down to series, defined by diagnostic horizons and quantitative soil properties so that any soil can be consistently named and placed.

Scope

This topic covers the major soil classification systems, especially USDA Soil Taxonomy with its twelve soil orders and the international World Reference Base, the diagnostic horizons and properties used to define classes, and the role of classification in soil survey and communication. It is the organizing framework that names the products of soil formation.

Core questions

What diagnostic horizons and properties define soil classes?
What are the twelve soil orders of USDA Soil Taxonomy?
How do USDA Soil Taxonomy and the World Reference Base differ?
How is classification used in soil survey and mapping?

Key concepts

Diagnostic horizons and properties
Soil orders and the hierarchy of categories
USDA Soil Taxonomy
World Reference Base for Soil Resources
Soil series and mapping units
Soil correlation

Key theories

Diagnostic-horizon classification: Modern soil taxonomy defines classes by quantitatively specified diagnostic surface and subsurface horizons and properties, replacing subjective genetic naming with reproducible criteria that any surveyor can apply.
Hierarchical categories: USDA Soil Taxonomy nests soils into orders, suborders, great groups, subgroups, families, and series, so that names carry increasingly specific information about a soil's properties.
International harmonization (WRB): The World Reference Base provides a globally agreed system of reference soil groups and qualifiers, enabling soils described under different national systems to be correlated and compared internationally.

Clinical relevance

Classification lets soil knowledge be transferred from one place to another: knowing a soil's order or series predicts its fertility, drainage, and management needs, supports land-use planning and crop suitability assessment, and provides the common language used in soil surveys and environmental regulation.

History

Early classifications mixed genetic and descriptive criteria; the comprehensive USDA Soil Taxonomy, developed largely under Guy Smith and first published in 1975, introduced rigorous quantitative diagnostic criteria. The FAO and IUSS later developed the World Reference Base to provide an international correlation system, and both continue to be revised.

Key figures

Guy D. Smith
Nyle C. Brady
Ray R. Weil

Seminal works

soilsurveystaff2014
iuss2015wrb
brady2016

Frequently asked questions

How many soil orders are there?: USDA Soil Taxonomy recognizes twelve soil orders, including Mollisols, Alfisols, Ultisols, Oxisols, Aridisols, Entisols, Inceptisols, Histosols, Vertisols, Andisols, Spodosols, and Gelisols, each defined by characteristic diagnostic horizons and properties.
Why are there competing soil classification systems?: Different countries developed systems suited to their own soils and traditions; USDA Soil Taxonomy and the international World Reference Base are the two most widely used, and the World Reference Base exists specifically to correlate soils described under different national systems.