Sabine Grunwald

U.S. Soil Taxonomy

Soil Classification
Simplified Key to Soil Orders
Soil Orders: Formative Elements
Suborders: Formative Elements
Great Groups: Formative Elements

Web Link
Keys to Soil Taxonomy (Soil Survey Staff) USDA-NRCS

Soil Classification

Six categories are distinguished according U.S. Soil Taxonomy:

Order (11 taxa): This category is based largely on soil forming processes as indicated by the presence or absence of major diagnostic horizons. A given order includes soils whose properties suggest that they are not dissimilar in their genesis. They are thought to have been formed by the same general genetic processes.
Suborder (60 taxa): Suborders are subdivisions of orders that emphasize genetic homogeneity . The presence or absence of properties associated with wetness, climatic environment, major parent material, and vegetation.
Great Group (approximate 303): Great groups are subdivisions of suborders according to similar kind, arrangement, and diagnostic horizons. The emphasis is on the presence or absence of specific diagnostic features, base status, soil temperature, and soil moisture regimes.
Subgroup (> 1,200): Subgroups are subdivisions of the great groups. The central concept of a great group makes up one group (Typic). Other subgroups may have characteristics that are intergrades between those of the central concept and those of the orders, suborders, or great groups. Extragradation is used to identify critical properties common in soils in several orders, suborders, and great groups.
Family: Families are found in soils with a subgroup having similar physical and chemical properties affecting their response to management and especially to the penetration of plant roots. Differences in texture, mineralogy, temperature, and soil depth are bases for family differentiation.
Series (approximate 17,000 in the U.S.): Its differentiating characteristics are based primarily on the kind an arrangement of horizons, color, texture, structure, consistence, reaction of horizons, chemical, and mineralogical properties of the horizons.
(Phase: technically not a class in Soil Taxonomy but used in field surveying)

Figure 1. Orders, suborders, great groups, subgroups, families, and series - U.S. Soil Taxonomy.

Soil Taxonomy is based on the properties of soils as they are found in the landscape. One objective of the system is to group soils similar in genesis, but the specific criteria used to place soils in these groups are those of soil properties. Because Soil Taxonomy is a hierarchical system each soil is grouped first in the broadest category first. When more details are added lower categories are defined. Differentiating characteristics are not uniformly applied to all soils at a given categorical level, because soils have an enormous complexity. Therefore, in Soil Taxonomy certain types of differentiating characteristics are applied only to certain taxa (of the level above which one is considering) to produce the desired taxa at the level with which one is dealing.

Simplified Key to Soil Orders

Table 1. Brief description of Soil Orders.

Soil Order	General Features
Alfisols	Alfisols develop in humid and subhumid climates, have average annual precipitation of 500-1300 mm. They are frequently under forest vegetation. Characteristic features: Clay accumulation in a Bt horizon, thick E horizon, available water much of the growing season, slightly to moderately acid.
Andisols	Andisols are soils with over 60 % volcanic ejecta (ash, cinder, pumice, basalt) with bulk densities below 900 kg/m³. Characteristic features: Dark A horizon, early-stage secondary minerals (allophane, imogolite, ferrihydrite clays), high adsorption and immobilization of phosphorus, very high cation exchange capacity.
Aridisols	Aridisols exist in dry climates. Characteristic features: horizons of lime or gypsum accumulation, salty layers, and/or A and Bt horizons.
Entisols	Entisols have no profile development except a shallow marginal A horizon. Many recent river floodplains, volcanic ash deposits, unconsolidated deposits with horizons eroded away, and sands are Entisols.
Gelisols	Gelisols are soils that contain within 200 cm of the ground surface the condition known as permafrost.
Histosols	Histosols are organic soils (peat and mucks) consisting of variable depths of accumulated plant remains in bogs, marshes, and swamps.
Inceptisols	Inceptisols, especially in humid regions, have weak to moderated horizon development. Horizon development have been retarded because of cold climate, waterlogged soils, or lack of time for stronger development. Characteristic feature: Texture has to be finer than loamy very fine sand.
Mollisols	Mollisols are frequently under grassland, but with some broadleaf forest-covered soils. Characteristic features: Deep, dark A horizons, they may have B horizons and lime accumulation.
Oxisols	Oxisols are excessively weathered, whereas few original minerals are left unweathered. They develop only in tropical and subtropical climates. Characteristic features: Often Oxisols are over 3 m deep, have low fertility, have dominantly iron and aluminum clays, and are acid.
Spodosols	Spodosols are typically the sandy, leached soils of cold coniferous forests. Characteristic features: O horizons, strongly acid profiles, well-leached E horizons, Bh or Bs horizons of accumulated organic material plus iron and aluminum oxides.
Ultisols	Ultisols are extensively weathered soils of tropical and subtropical climates. Characteristic features: Thick A horizon, clay accumulation in a Bt, strongly acid.
Vertisols	Vertisols exist most in temperate to tropical climate with distinct wet and dry seasons. They have a high content of clays that swell when wetted and show cracks when dry. Characteristic features: Deep self-mixed A horizon , top soil falls into cracks seasonally, gradually mixing the soil to the depth of the cracking.

