5.1 - USDA Soil Classification System

Goals: To consider the classification of soils at the broadest taxonomic level (USDA system) and to identify dominant environmental factors that contribute to soil formation at this level on a regional basis.

Humans tend to classify and categorize almost everything we encounter in our natural world. From rocks to soils, from landscapes to living things on the land and in the water, we have systems of classification to describe these things in uniform terms.  These systems then enable us to communicate with each other about these subjects in terms that are understandable and consistent. Classification systems and taxonomic conventions allow us to describe a thing or phenomenom in a way that can then be understood by those in remote locations and without direct experience of the subject.

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 (Principles 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.

Thinking questions to prepare for the lesson:

Question 1:  

a.  What types of classification systems are used for things that you are familiar with? (e.g., cars, homes, areas of a department store, or food groups)  

b.  How would your life be different without these everyday classification schemes?

Question 2:  How could a map of soil types and properties be useful to:

• a land developer?
• a construction site manager?
• a landscape designer?
• an agricultural producer?

Soil taxonomic classifications reflect the dominant Soil Forming Factors active during soil formation at a particular location.  The USDA system of Soil Taxonomy (soil naming) consists of a hierarchy of six levels. These levels, in order from most general to most specific, are:

• Order
• Suborder
• Great Group
• Subgroup
• family
• Series

This system of Soil Taxonomy is comparable to the Linnean system used in biology to classify living things (kingdom, phylum, class, order, family, genus, and species).  

This system of soil classification provides information that can be used by land managers to make inferences regarding a particular soil’s utility for plant production, urban/residential usages, waste management, and construction sites.

The system of soil classification used by the National Cooperative Soil Survey has six categories (Soil Survey Staff, 1999). Beginning with the broadest, these categories are the Order, Suborder, Great Group, Subgroup, family, and Series.  These categories are defined in the following paragraphs.

Order – Twelve soil orders are recognized. The differences among orders reflect the dominant soil forming processes and the degree of soil formation. Each order is identified by a word ending in 'sol.' An example is Alfisols.

Suborder - Each order is divided into suborders primarily on the basis of properties that influence soil formation and/or are important to plant growth.

Great Group – Each suborder is divided into great groups on the basis of similarities in horizons present, soil moisture or temperature regimes, or other significant soil properties.

Subgroup – Each great group has a ‘typic’ (typical) subgroup which is basically defined by the Great Group. Other Subgroups are transitions to other orders, suborders, or great groups due to properties that distinguish it from the great group.

Family – Families are established within a subgroup on the basis of physical and chemical properties along with other characteristics that affect management.

Series – The series consists of soils within a family that have horizons similar in color, texture, structure, reaction, consistence, mineral and chemical composition, and arrangement in the profile.

For a more complete description of the soil classification system, go to the USDA-NRCS website.