4 Soil Classification and Mapping

The word, “taxonomy” is based on the Greek words “taxis”, meaning arrangement; and “nomia”, meaning method. In biology, taxonomy refers to a hierarchical system in which organisms are grouped based on shared characteristics, with domains and kingdoms at the top of the hierarchy, and genus and species at the lowest levels. Similarly, Soil Taxonomy is a hierarchical system used to group soils based on observable or measurable characteristics. A common application of soil classification (the act of identifying the taxonomic classification for a given soil) is to develop models of how soils of different classifications associate with one another within a landscape, which can eventually be used in soil mapping. The primary concepts of soil classification using Soil Taxonomy will be reviewed in this lab, followed by an overview of the Web Soil Survey (United States Department of Agriculture Natural Resources Conservation Service, 2016).

Learning Objectives

  • Become familiar with the 12 soil orders.
  • Understand the structure of Soil Taxonomy.
  • Become familiar with the contents of a county soil survey report.
  • Use a soil survey report for land use evaluation.

Materials

  • Four soil monoliths (Crete, Clark, Morrill, and Chase soil series)
  • Printed county soil survey reports
  • Computer with internet access and a projector

Note

For this lab, you will need to bring a laptop or tablet to use. If you do not have a laptop or tablet, please share with a partner.

Prelab Assignment

Using the assigned readings and the introduction to this lab, consider the questions listed below. These definitions/questions will provide a concise summary of the major concepts addressed in the lab. They are also relevant to the soil survey report and are useful study notes for exams.

  1. Explain the difference between a profile, a pedon, and a polypedon.
  2. Describe the key properties or diagnostic features for each of the following diagnostic epipedons:
    • Mollic:
    • Umbric:
    • Histic:
    • Ochric:
  3. Describe the key properties or diagnostic features of the following diagnostic subsurface horizons:
    • Argillic:
    • Kandic:
    • Natric:
    • Calcic:
    • Spodic:
    • Oxic:
    • Cambic:
  4. List the soil moisture regimes from dries to wettest, and note the criteria for each.
  5. Explain the difference between a soil phase and a soil consociation.
  6. What are the 12 soil orders? Describe the key properties or diagnostic features of each.
  7. What are the six formal categories of Soil Taxonomy?

 

Introduction

Soils can vary widely in their properties, and each has a unique arrangement of layers or horizons. The soil profile description provides the information that distinguishes one soil from another. Review the following example of a profile description, and note the explanation of terms in Table 4.1.

Harney silt loam [adapted from the National Cooperative Soil Survey (1997)]

Ap – 0 to 9 in; dark grayish brown (10YR 4/2) silt loam, very dark grayish brown (10YR 3/2) moist; moderate medium granular structure; slightly hard, very friable; many fine roots; slightly acid; clear smooth boundary. (4 to 14 in thick)

AB – 9 to 12 in; dark grayish brown (10YR 4/2) silt loam, very dark grayish brown (10YR 3/2) moist; moderate fine subangular blocky structure; hard, friable; many fine roots; neutral; clear smooth boundary. (0 to 10 in thick)
Bt1 – 12 to 18 in; grayish brown (10YR 5/2) silty clay loam, dark grayish brown (10YR 4/2) moist; moderate medium subangular blocky structure; very hard, very firm; few fine roots; moderately alkaline; clear smooth boundary.
Bt2 – 18 to 28 in; grayish brown (10YR 5/2) silty clay loam, dark grayish brown (10YR 4/2) moist; strong medium subangular blocky structure; very hard, very firm; few fine roots; moderately alkaline; gradual smooth boundary. (Combined thickness of the Bt horizon is 10 to 26 in)
BCk2 – 8 to 35 in; brown (10YR 5/3) silty clay loam, brown (10YR 4/3) moist; moderate medium subangular blocky structure; hard, firm; few fine roots; many soft accumulations of carbonates; strong effervescence; moderately alkaline; gradual smooth boundary. (0 to 16 in thick)
Ck3 – 5 to 47 in; pale brown (10YR 6/3) silt loam, brown (10YR 5/3) moist; massive; slightly hard, friable; common soft accumulations of carbonates; strong effervescence; moderately alkaline; gradual smooth boundary. (0 to 20 in thick)
C – 47 to 60 in; pale brown (10YR 6/3) silt loam, brown (10YR 5/3) moist; massive; slightly hard, friable; strong effervescence; moderately alkaline.

