An Abbreviated Key to Soil Orders
SOILS WITH-1. A. permafrost at <1 m; OR
B. gelic materials at <1m and permafrost at <2m
GELISOLS
2. organic materials that are—
A. 2/3 of thickness over lithic/paralithic contact, with no mineral
horizons >10 cm; OR
B. saturated >30 days/year, AND are >60 cm thick if fibrous OR
>40 cm thick if sapric
HISTOSOLS
3. a spodic horizon >10 cm thick (with certain depth requirements)
SPODOSOLS
4. andic soil properties in 60% of the soil within 60 cm of the
surface or a lithic/paralithic contact
ANDISOLS
5. an oxic horizon within 150 cm of soil surface AND no kandic
horizon; OR
>40% clay in top 18 cm AND kandic horizon with low
weatherable minerals within 1 m of surface
OXISOLS
6. a layer >25 cm thick within 1 m of surface with slickensides or
wedge-shaped peds, AND
>30% clay in all horizons to depth of 50 cm or a lithic/paralithic
contact, AND
periodic cracking at soil surface
VERTISOLS
7. aridic moisture regime AND ochric epidpedon AND one of
following within 1 m of surface: cambic, gypsic, calcic,
petrocalcic, petrogypsic, natric, OR argillic
ARIDISOLS
8. argillic or kandic horizon AND % BS (ECEC) <35% in all
horizons 125 below the top of argillic/kandic or to depth 180
below soil surface or to lithic/paralithic contact
ULTISOLS
9. mollic epipedon AND %BS (pH 7)>50% in ALL horizons to 180
cm depth or to lithic/paralithic contact
MOLLISOLS
10. argillic, kandic, or natric horizon, OR fragipan with clay films in
some part
ALFISOLS
11. cambic, calcic, gypsic, petrocalcic, OR petrogypsic horizon
within 1 m of surface OR oxic or spodic horizon within 2 m of
surface, OR sulfuric horizon
INCEPTISOLS
12. All other soils
ENTISOLS
Lower Levels in Soil Taxonomy
ORDER
SUBORDER
GREAT GROUP
SUBGROUP
FAMILY
SERIES
FORMATIVE SUFIXES

Orders: names of orders end in "sol"
alf - Alfisol
ept - Inceptisol
and - Andisol
oll - Mollisol
id - Aridisol
ox - Oxisol
ent - Entisol
od - Spodosol
el - Gelisol
ult - Ultisol
ist - Histisol
ert - Vertisol
Nomenclature of Lower Levels

Suborders: names of suborders have two syllables



First connotes something about the soil
Second is the formative element from the order
 Udalf - an Alfisol with a udic moisture regime
 Psamment - (psamm: sandy) an Entisol with a sandy particle
size throughout
 Aquult - an Ultisol with an aquic moisture regime
Great Groups: name consists of the suborder and a prefix
that suggests something of the diagnostic properties of the
soil



Paleudalf - (Pale: old); an old (deeply weathered) Udalf
Udipsamment - sandy Entisol with a udic moisture regime.
Plinthaquult - wet Ultisol with plinthite.

Subgroups: name consists of the great group modified
by one or more adjectives


"Typic" - central concept of the great group
Other types of subgroups are:

Intergrades toward other great groups


Spodic Udipsamment is an intergrade to Spodosols,
Extragrades - subgroups not intergrading toward any known kind
of soil

Lithic Udipsamment is a intergrade to not soil (hard rock within 50 cm
of the soil surface)
Suborder Criteria
Vary WIDELY with particular order
--Meant to delineate MAJOR GROUPINGS within each order
HISTOSOLS:
not saturated: FOLISTS
saturated:
high fiber content: FIBRISTS
low fiber content: SAPRISTS
intermediate fiber: HEMISTS
For MANY orders, suborder criteria are based on soil moisture
regimes as way to separate broad classes within the order
Soil Moisture Regimes





Determine the potential use of the soil if use depends
on water in the soil
Suggest processes that have occurred or that are
ongoing in the soil
The moisture regime, except for the aquic moisture
regime, is the result of climate but is based on soil
properties: soil moisture content.
Dry soil - water content less than that held against 1500
kPa tension
Moist soil - water content is greater than that held
against 1500 kPa tension but less than saturation.
Soil Moisture Regimes: Control Section



