1. National Soil Survey Center
Helping People Understand Soils
Soil Taxonomy
NRCS
National Soil Survey Center
Soil Survey Standards
Introductory Slide

2. Why is this topic important?
Soil Survey
Helping People Understand Soils
Why is this topic important?
Soil Taxonomy (ST) is the basic language of soil classification for your profession.
You should become proficient in the parts of Soil Taxonomy that apply to the soils in your work area.
Motivational Slide .

3. Objectives You will be able to: Soil Survey
Helping People Understand Soils
Objectives
You will be able to:
List 3 reasons we use Soil Taxonomy
List the 8 attributes of Soil Taxonomy
Describe the general concept for the categorical levels
Use the 2nd edition of ST and the Keys to Soil Taxonomy (KST) properly to classify soils
Objectives

4. Purpose of Soil Taxonomy
Soil Survey
Helping People Understand Soils
Purpose of Soil Taxonomy
Provides a way to group soils that have similar properties and genesis
Groupings made at various map scales
Soil maps of large areas use small map scales (e.g., 1: 7.5 million) and the higher category taxa of order through subgroup
Soil maps of small areas use large map scales (e.g., 1:12,000) and lower category taxa such as family and series
With ~23,500 soil series in the USA, The human mind can’t grasp all of this. Taxonomy allows us to group and sort them in ways we comprehend.
You can find competing series for possible use in your survey.
It provides a rational basis to group soils into progressively fewer classes in the higher levels.
Groupings can be made at various map scales by using different levels of the hierarchy. The higher categories of the system, (i.e., order through subgroup) are usually displayed at small map scales for large areas; the lower categories (i.e., family and series) are usually displayed at large map scales for small areas.
For the most detailed soil surveys, soils are grouped into series level components.
Some order 4 surveys may use families, or phases of higher taxa.
At still smaller scales, soils can be grouped at the great group level.
At small scales, order or suborder can be used.

5. Soil Survey Helping People Understand Soils
This small scale map shows soil orders mapped across the USA.

6. Purpose of Soil Taxonomy
Soil Survey
Helping People Understand Soils
Purpose of Soil Taxonomy
Facilitates communication among soil scientists
Ustert
Aquiturbel
Loamy, mixed, superactive, frigid Lithic Argixeroll
Names of taxa are based on formative elements
The names of soil orders begin with a formative element and end in the Latin root sol (e.g., Aridisols)
Underlying taxa also contain the formative element (e.g., Vertic Natrigypsids)
Uses connotative names that convey:
the position in the taxonomic system
and distinct meanings
Examples:
Ustert – Suborder of Vertisols:
Has cracking pattern typical of a seasonally dry moisture regime (i.e., ustic)
Aquiturbel – Great group:
Permafrost; frost churning; wetness.
Loamy, mixed, superactive, frigid Lithic Argixeroll – Family:
Mollisol with argillic horizon; shallow hard bedrock; cold, wet winters and warm, dry summers; loamy textures; weatherable minerals; clay fraction has high CEC

7. Purpose of Soil Taxonomy
Soil Survey
Helping People Understand Soils
Purpose of Soil Taxonomy
Transfer of Information and Technology
“A Basic System of Soil Classification for Making and Interpreting Soil Surveys” (USDA, AH-436)
Estimated soil property data from sampling a few locations
Estimated productivity (crop yields, tree growth, etc.)
Estimated soil performance (suitability for houses, need for irrigation, need for lime and fertilizer, etc.)
Information about soil properties and performance gathered at one location can be transferred to other locations where the same soil occurs.
More generalization and less specificity at the higher levels, greater detail and more specificity at the lower levels.
Example – Soil sampling and other data gathering is used to populate NASIS data mapunits with estimated properties and interpretations.

8. 8 Fundamental Attributes of Soil Taxonomy (2nd ed. Chapter 2)
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy (2nd ed. Chapter 2)
1.) Definitions are operational
Operational Definitions
Example, the Fine-loamy particle-size class requires 18 to 35 % clay. But you must also specify the method to measure clay. We use the pipette method as defined in Soil Survey Laboratory Methods manual (SSIR No. 42, ver.4.0, 2004).

9. 8 Fundamental Attributes of Soil Taxonomy
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy
2.) Multicategoric System
The system has 6 categories or levels (order through series).
Orders are the most general level; series are the most specific.
Limits of each category are constrained by the category above.
Results in increased ability to transfer technology and experience as you go from higher to lower categories.