Table 4.1. Explanation of the Ap horizon description.

Morphological property Description Morphological property Description
Horizon designation Ap Structure grade Moderate
Upper depth 0 in Structure size Medium
Lower depth 9 in Structure type Granular
Color name Dark grayish brown Moist consistence Very friable
Munsell hue 10YR Roots Many fine roots
Munsell value 4 pH Slightly acid
Munsell chroma 2 Boundary Clear smooth
Textural class Silt loam

Table adapted from King et al. (2003).

Completing a soil profile description involves a systematic approach:

  1. Observing the landscape setting.
  2. Examining the morphological features like texture, structure, color, consistence, etc. of the soil to distinguish any layers or horizons
  3. Describing in detail the texture, structure, color, consistence, and other important features of each horizon.
  4. Assigning horizon designations to each layer.
  5. Classifying the soil on the basis of its morphology and horizonation.

Soil Morphology and Land Use

Criteria that rate soils for a particular use are important to land use planning and land management decisions. Guidelines based on these criteria facilitate uniform and consistent land evaluations. Soil-based criteria can be developed for nearly any land use. To prepare a soil rating scheme, the following are required:

  • Precise definition of the land use
  • A list of soil properties affecting the use
  • Limits for each soil property that would be favorable or unfavorable for the land use.

A comprehensive classification system is important for any science: soil science, plant science, biology, geology, among many others. Effective taxonomy allows us to organize knowledge and learn new relationships. Soil Taxonomy helps in extrapolating soil management research among similar soils around the world. Soil Taxonomy is a quantitative system based on soil properties that can be observed or measured, organized in a hierarchy based on six categories beginning with 12 broad soil orders and narrowing in specificity to more than 23,000 series. The following diagram illustrates the organization of a taxonomic name by category.

Diagram depicting the USDA Soil Taxonomy classifications for four soil series from soil order down to soil series.
Figure 4.1. An example of USDA Soil Taxonomy classifications for four soil series within the Mollisol soil order.

Table 4.2 Simplified key to the 12 soil orders

Order Major Diagnostic Features Formative Element
Gelisols Soils with permafrost or gelic material within 100 cm el
Histosols Other soils with >30% organic matter (>12% organic carbon) content to a depth of 40 cm or
more
ist
Spodosols Other soils with a spodic horizon (illuvial humus, iron) within a depth of 200 cm od
Andisols Other soils with andic soil properties (low density, volcanic glass, pumice, etc.) in >= 50% of
the upper 60 cm
and
Oxisols Other soils with an oxic horizon, or containing more than 40% clay in the surface 18 cm and
a kandic horizon with less than 10% weatherable minerals (highly weathered)
ox
Vertisols Other soils containing more than 30% clay in all horizons and cracks that open and close
periodically (shrinking/swelling)
ert
Aridisols Other soils with some diagnostic subsoil horizon(s) and an aridic soil moisture regime id
Ultisols Other soils with an argillic or kandic horizon and a base saturation at pH 8.2 of <35% at a
depth of 180 cm
ult
Mollisols Other soils with a Mollic epipedon and a base saturation at pH 7 of 50% in all depths
above 180 cm
oll
Alfisols Other soils with an argillic, kandic, or natric horizon (and a base saturation at pH 8.2 of >35%
at a depth of 180 cm)
alf
Inceptisols Other soils with an umbric, mollic, or plaggen epipedon or a cambic horizon ept
Entisols Other soils ent

The formative elements are used in the names of suborders and lower taxonomic levels. (Table courtesy of R. Weil)

Many other formative elements can specify unique soil properties at each taxonomic level. Each formative element has a connotation for a given soil. These connotations of the formative elements used for suborders and great groups are listed in Table 4.3 and Table 4.4.

Table 4.3. Formative elements used to identify various suborders in Soil Taxonomy.