Upper boundary - depth to which a dry soil will be
moistened by 2.5 cm of water within 24 hours.
Lower boundary - depth to which a dry soil will be
moistened by 7.5 cm of water within 48 hours.
These depths are approximately:



10 to 30 cm for loams, silt loams, silts, clay loams, very fine
sandy loams, silty clay loams, silty clays, sandy clay loams,
sandy clays, and clays;
20 to 60 cm for fine sandy loams and coarser sandy loams;
30 to 90 cm for loamy sands and sands.
Moisture Regimes--Overview
Aridic: MCS dry >180 d, moist for < 90 d
no cropping possible; little soil development
Xeric: MCS dry >90 d, moist >180 d; winter rain/dry summer
Mediterranean; winter crops; some soil development
Ustic: MCS dry >90 d, moist >180 d; summer/uniform rain
cotton, irrigated crops; some soil development
Udic: MCS dry for < 90 d; typically uniform rain
corn cultivation; optimum soil development
Perudic: udic, with monthly PPT>monthly ET
cooler or very high rainfall
Aquic: redox features (<2 chroma) directly below A
Aridic Moisture Regime
Blaine County, Montana
Soil water holding capacity = 22 cm
16
Precipitation
ET
Precipitation or ET, cm
12
Deficit
8
4
Utilization
Recharge
0
Sep
Nov
Jan
Mar
May
Jul
Sep
Udic Moisture Regime
Tallulah, Louisanna
Soil water holding capacity = 22 cm
20
Precipitation
Evaptranspiration
Precipitation or ET, cm
16
Utilization
12
Deficit
8
Surplus
(leaching)
Recharge
4
0
Sep
Nov
Jan
Mar
May
Jul
Sep
Ustic Moisture Regime
Lynn County, Texas
Soil available water capacity = 22 cm
Annual Precipitation = 65 cm
16
Precipitation
ET
12
Precipitation or ET, cm
Deficit
Utilization
8
4
Recharge
0
Sep
Nov
Jan
Mar
May
Jul
Sep
Ustic Moisture Regime
Lagos, Nigeria
Soil water holding capacity = 22 cm
40
Precipitation
ET
Precipitation or ET, cm
30
Recharge
20
Utilization
10
Deficit
0
Sep
Nov
Jan
Mar
May
Jul
Sep
Xeric Moisture Regime
Solano County, California
Soil available water capacity = 22 cm
Annual Pecipitation = 68 cm
16
Precipitation or ET, cm
12
8
Recharge Surplus
Deficit
Utilization
4
Precipitation
ET
0
Sep
Nov
Jan
Mar
May
Jul
Sep
Aquic Moisture Regime


Soils with an aquic moisture regime have aquic
conditions immediately below an ochric epipedon or
albic horizon
Aquic conditions:


Horizons with aquic conditions are those that experience
continuous or periodic saturation and reduction
Presence of aquic conditions is indicated by redoximorphic
features and is verified by measurement of saturation and
reduction in the field.
Aquic Moisture Regime - Saturation


Saturation - zero or positive pressure in the soil water. This can be
determined be observing "free water" in an unlined auger hole.
 Duration of saturation is not specified.
Three types of saturation are recognized:
 Endosaturation – saturated in all horizons from the upper boundary of
saturation to 2 m or more.
 Episaturation – saturated in one or more horizons above horizons
within 2 m that are not saturated (perched water table).
 Anthric saturation - variant of episaturation associated with controlled
flooding for wetland rice.