Figure 2. Soil profiles: Vertisol, Spodosol, Alfisol, and Ultisol (Foth, 1984).

Figure 3. Soil profiles: Oxisol, Mollisols, and Andisol (Foth, 1984).

Reference
Foth H.D., 1984. Fundamentals of Soil Science. John Wiley & Sons, Inc.

Soil Orders: Formative Elements

Table 2. Formative elements of soil orders.

Soil Order	Derivation	Formative element
Alfisols	Nonsense symbol	alf
Andisols	Jap. ando, black soil	and
Aridisols	L. aridus, dry	id
Entisols	Nonsense symbol	ent
Gelisols	Gr. gelid, very cold	el
Histosols	Gr. histos, tissue	ist
Inceptisols	L. inceptum, beginning	ept
Mollisols	L. mollis, soft	oll
Oxisols	Fr. oxide, oxide	ox
Spodosols	Gr. Spodos, wood ash	od
Ultisols	L. ultimus, last	ult
Vertisols	L. verto, turn	ert

Suborders: Formative Elements

Table 3. Suborder names in Soil Taxonomy - formative elements and meaning.

Formative element	Derivation	Meaning or Connotation
alb	L. albus, white	Presence of an albic horizon
aqu	L. aqua, water	Characteristics associated with wetness
ar	L. arare, to plow	Mixed horizon
arg	L. argilla, white clay	Presence of an argillic horizon
bor	Gr. boreas, northern	Cool climate
calc	L. calcis, lime	Presence of a calcic horizon
camb	L. cambiare, to exchange	Presence of a cambic horizon
cry	Gr. kryos, cold	Cold climate
dur	L. duras, hard	Presence of a duripan
fibr	L. fibra, fiber	Least decomposed stage
fluv	L. fluvius, river	Flood plains
fol	L. folia, leaf	Mass of leaves
gyps	L. gypsum, gypsum	Presence of a gypsic horizon
hem	Gr. hemi, half	Intermediate state of decomposition
hist	Gr. histos, tissue	Organic soil material
hum	L. humus, earth	Presence of organic matter
ochr	Gr. base of ochros, pale	Presence of an ochric epipedon
orth	Gr. orthos, true	The common ones
plagg	Ger. Plaggen, sod	Presence of plaggen epipedons
per	L. perennis, all year	Perudic soil moisture regime
psamm	Gr. psammos, sand	Sand textures
rend	Polish Rendzina, limestone soil	Rendzinalike
sal	L. sal, salt	Presence of a salic horizon
sapr	Gr. sapros, rotten	Most decomposed stage
stat	Gr. statiskos, stationary	No congellipedoturbation
torr	Gr. torridus, hot, dry	Torric soil moisture regime
trop	Gr. tropikos, of the solstice	Continually warm
turb	L. turbidus, disturbed	Active congellipedoturbation
ud	L. udus, humid	Udic soil moisture regime
umbr	L. umbra, shade	Presence of an umbric epipedon
ust	L. ustus, burnt	Ustic soil moisture regime
vitr	L. vitrum, glass	Presence of glass
xer	Gr. xeros, dry	Xeric soil moisture regime