Formative Element Connotation Formative Element Connotation
alb Presence of albic horizon (a
bleached eluvial horizon)
hist Presence of histic epipedon
anthr Presence of anthropic or plaggen
epipedon
hum Presence of organic matter
aqu Characteristics associated with
wetness
orth The common ones
ar Mixed horizons per Of year-round humid climates,
perudic moisture regime
arg Presence of argillic horizon (a
horizon with illuvial clay)
psamm Sand textures
calc Presence of calcic horizon rend Rendzinalike-high in carbonates
camb Presence of cambric horizon sal Presence of salic (saline) horizon
cry cold sapr Most decomposed stage
dur Presence of a duripan torr Usually dry
fibr Least decomposed stage turb Cryoturbation
fluv Floodplains ud Of humid climates
fol Mass of leaves ust Of dry climates, usually hot in summer
gyps Presence of gypsic horizon vitr Resembling glass
hem Intermediate stage of decomposition xer Dry summers, moist winters

Table from King et al. (2003)

Table 4.4 Formative elements for names of great groups and their connotations

Formative Element Connotation Formative Element Connotation
acr Extreme weathering hist Presence of organic materials
aer Chroma >2, non-reducing fragi Fragipan
agr Agric horizon hum Humus
al High aluminum, low iron hydr Water
alb Albic horizon kand Low activity 1:1 silicate clay
and Ando-like lithic Near stone
anhy Anhydrous luv, lu Illuvial
aqu Water saturated melan Melanic epipedon
aren Sandy molli With a mollic epipedon
argi Argillic horizon natr Presence of a natric horizon
calc, calci Calcic horizon pale Old development
camb Cambric horizon petr Cemented horizon
chrom High chroma plac Thin pan
cry Cold plagg Plaggen horizon
dur Duripan plinth Plinthite
dystr, dys Low base saturation psamm Sand texture
endo Fully water saturated quartz, quartzi High quartz
epi Perched water table rhod Dark red colors
eutr High base saturation sal Salic horizon
ferr Iron sapr Most decomposed
fibr Least decomposed somb Dark horizon
fluv Floodplain sphagn Sphagnum moss
fol Mass of leaves sulf Sulfur
fragloss See frag and gloss torr Usually dry and hot
fulv light-colored melanic horizon ud Humid climates
gyps gypsic horizon umber Umbric epipedon
gloss Tongued ust Dry climate, usually hot in summer
hal Salty verm Wormy or mixed by animals
hapl Minimum horizon vitr Glass
hem Intermediate decomposition xer Dry summers, moist winters

Table adapted from King et al. (2003)

A complete taxonomic name communicates a great deal of information about the soil if we understand each part of the name. As an example of the quantitative information revealed in a taxonomic name, the following classification name will be dissected by category. Consider, for example, the Harney soil, with a taxonomic classification of fine, smectitic, mesic Typic Argiustoll.

A diagram depicting the taxonomy of the Harney soil series using the USDA Soil Taxonomy classification system.he Hearney soil series.
Figure 4.2. Formative elements in the taxonomic classification of the Harney series.

Table 4.5.Translation of the taxonomic classification of the Harney Series.

Categories Properties connoted
ORDER: Mollisol Has a mollic epipedon and a base saturation of >50% to a
depth of 1.8 m from the soil surface or to an impermeable
layer
SUBORDER: Ustoll has an ustic moisture regime; dry for as long as 90 days
cumulatively per year
GREAT GROUP: Argiustoll has an argillic horizon
SUBGROUP: Typic Argiustoll typical of an Argiustoll, not intergrading toward another great
group condition
FAMILY: fine, smectitic, mesic the upper 50 cm of the argillic horizon has 35-60% clay; the
dominant clay minerals are smectite minerals
(montmorillonite, beidellite, and nontronite); the mean annual
soil temperature at 50 cm is 8°C to 15°C (47°F to 59°F)
SERIES: Harney differs from soils in the same family in based on color, parent
material (loess), and calcium accumulation below 28 in.

Table courtesy of C. J. Moorberg, adapted from King et al. (2003)

Activity 1: Practice Key to Soil Orders

    Now that you have studied the characteristics of the 12 soil orders, enter the most appropriate soil order name in each rectangle.