Produces reduction in the saturated surface soil and oxidation of reduced
Fe and Mn in the oxidized subsoil.
Suborder Example: Ultisols
Ultisols that have—
1. Aquic conditions: redox features (including depletions) directly
below the epipedon.
AQUULTS
2. >0.9% organic carbon in the upper 15 cm of the argillic/kandic
HUMULTS
3. A udic moisture regime
UDULTS
4. A ustic moisture regime
USTULT
5. Other ultisols (presumably, xeric moisture regime)
XERULTS
Some other suborder designations…
Temperature regimes:
cryalf, torrand, geland
Soil moisture regimes:
udalf, xeralf, ustalf, perox; aq- (wet): aqoll, aqod
Specific soil properties/horizons:
vitrand
turbel
humult
durid, cambid, salid, gypsid, argid, calcid
hemist, folist, fibrist, saprist
rendoll, alboll
arent, psamment, fluvent, orthent
Great Group Example: Udults…
Have a subsurface horizon with >50% (vol) plinthite: Plinthudults
Have a fragipan within 1 m of surface:
Fragiudults
Have a kandic horizon, and do not have a clay decrease of >20%
(relative) within 150 cm of surface:
Kanidudults
Have a kandic horizon (with clay decrease)
Kanhapludults
(Have an argillic horizon and) do not have a clay decrease of >20%
(relative) within 150 cm of surface:
Paleudults
Have an epipedon with value of 3 or less throughout, AND in the
upper 1 m of the argillic horizon, at least 50% of the volume has
color hues of 2.5YR or redder and values of 3 or less:
Rhodudults
All other Udults:
Hapludults
Subgroup Examples (for Hapludults)
Lithic contact at <50cm
Lithic
Surface cracks and slickensides in subsurface
Vertic
<2 chroma depletions in upper 60 cm of argillic
Aquic
Fragic properties in 30% of subsurface layer >15 cm thick
Fragic
Sandy particle size in upper 75 cm of argillic
Psammentic
Sandy particle size from surface to top of argillic at
depth of 50-100 cm
Arenic
Sandy particle size from surface to top of argillic at
depth of >100 cm
Grossarenic
….. (6 other ones: total of 14)
OTHER Hapludults
Typic
Family Criteria


Soil families serve purposes that are largely pragmatic and to
enhance interpretation ability
Family names include and are listed as part of the classification in the
following order:
 Particle-size class
 Mineralogy class
 Cation-exchange activity class*
 Calcareous and reaction class *
 Soil temperature class
 Soil depth class *
 Soil rupture resistance or consistence class *
 Class of coatings (on sands) *
 Class of cracks *
“Fine, kaolinitic, thermic”
“Loamy-skeletal, mixed, mesic”
Key to Particle Size Control Section:
A. For mineral soils that have a root-limiting layer within 36 cm of the mineral soil surface:
From the mineral soil to the root-limiting layer; or
B. For those Alfisols, Ultisols, and great groups of Aridisols and Mollisols that have an argillic, kandic, or natric horizon that
has its upper boundary within 100 cm of the mineral soil surface and its lower boundary at a depth of 25 cm or
more below the mineral soil surface or that are in a Grossarenic or Arenic subgroup:
Either the entire argillic, kandic, or natric horizon if 50 cm or less thick, or
the upper 50 cm of the horizon if more than 50 cm thick; or
C. For those Alfisols, Ultisols, and great groups of Aridisols and Mollisols have an argillic, kandic, or natric horizon that has
its upper boundary at a depth of 100 cm or more from the mineral surface and that are not in a Grossarenic or
Arenic subgroup:
Between 25 cm from the mineral soil surface and 100 cm below the mineral soil surface or a root-limiting layer,
whichever is shallower; or
D. For other soils that have an argillic or natric horizon that has its lower boundary at a depth of less than 25 cm from the
mineral soil surface:
Between the upper boundary of the argillic or natric horizon and a depth of 100 cm below the mineral soil surface or a
root-limiting layer, whichever is shallower; or
E. All other mineral soils:
Between 25 cm below the mineral soil surface and either 100 cm below the mineral soil surface or a root limiting layer,
whichever is shallower.
Family Particle-Size Classes
100
90
10
80
20
very fine
70
30
50
t
Sil
%
40
%
Cl
ay
60
50
fine
40
60
30
fine
silty
fine loamy
20
70
80
10
coarse
silty
coarse loamy
90
sandy
100
90
80
70
60
50
% Sand
40
30
20
10
100
PARTICLE SIZE FAMILIES
For soils that are <50 cm to root-limiting layer, or are in a Lithic, Arenic, or Grossarenic subgroup:
<35% clay in control section: “loamy”
≥35% clay in control section: “clayey”
Soils that are >50 cm to a root-limiting layer use the following family names:
35-60% clay in control section: “fine”
>60% clay in control section: “very fine”
NOTE clay % for “textural classes” do NOT correspond exactly to this triangle….
Control sections that have >35% fragments use the following family names:
--sand or loamy sand textures: “sandy-skeletal”
--<35% clay: “loamy-skeletal”
--≥35% clay: “clayey-skeletal”
Family particle size name is NOT used with “psamments” (“sandy” family is implied)
NOTE: “Lithic” families have lithic contact at <50 cm;
“Arenic” families have 50-100 cm of sandy particle size class at surface (A+E);
“Grossarenic” have >100 cm sand at surface.
Particle-Size Class