Great Groups: Formative Elements

Table 4. Great group names in Soil Taxonomy - formative elements and meaning.

Formative element	Derivation	Meaning or connotation
Acr	Gr. akros, at the end	Extreme wethering
Agr	L. ager, field	An agric horizon
Alb, Al	L. albus, white	An albic horizon
Anhyd	Gr. anhydros, dry	Without water
Aqui, Aqu	L. aqua, water	Wetness
Arg	L. argilla, white clay	An argillic horizon
Bor	Gr. boreas, northern	Cool climate
Calc	L. calcic, lime	A calcic horizon
Camb	L. cambiare, to exchange	A cambic horizon
Cry	Gr. Kryos, cold	Cold climate
Duri, Dur	L. durus, hard	A duripan
Dystr	Gr. dys, ill; dystrophic, infertile	Low base saturation
Endo	Gr. endon, within	Groundwater
Eutr	Gr. eu, good; eutrophic, fertile	High base saturation
Epi	Gr. epi, upon	Perched water table
Ferr	L. ferrum, iron	Presence of iron
Fluv	L. fluvus, river	Flood plain
Fulv	L. fulvus, deep yellow	Dark yellow epipedon
Frag	L. fragillis, brittle	A fragipan
Fragloss	Compound of fra(g) and gloss	See the formative elements frag and gloss
Geli	Gr. gelid, very cold	Permafrost
Gloss	Gr. glossa, tongue	Interfingered horizon boundary
Gyps	L. gypsum, gypsum	Gypsic horizon
Hal	Gr. hals, salt	Salty
Hapl	Gr. haplous, simple	Minimum horizon
Hum	L. humus, earth	Presence of humus
Hydr	Gr. hydro, water	Presence of water
Kandi	Modified from kandite	A kandic horizon
Kanhapl	Compound kan(di) and hapl	Thin kandic horizon
Luvi, Lu	Gr. louo, to wash	Illuvial
Med	L. meda, middle	Temperate climate
Melan, Melano	Gr. melas, black	Melanic epipedon
Natr	Modified form natrium, sodium	A natric horizon
Pale	Gr. paleos, old	Old development
Petro	Gr. petra, rock	Petrocalcic horizon
Plac	Gr. base of plax, flat stone	Presence of a thin pan
Plinth	Gr. plinthos, brick	A plinthite
Psamm	Gr. psammos, sand	Sand texture
Quartzi	Ger. quarz, quartz	High quartz content
Rhod	Gr. base of rhodon, rose	Dark red colors
Sali, Sal	L. base of sal, salt	Salic horizon
Sombri	Fr. sombre, dark	A dark horizon
Sphagno	Gr. sphagnos, bog	Presence of sphagnum moss
Sulfo, Sulf	L. sulfur, sulfur	Presence of sulfides
Torri	L. torridus, hot and dry	Usually dry
Trop	Gr. tropikos, of the solstice	Continually warm
Udi, Ud	L. udus, humid	Udic soil moisture regime
Umbr	L. base of umbra, shade	Umbric epipedon
Usti, ust	L. base of ustus, burnt	Ustic soil moisture regime
Vermi, Verm	L. base of vermes, worm	Wormy, or mixed by animals
Vitri, Vit	L. vitrum, glass	Presence of glass
Xero, Xer	Gr. xerox, dry	Xeric soil moisture regime