Flow chart activity that identifies the primary characteristics of each of the 12 soil orders.
Figure 4.3. General characteristics of the 12 soil orders.


Activity 2: Structure of Soil Taxonomy

    To illustrate the structure of Soil Taxonomy, separate a complete taxonomic name into the six categories. Follow the example of the Harney silt loam in figure 4.2.

Colby

Taxonomic Name: Fine-silty, mixed, superactive, calcareous, mesic Aridic Ustorthents

Order  
Suborder
Great Group
Subgroup
Family
Series

Goessel

Taxonomic Name: Fine, smectitic, mesic Typic Haplusterts

Order  
Suborder
Great Group
Subgroup
Family
Series

Wymore

Taxonomic Name: Fine, smectitic, mesic Aquertic Argiudolls

Order  
Suborder
Great Group
Subgroup
Family
Series

Activity 3: Interpreting Taxonomy

As a further exercise in understanding taxonomic names, complete the following questions. Use the list of taxonomic names of soils representative of Mollisols from the prairie pothole region of Iowa below to answer these questions.

The Des Moines lobe of the Wisconsin glaciation covered north-central Iowa with a deep layer of glacial deposits, and provides a good example of how taxonomic names depict important soil properties. The Clarion-Nicollet-Webster-Glenco topo-sequence, or “catena” (Figure 4.4), illustrates how Soil Taxonomy reflects wetness, or depth to a water table.

    Fill in the subgroup taxonomic name for soils in Table 4.6, and study how the terms change with wetness.

Clarion Nicollet Webster Glencoe catena
Figure 4.4. A cross-section depicting the Clarion-Nicollet-Webster-Glencoe catena. Figure courtesy of C. J. Moorberg.

Clarion series: Fine-loamy, mixed, superactive, mesic Typic Hapludolls

Nicollet series: Fine-loamy, mixed, superactive, mesic Aquic Hapludolls

Webster series: Fine-loamy, mixed, superactive, mesic Typic Endoaquolls

Glencoe series: Fine-loamy, mixed, superactive, mesic Cumulic Endoaquolls

Table 4.6. Clarion-Nicollet-Webster-Glenco topo-sequence Taxonomy.

Series Drainage Class Depth to Seasonal High Water Table Subgroup Taxonomic Name
Clarion Moderately Well 61 - 102 cm (24 - 48 in)
Nicollet Somewhat Poorly 30 – 61 cm (12 – 24 in)
Webster Poorly < 30 cm (< 12 in)
Glencoe Very Poorly < 30 cm (< 12 in), and
accumulation of organic matter

Table courtesy of C. J. Moorberg

Notice that the wetter the drainage class (that is, the shallower the depth to the seasonal high water table), the higher the “aqu” formative element becomes in the overall classification. That is because Soil Taxonomy prioritizes soil management considerations. The depth to the seasonal high water table would be a management concern for most land uses for the Nicollet, Webster, and Glencoe series; it would be of less concern for the Clarion series, and thus “aqu” is not included in the classification.

Also note that for the Glencoe series, in addition to having the “aqu” formative element as part of the suborder, the “cumulic” formative element has been designated in the subgroup. That formative element alludes to a “thickened epipedon” caused by the accumulation of organic matter. Because the water table is so shallow, little oxygen is available at the surface for a significant portion of the growing season. This slows decomposition, allowing organic matter to build, thus creating a thickened epipedon with lots of organic matter.

Activity 4: Practicing Soil Taxonomy Interpretations with State Soils of the US

State soils have been selected for all 50 states and three territories in the U.S. The group of soils represents a diverse sample of soil conditions and classifications. It serves as an interesting focus for a little practice at deciphering and understanding Soil Taxonomy. Use the attached list of state soils in Table 4.7 along with Table 4.2, Table 4.3, and Table 4.4 to answer the following questions:

    What is the most commonly recognized ORDER among the state soils?

 

    Which of the soil ORDERS is not represented in the list of state soils?

 

    How many Oxisols are represented by the 53 soils?

 

    What is the complete SUBORDER name for the state soil of Alaska?

 

    How many Vertisols are represented in the state soils?

