Soil Survey Helping People Understand Soils Soil Taxonomy NRCS National Soil Survey Center Soil Survey Standards

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Helping People Understand Soils

Soil TaxonomyNRCS

National Soil Survey Center

Soil Survey Standards

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• 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.

Why is this topic important?

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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

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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

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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)

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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.)

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8 Fundamental Attributes of Soil Taxonomy

(2nd ed. Chapter 2)

1.) Definitions are operational

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2.) Multicategoric System

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3.) Taxa represent real bodies of soil

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4.) Differentia are observable/inferable properties

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5.) Soil Taxonomy is (relatively) easy to modify as new knowledge & experience is gained

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6.) Native & cultivated pedons classified the same

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7.) All soils in the landscape can be accommodated

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8.) All soils known in the world are included

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Concepts by Category (Level)

Orders (12)

Differentiated by properties resulting from major soil-forming processes

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Suborders (70)

Differentiated by important properties, such as soil climate, exerting major controls on the current set of soil-forming processes

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Great Groups (344)

Presence of key horizons, or other properties exerting subordinate controls

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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)

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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

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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

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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

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– 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

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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

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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

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Using the Keys CorrectlyChapter 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

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Using the Keys CorrectlyAppendix

• 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

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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

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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

Documents

Soil Survey Helping People Understand Soils Soil Taxonomy NRCS National Soil Survey Center Soil Survey Standards