The particle-size class is determined from the
weighted average particle size within the control
section.
The weighted average is calculated by:
SUM(% fraction horizon X thickness of horizon) / control
section thickness
Particle-Size Control Section
Horizon
Ap
E1
E2
BE
Bt1
Bt2
Depth,
cm
0-15
15-33
33-47
47-62
62-88
88-132
Sand, Clay, %
%
68
12
72
13
74
11
45
25
48
39
53
32
Weighted average clay = [(15 X 25) + (26 X 39) + (9 X 32)] / 50 = 33.5
Weighted average sand = [(15 X 45) + (26 X 48) + (9 X 53)] / 50 = 48.0
Family particle-size class is fine-loamy
Particle-Size Classes


Three generalized particle-size classes
 clayey - >35% clay
 loamy - <35% clay
 sandy – weighted average particle size is sand or loamy sand
 used for soils in shallow families and for soils in arenic, grossarenic,
lithic, and pergelic subgroups
Other soils (fall-out key in Soil Taxonomy)
 Fragmental - <10% fine earth material (<2mm); >90% coarse
fragments
 Sandy-skeletal - sand or loamy sand texture and 35-90% coarse
fragments
 Loamy-skeletal - <35% clay and 35-90% coarse fragments
 Clayey-skeletal - >35% clay and 35-90% coarse fragments
Particle-Size Classes

There are two situations in which particle-size class names are not
used.
 The name is redundant, e.g. Psamments
 The soil is such that particle-size analysis is difficult to apply and
particle-size does not adequately describe particle characteristics

For anisols: terms used to replace particle-size classes include ashy,
pumiceous, medial, hydrous, and others.
Strongly contrasting particle-size classes:
 If the particle size changes significantly within the control section and
the transition is less than 12.5 cm, terms are used to describe the
strongly contrasting particle size classes, e.g. clayey over loamy.


Soils with andic properties
Mineralogy Classes





Control section is the same as that for particle size.
Mineralogy placement is dependent on particle-size class which
determines the fraction of the soil used to determine mineralogy
For many soils, the mineralogy class is determined either for the
whole soil (<2 mm) or for the sand and silt fractions
Clay mineralogy specified only for
 Oxisols
 Kandi and Kanhap great groups
 Soils with fine, very-fine, clayey, or clayey-skeletal particle-size class
Mineralogy class is determined from the key for mineralogy classes
Common Mineralogy Classes for SE Soils

Sandy or loamy particle-size class

Mineralogy of coarse silt and sand (0.02-2 mm)



Siliceous – >90% quartz and other resistant minerals
Mixed – other soils
Fine or very-fine particle size class and kandi or
kanhap great groups

Mineralogy of clay separate (<0.002 mm)



Kaolinitic – >50% kaolinite and <10% smectite
Smectitic – more smectite than any other clay mineral
Mixed – other soils
Cation-Exchange Activity Classes




Intended to help interpret mineralogy and nutrient retention capacity
of soils in mixed and siliceous mineralogy classes
Not used in Histosols, Oxisols, kandi and kanhap great groups, or in
soils with sandy or sandy-skeletal particle size classes
Control section same as particle-size and mineralogy
Based on CEC (pH 7.0) to clay ratio
 superactive - >0.60; smectite and other reactive clays
 active - 0.40 to 0.60
 semiactive - 0.24 to 0.40
 subactive - <0.24; kaolinite and other low-activity clays
Calcareous and Reaction Classes



Only used for selected taxa
Control section - 25 to 50 cm
Classes




allic – high extractable Al
calcareous - effervesces with HCl in all of control section
acid - pH <5.0 in 0.01 M CaCl2 (about 5.5 in water) throughout
the control section.
non-acid - pH >5.0 in 0.01 M CaCl2 in some or all of the control
section - not used in family name of calcareous soils.
Soil Temperature Classes


Mean annual soil temperature estimated by adding 1o C to mean annual air
temperature.
Classes of soil temperature:

Pergelic - mean annual soil temperature <0o C




Cryic - mean annual soil temperature between 0 and 8o C and mean summer soil
temperature is <15o
Frigid - mean annual soil temperature between 0 and 8o C (mean summer temperature >15o
C is implied)
Mesic - mean annual soil temperature between 8 and 15o C


Cotton and yellow pine
Hyperthermic - mean annual soil temperature is 22o C or higher


Corn and winter wheat
Thermic - mean annual soil temperature between 15 and 22 o C


Permafrost at a shallow depth.
Citrus
For frigid, mesic, thermic, and hyperthermic temperature regimes; an iso prefix is
used if the mean summer and mean winter soil temperature differ by less than 6o C,
i.e. isofrigid, isomesic, isothermic, and isohyperthermic.