 

    In what soil property does the Downer soils of New Jersey differ from the Greenwich soils or Deleware?

 

    The state soil of South Carolina has a soil condition identified by its great group. What element is present in the upper 50 cm of this soil? (Hint: use Table 4.4)

 

    What is the complete taxanomic name for the state soil of Kansas?

 

Table 4.7. Soil Taxonomy classifications of state soils of the U.S.

Series State Family Classification
Tanana AK coarse-loamy, mixed, superactive, subgelic Typic Aquiturbels
Bama AL fine-loamy, siliceous, subactive, thermic Typic Paleudults
Stuttgart AR fine, smectitic, thermic Albaquultic Hapludalfs
Casa Grande AZ fine-loamy, mixed, superactive, hyperthermic Typic Natrargids
San Joaquin CA fine, mixed, active, thermic Abruptic Durixeralfs
Seitz CO clayey-skeletal, smectitic Ustic Glossocryalfs
Windsor CT mixed, mesic Typic Udipsamments
Greenwich DE coarse-loamy, mixed, semiactive, mesic Typic Hapludults
Myakka FL sandy, siliceous, hyperthermic Aeric Alaquods
Tifton GA fine-loamy, kaolinitic, thermic Plinthic Kandiudults
Akina GU very-fine, kaolinitic, isohyperthermic Inceptic Haplustox
Hilo HI medial over hydrous, ferrihydritic, isohyperthermic Acrudoxic
Hydrudands
Tama IA fine-silty, mixed, superactive, mesic Typic Argiudolls
Threebear ID medial over loamy, amorphic over mixed, superactive, frigid
Oxyaquic Udivitrands
Drummer IL fine-silty, mixed, superactive, mesic Typic Endoaquolls
Miami IN fine-loamy, mixed, active, mesic Oxyaquic Hapludalfs
Harney KS fine, smectitic, mesic Typic Argiustolls
Crider KY fine-silty, mixed, active, mesic Typic Paleudalfs
Ruston LA fine-loamy, siliceous, semiactive, thermic Typic Paleudults
Paxton MA coarse-loamy, mixed, active, mesic Oxyaquic Dystrudepts
Sassafras MD fine-loamy, siliceous, semiactive, mesic Typic Hapludults
Chesuncook ME coarse-loamy, isotic, frigid Aquic Haplorthods
Kalkaska MI sandy, isotic, frigid Typic Haplorthods
Lester MN fine-loamy, mixed, superactive, mesic Mollic Hapludalfs
Menfro MO fine-silty, mixed, superactive, mesic Typic Hapludalfs
Natchez MS coarse-silty, mixed, superactive, thermic Typic Eutrudepts
Scobey MT fine, smectitic, frigid Aridic Argiustolls
Cecil NC fine, kaolinitic, thermic Typic Kanhapludults
Williams ND fine-loamy, mixed, superactive, frigid Typic Argiustolls
Holdrege NE fine-silty, mixed, superactive, mesic Typic Argiustolls
Marlow NH coarse-loamy, isotic, frigid Oxyaquic Haplorthods
Downer NJ coarse-loamy, siliceous, semiactive, mesic Typic Hapludults
Panistaja NM fine-loamy, mixed, superactive, mesic Ustic Haplargids
Orovada NV coarse-loamy, mixed, superactive, mesic Durinodic Xeric
Haplocambids
Honeoye NY fine-loamy, mixed, semiactive, mesic Glossic Hapludalfs
Miamian OH fine, mixed, active, mesic Oxyaquic Hapludalfs
Port OR fine-silty, mixed, superactive, thermic Cumulic Haplustolls
Hazleton PA loamy-skeletal, siliceous, active, mesic Typic Dystrudepts
Bayamon PR very-fine, kaolinitic, isohyperthermic Typic Hapludox
Narragansett RI coarse-loamy over sandy or sandy-skeletal, mixed, active, mesic
Typic Dystrudepts
Bohicket SC fine, mixed, superactive, nonacid, thermic Typic Sulfaquents
Houdek SD fine-loamy, mixed, superactive, mesic Typic Argiustolls
Dickson TN fine-silty, siliceous, semiactive, thermic Glossic Fragiudults
Houston Black TX fine, smectitic, thermic Udic Haplusterts
Taylorsflat UT fine-loamy, mixed, superactive, mesic Xeric Haplocalcids
Pamunkey VA fine-loamy, mixed, semiactive, thermic Ultic Hapludalfs
Victory VI loamy-skeletal, mixed, superactive, isohyperthermic Typic
Haplustepts
Tunbridge VT coarse-loamy, isotic, frigid Typic Haplorthods
Tokul WA medial, amorphic, mesic Aquic Vitrixerands
Antigo WI coarse-loamy over sandy or sandy-skeletal, mixed, superactive,
frigid Haplic Glossudalfs
Monongahela WV fine-loamy, mixed, semiactive, mesic Typic Fragiudults
Forkwood WY fine-loamy, mixed, superactive, mesic Ustic Haplargids

