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Soil Formation and Classification

The National Cooperative Soil Survey identifies and maps over 20,000 different kinds of soil in the United States. Most soils are given a name, which generally comes from the locale where the soil was first mapped. Named soils are referred to as soil series.

Soil survey reports include the soil survey maps and the names and descriptions of the soils in a report area. These soil survey reports are published by the National Cooperative Soil Survey and are available to everyone.

Soils are named and classified on the basis of physical and chemical properties in their horizons (layers). “Soil Taxonomy” uses color, texture, structure, and other properties of the surface two meters deep to key the soil into a classification system to help people use soil information. This system also provides a common language for scientists.

Soils and their horizons differ from one another, depending on how and when they formed. Soil scientists use five soil factors to explain how soils form and to help them predict where different soils may occur. The scientists also allow for additions and removal of soil material and for activities and changes within the soil that continue each day.

Soil Forming Factors

Parent material. Few soils weather directly from the underlying rocks. These “residual” soils have the same general chemistry as the original rocks. More commonly, soils form in materials that have moved in from elsewhere. Materials may have moved many miles or only a few feet. Windblown “loess” is common in the Midwest. It buries “glacial till” in many areas. Glacial till is material ground up and moved by a glacier. The material in which soils form is called “parent material.” In the lower part of the soils, these materials may be relatively unchanged from when they were deposited by moving water, ice, or wind.

Sediments along rivers have different textures, depending on whether the stream moves quickly or slowly. Fast-moving water leaves gravel, rocks, and sand. Slow-moving water and lakes leave fine textured material (clay and silt) when sediments in the water settle out.

Climate. Soils vary, depending on the climate. Temperature and moisture amounts cause different patterns of weathering and leaching. Wind redistributes sand and other particles especially in arid regions. The amount, intensity, timing, and kind of precipitation influence soil formation. Seasonal and daily changes in temperature affect moisture effectiveness, biological activity, rates of chemical reactions, and kinds of vegetation.

Topography. Slope and aspect affect the moisture and temperature of soil. Steep slopes facing the sun are warmer, just like the south-facing side of a house. Steep soils may be eroded and lose their topsoil as they form. Thus, they may be thinner than the more nearly level soils that receive deposits from areas upslope. Deeper, darker colored soils may be expected on the bottom land.

Biological factors. Plants, animals, micro-organisms, and humans affect soil formation. Animals and micro-organisms mix soils and form burrows and pores. Plant roots open channels in the soils. Different types of roots have different effects on soils. Grass roots are “fibrous” near the soil surface and easily decompose, adding organic matter. Taproots open pathways through dense layers. Micro-organisms affect chemical exchanges between roots and soil. Humans can mix the soil so extensively that the soil material is again considered parent material.

The native vegetation depends on climate, topography, and biological factors plus many soil factors such as soil density, depth, chemistry, temperature, and moisture. Leaves from plants fall to the surface and decompose on the soil. Organisms decompose these leaves and mix them with the upper part of the soil. Trees and shrubs have large roots that may grow to considerable depths.

Time. Time for all these factors to interact with the soil is also a factor. Over time, soils exhibit features that reflect the other forming factors. Soil formation processes are continuous. Recently deposited material, such as the deposition from a flood, exhibits no features from soil development activities. The previous soil surface and underlying horizons become buried. The time clock resets for these soils. Terraces above the active floodplain, while genetically similar to the floodplain, are older land surfaces and exhibit more development features.

These soil forming factors continue to affect soils even on “stable” landscapes. Materials are deposited on their surface, and materials are blown or washed away from the surface. Additions, removals, and alterations are slow or rapid, depending on climate, landscape position, and biological activity.

When mapping soils, a soil scientist looks for areas with similar soil-forming factors to find similar soils. The colors, texture, structure, and other properties are described. Soils with the same kind of properties are given taxonomic names. A common soil in the Midwest reflects the temperate, humid climate and native prairie vegetation with a thick, nearly black surface layer. This layer is high in organic matter from decomposing grass. It is called a “mollic epipedon.” It is one of several types of surface horizons that we call “epipedons.” Soils in the desert commonly have an “ochric” epipedon that is light colored and low in organic matter. Subsurface horizons also are used in soil classification. Many forested areas have a subsurface horizon with an accumulation of clay called an “argillic” horizon.

Soil Orders

Soil taxonomy at the highest hierarchical level identifies 12 soil orders. The names for the orders and taxonomic soil properties relate to Greek, Latin, or other root words that reveal something about the soil. Sixty-four suborders are recognized at the next level of classification. There are about 300 great groups and more than 2,400 subgroups. Soils within a subgroup that have similar physical and chemical properties that affect their responses to management and manipulation are families. The soil series is the lowest category in the soil classification system.