Designates tropical climates
Other Family Groupings






Used to provide reasonable groupings of certain series
Many of the characteristics are poorly understood
Depth of soil
 “shallow” - less than 50 cm (100 cm in Oxisols) to upper boundary of
rock or root limiting horizon and not in lithic subgroup
Rupture Resistance
 “ortstein” and “noncemented” - used only for Spodosols.
Classes of coatings - only used for Psamments
 “coated” - silt + (2 X clay) > 5
 “Uncoated” - silt + (2 X clay) < 5
Classes of permanent cracks
 “Cracked” is the only class
 Used only in Fluvaquents and Humaquepts
Series

Lowest category in Soil Taxonomy


Purpose is mainly pragmatic


More than 19,000 series in the United States
Closely related to interpretations
Differentiae used for series are the same as those used
for classes in other categories

Series properties cannot range across limits of classes in
higher categories




Cecil must be a fine, kaolinitic, thermic Typic Kanhapludult
Rion must be a fine-loamy, siliceous, themic Typic Kanhapludult
Cataula must be a fine, kaolinitic, themic Oxyaquic Kanhapludult
Many families only have one series
Greater Restriction within a Family

Fine, kaolinitic, thermic Typic Kanhapludults



fine-loamy, siliceous, thermic Plinthic Kandiudults



Cecil – Bt horizon has hue of 2.5YR or redder
Appling – Bt horizon has hue of 5YR or yellower
Tifton – common Fe stone in upper Bt
Dothan – no Fe stone in upper Bt
Distinctions within a family are restrictions in the range
of one or more properties of the family

Only those differences that serve to distinguish one series from
another are included in statements of series differences
3 Tests of Series Differentiae



Properties serving as differentiae can be observed or can be inferred
with reasonable assurance
Differentiae must create soil series having a unique range of
properties
 Difference among series should be greater than normal errors made
by qualified pedologists
Differentiae must reflect a property of the soils
 Can be reflected in the nature or degree of expression of one or more
horizons.


May also be




Can be almost any horizon or soil property
Landscape property
Commonly associated soils
Climate
We may not be as smart as we think
Phase

Properties that may influence certain but not all uses of
a soil






Slope
Stoniness
Aspect
“Wind swept”
Not a part of Soil Taxonomy
Utilitarian classification that can be superimposed at
any categorical level to permit more precise
interpretations for soil use
Competing Series Statement - Cecil
These are the Appling, Bethlehem, Madison, Nankin, Pacolet, Tumbleton, and
Wedowee series in the same family
Those in closely related families are the Aragon, Braddock, Cataula, Chestatee,
Cullen, Georgeville, Hayesville, Herndon, Hulett, Kolomoki, Lloyd, Mayodan,
Mecklenburg, Spotsylvania, Tatum and Wedowee series
Appling soils have dominant hue of 7.5YR or yellower or where hue is 5YR it
has evident patterns of mottling in a subhorizon of the Bt or BC horizon
Aragon soils contain fragments of chert, and have a cherty limestone C horizon.
Bethlehem soils are moderately deep to weathered bedrock of sillimanite schist,
phyllite schist, or mica schist.
Braddock and Hayesville soils are mesic
Cataula soils have a fragipan, Chestatee soils contain more than 15 percent, by
volume, of coarse fragments throughout the pedon . . . . . . .
Piedmont Series Key

Upper 50 cm of Bt horizon has >35% clay

Bt horizon has hue of 5YR or redder



Upper part of Bt horizon has common mica flakes - Madison
Bottom of Bt horizon with more than 35% clay is 45 to 75 cm
below the surface – Pacolet
Bottom of Bt horizon with more than 35% clay is more than
75 cm below the surface – Cecil