2.2: Soil Classification and Mapping

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.

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).

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.

Recommended Reading

• Illustrated Guide to Soil Taxonomy (Soil Survey Staff, 2015)
• From the Surface Down: An Introduction to Soil Surveys for Agronomic Use (USDA NRCS, 2010)

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.

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.

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.

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.

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.