Table courtesy of J. Kleiss and D. Lindbo

Activity 5: Soil Survey Reports

As an introduction to soil reports, look through a typical printed county soil survey report; take note of the manual’s organization and the extensive content. The report begins with some background information on the county, along with an overview of how the survey was conducted. The county soil conditions are described, and the soil mapping units are discussed in detail. A colored map displays these general soil units.

Following the brief overview are detailed soil map unit descriptions. These show the symbol that is on the soil map, the dominant soil series, topsoil texture and range of slope found in the unit. The descriptions of each map unit details the landscape setting, general properties, and major use and management considerations. If other soils are present in the map unit, this is an important part of map unit description. The next section of the soil report offers specific use and management suggestions and discusses how specific types of land use ratings were formulated. This is followed with an overview of what specific kinds of soil data are included.

Specific information on soil classification and detailed profile descriptions for each soil are followed by a glossary of terms used in the report. A sequence of tables provides detailed ratings on a wide range of land uses. This interpretative information is offered for each soil map unit. The last section of the report shows the soil maps on an aerial photograph base.

To become familiar with county soil survey reports select one provided and review the table of contents and the summary list of tables. Leaf through the report and note the following sections:

  • Map Unit descriptions
  • Use and management of soils
  • Classification and profile descriptions
  • Interpretive tables
  • General soil map
  • Soil legend
  • Soil maps

The United States Department of Agriculture Natural Resource Conservation Service (USDA NRCS) today provides these soil surveys in a digital format through the Web Soil Survey (United States Department of Agriculture Natural Resources Conservation Service, 2016). The Web Soil Survey provides all the information previously contained in the county soil survey reports. It also contains additional tools and information that has not been available in printed versions of the soil surveys. Another advantage of the Web Soil Survey is that the information contained in it can be updated as needed, instead of being updated following new surveys of the same county, which take 30 to 60 years! Your instructor will walk you through some of the main features of the Web Soil Survey and show you how to request a PDF copy of a soil survey report for a designated area. You will use these skills for your Soil Survey Report assignment.

 

Assignment: Soil Survey Report

For this lab, you will be preparing a soil survey report. The report assignment will be provided to you at the beginning of the lab. Your instructor will go over what to include in the report and where to collect the necessary information from the Web Soil Survey.

Subsequent Lab Set-Up

Some activities require preparation beyond the lab period and must be set up ahead of time. The soil texture by hydrometer activity in the  Soil Texture and Structure lab involves dispersing soil particles chemically, which requires time for the reactions to take place. We will do this now, so the samples are ready next week.

For each of the three soils provided, do the following:

Weigh out 30.0 g of dry soil (assume oven-dry) into a 250-ml Erlenmeyer flask.

Wash sides of flask with distilled water from a wash bottle.

Add 100 ml of distilled water using a graduated cylinder. Then add 10 ml of sodium hexametaphosphate solution (500 g/L) from the dispenser on the sodium hexametaphosphate bottle.

Swirl to mix.

Wash sides of flask with distilled water from a wash bottle.

Cover the flask with Parafilm and label the flask with your lab section, table number, and soil type. Store the flasks in the location specified by your instructor for the next laboratory period.

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Soils Laboratory Manual by Colby J. Moorberg & David A. Crouse is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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