10. 8 Fundamental Attributes of Soil Taxonomy
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy
3.) Taxa represent real bodies of soil
The classes and their definitions reflect soils that are known to exist.
The system does not try to anticipate all possible combinations that MIGHT exist.
Proposals must be developed and submitted for review before new taxa are considered for addition to the keys.

11. 8 Fundamental Attributes of Soil Taxonomy
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy
4.) Differentia are observable/inferable properties
Classes are defined by quantitative soil properties.
Does not use properties outside of the soil.
Does not use theories of soil genesis. But properties are generally selected because they are thought to be the result of, or contribute to, genetic process.

12. 8 Fundamental Attributes of Soil Taxonomy
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy
5.) Soil Taxonomy is (relatively) easy to modify as new knowledge & experience is gained
Change at one level does not impact the entire system.
Change impacts that class and those below.
We have a process for proposing changes. The process is described in the National Soil Survey Handbook, Part 614 Applying Soil Taxonomy.

13. 8 Fundamental Attributes of Soil Taxonomy
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy
6.) Native & cultivated pedons classified the same
Criteria are mostly based on relatively stable subsurface horizons and characteristics that are below the depth of disturbance from human activities.
Mollisols, Andisols, and Histosols are exceptions in that they require certain soil properties to be at or near the soil surface.
The intent of using properties in the subsurface is that both undisturbed and cultivated soils are classified in the same taxon, insofar as possible.
Severe disturbance of a native soil may result in a change in taxonomic classification.

14. 8 Fundamental Attributes of Soil Taxonomy
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy
7.) All soils in the landscape can be accommodated
There are no soils that cannot be classified in Soil Taxonomy.
The keys are constructed so every soil falls into an existing taxon.
Some keys are improved by adding new proposed taxa.

15. 8 Fundamental Attributes of Soil Taxonomy
Soil Survey
Helping People Understand Soils
8 Fundamental Attributes of Soil Taxonomy
8.) All soils known in the world are included
This allows us to have a better perspective regarding soils that occur only on a limited basis in the USA, with their main occurrence elsewhere.
Oxisols is an example of a soil order that is not well represented in the USA.

16. Concepts by Category (Level)
Soil Survey
Helping People Understand Soils
Concepts by Category (Level)
Orders (12)
Differentiated by properties resulting from major soil-forming processes
Examples:
Mollisols – Organic matter accumulation and limited leaching of bases.
Gelisols – Presence of permafrost, freezing and thawing.
Oxisols – intensely weathered to kaolin and free oxides. Very low clay activity.

17. Concepts by Category (Level)
Soil Survey
Helping People Understand Soils
Concepts by Category (Level)
Suborders (70)
Differentiated by important properties, such as soil climate, exerting major controls on the current set of soil-forming processes
Examples
Aquods – Spodosols with aquic moisture regime (saturation and reduction)
Many of the suborders use moisture regime, but not all.
Psamments – Entisols with coarse-textured (i.e., sandy) parent materials.
Cryands – Andisols with cold soil temperatures

18. Concepts by Category (Level)
Soil Survey
Helping People Understand Soils
Concepts by Category (Level)
Great Groups (344)
Presence of key horizons, or other properties exerting subordinate controls
Examples
Molliturbels – Have a mollic epipedon.
Natricryolls – natric horizon
Plinthudults – presence of plinthite.

19. Concepts by Category (Level)
Soil Survey
Helping People Understand Soils
Concepts by Category (Level)
Subgroups (2,664)
Represent:
1) Overlapping of processes recognized in other taxa (intergrades)
2) Processes or conditions not recognized in other taxa (extragrades)
3) Others (Typic)
Example of types of subgroups
Intergrade: Vertic Epiaquolls (grading toward Vertisols)
Extragrade: Lithic Haplustolls
Typic subgroups – not necessarily the common or extensive ones, or the central concept. They catch everything that does not fall out in the other subgroups.

20. Concepts by Category (Level)
Soil Survey
Helping People Understand Soils
Concepts by Category (Level)
Families (~11,252)
- Groups soils in a subgroup that have similar physical and chemical properties that affect their response to management and manipulation for use
The number of soil families in the US is currently about 11,250. This number can change daily as new series are added to the database that represent newly recognized families.
Example: Fine, smectitic, mesic Vertic Natrigypsids
Classes for Family Differentiae
- Particle-size and their substitutes
- Mineralogy
- Cation-exchange activity
- Calcareous and reaction
- Soil temperature
- Soil depth
- Rupture-resistance (used in Spodosols that have ortstein)
- Coatings on sands (used in Quartzipsamments)
- Permanent cracks (used in Fluvaquents or Humaquepts)

