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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/m 3. 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