21. Concepts by Category (Level)
Soil Survey
Helping People Understand Soils
Concepts by Category (Level)
Series (~23,500)
- Properties are many of the same ones used as differentia at higher levels, whether used as the formative elements in the names of taxa
- Used to differentiate natural bodies of soils within a family
- Phases of series are not a category of ST
We have over 23,000 series in the USA today.
Series are stored in the soil classification (SC) database (
and OSD fileshare system (
The quality control of content for official soil series descriptions (OSDs) is performed by the MLRA Soil Survey Offices (MLRA-SSO).
The quality assurance on OSDs and processing of changes in taxonomic classification for soil series is the responsibility of the 18 MLRA Soil Survey Regional office (MO) staffs.

22. Using the Keys Correctly
Soil Survey
Helping People Understand Soils
Using the Keys Correctly
Know what to Classify (Ch. 1, KST)
Top (boundary between soil and air, shallow water, live plants, or undecomposed plant material)
Bottom (200 cm, lithic contact, 25 cm into paralithic materials)
There are rules for buried soils
Review slide items briefly.

23. Using the Keys Correctly
Soil Survey
Helping People Understand Soils
Using the Keys Correctly
Definitions of the diagnostic horizons and required characteristics for mineral and organic soils are in Ch. 3 of KST
You need to be familiar with these for the soils in your area
Review slide items briefly.

24. Using the Keys Correctly
Soil Survey
Helping People Understand Soils
Using the Keys Correctly
2nd edition of ST has more in-depth discussions than KST on the diagnostic horizons and required characteristics
Realize that each new edition of KST updates and supersedes the older version of the text or keys in ST
The second edition of Soil Taxonomy was published in 1999.
Examples:
1.) The text for the argillic horizon is on 6 pages in the 2nd edition of Soil Taxonomy versus ½ page in the Keys to Soil Taxonomy.
2.) The keys to great groups for many Inceptisols in the 11th ed. of KST take precedence over the corresponding but older versions of the keys in the 2nd ed. of ST. The major difference is the additional new “Humi-” great groups (in Gelepts, Ustepts, Xerepts, and Udepts) that were added in 2010.

25. Using the Keys Correctly
Soil Survey
Helping People Understand Soils
Using the Keys Correctly
With a key, you must start at the beginning and systematically work your way down comparing your soil to the criteria
Don’t make a habit of jumping into the middle
Taxa are listed in each key in an order of importance or interpretive significance.
You cannot skip the first one your soil meets just because it could meet the following ones as well.

26. Using the Keys Correctly
Soil Survey
Helping People Understand Soils
Using the Keys Correctly
Chapter 17 (KST) has the information needed for family level differentiae
Separate sections for mineral and organic soils
9 family classes, but there are different rules for when they are used
There are varying rules for identifying control sections
Each is arranged as a key
Review points on the slide.

27. Using the Keys Correctly
Soil Survey
Helping People Understand Soils
Using the Keys Correctly
Appendix
Gives a brief description of the physical, chemical, and mineralogical laboratory determinations as used in ST
Remember: We use “operational definitions”
Full details are in the Lab Methods Manual
Rounding
Pay attention to decimal points in criteria and use conventional rules for rounding
NCSS laboratory procedures are contained in the Soil Survey Laboratory Methods Manual (SSIR 42) available at:
Rounding Example:
Fine-loamy soils have 18 to 35 % clay (note that no decimal point is in the criteria). So 17.6% measured clay meets the definition.
Conventional rules for rounding
- If the digit immediately to the right of the last significant figure is more than 5, round up to the next higher digit. (e.g., 34.8=35)
- If the digit immediately to the right of the last significant figure is less than 5, round down to the next lower digit. (e.g., 34.4 = 34)
- If the digit immediately to the right of the last significant figure is equal to 5, round to the adjacent even number, either up or down. (e.g., 34.5 = 34)

28. Soil Survey
Helping People Understand Soils
Summary
Soil taxonomy helps us organize our knowledge, communicate about soils, and transfer experience
Soil taxonomy adheres to 8 fundamental attributes
It is a 6-category hierarchical system with general concepts for each level
Definitions of criteria are operational
Review points on slide briefly.

29. Soil Survey
Helping People Understand Soils
Summary
The 2nd edition of Soil Taxonomy (1999) has in-depth discussion of many aspects of the system not present in Keys to Soil Taxonomy but
Each new edition of the Keys to Soil Taxonomy updates and supersedes the older version of the text or keys in ST
Review points on slide briefly.