Soil Order Formative Terms Pronunciation
Alfisols Alf, meaningless syllable Pedalfer
Andisols Modified from ando Ando
Aridisols Latin, aridies, dry Arid
Entisols Ent, meaningless Recent
Gelisols Latin gelare, to freeze Jell
Histosols Greek, histos, tissue Histology
Inceptisols Latin, incepum, beginning Inception
Mollisols Latin, mollis, soft Mollify
Oxisols French oxide Oxide
Spodosols Greek spodos, wood ash Odd
Ultisols Latin ultimus, last Ultimate
Vertisols Latin verto, turn Invert


The distribution of these soil orders in the United States corresponds with the general patterns of the soil forming factors across the country. A map of soil orders is useful in understanding broad areas of soils. Detailed soil maps found in soil survey reports, however, should be used for local decision making. Soil maps are like road maps, for very general overview, a small scale map in an atlas is helpful, but for finding a location of a house in a city, a large scale detailed map should be used.

More detailed information on soil orders is available in “Soil Taxonomy” pp. 837-850, Chapter 22.

Formative Elements in Names of Soil Suborders

Formative Element Derivation Sounds Like Connotation
Alb L, albus, white Albino Presence of albic horizon
Anthr Modified from Gr. anthropes, human Anthropology Modified by humans
Aqu L. aqua, water Aquifer Aquic conditions
Ar L. Arare, to plow Arable Mixed horizons
Arg Modified from argillic horizon; L. argilla, white clay Argillite Presence of argillic horizon
Calc L. calcis, lime Calcium Presence of a calcic horizons
Camb L. cambiare, to exchange Am Presence of a cambic horizon
Cry G. kryos, icy cold Cry Cold
Dur L. durus, hard Durable Presence of a duripan
Fibr L. fibra, fiber Fibrous Least decomposed stage
Fluv L. fluvius, river Fluvial Flood plain
Fol L. folia, leaf Foliage Mass of leaves
Gyps L. gypsum, gypsum Gypsum Presence of a gypsic horizon
Hem Gr hemi, half Hemisphere Intermediate stage of decomposition
Hist Gr. histos, tissue Histology Presence of organic materials
Hum L. humus, earth Humus Presence of organic matter
Orth Gr. orthos, true Orthodox The common ones
Per L. Per, throughout in time Perennial Perudic moisture regime
Psamm Gr. psammos, sand Sam Sandy texture
Rend Modified from Rendzina End High carbonate content
Sal L. base of sal, salt Saline Presence of a salic horizon
Sapr Gr. sapros, rotten Sap Most decomposed stage
Torr L. torridus, hot and dry Or Torric moisture regime
Turb L. Turbidis, disturbed Turbulent Presence of cryoturbation
Ud L. udus, Humid You Udic moisture regime
Vitr L. vitrum, glass It Presence of glass
Ust L. ustus, burnt Combustion Ustic moisture regime
Xer Gr. xeros, dry Zero Xeric moisture regime

Formative Elements in Names of Soil Great Groups

Formative Element Derivation Sounds Like Connotation
Acr Modified from Gr. Akros, at the end Act Extreme weathering
Al Modified from aluminum Algebra High aluminum, low iron
Alb L. Albus, white Albino An albic horizon
Anhy Gr. anydros, waterless Anhydrous Very dry
Anthr Modified from Gr. anthropos, human Anthropology An anthropic epipedon
Aqu L. aqua, water Aquifer Aquic conditions
Argi Modified from argillic horizon; L. argilla, white clay Argillite Presence of an argillic horizon
Calci, calc L. calcis,lime Calcium A calcic horizon
Cry Gr. kryos, icy cold Cry Cold
Dur L. durus, hard Durable A duripan
Dystr, dys Modified from Gr. dys, ill; dystrophic infertile Distant Low base saturation
Endo Gr. endon, endo, within Endothermic Implying a ground water table
Epi Gr. epi, on, above Epidermis Implying a perched water table
Eutr Modified from Gr. eu, good; euthrophic, fertile You High base saturation
Ferr L. ferrum, iron Fair Presence of iron
Fibr L. fibra, fiber Fibrous Least decomposed stage
Fluv L. fluvius, river Fluvial Flood plain
Fol L. folia, leaf Foliage Mass of leaves
Fragi Modified from L. fragilis, brittle Fragile Presence of fragipan
Fragloss Compound of fra (g) and gloss   See the formative elements "frag" and "gloss"
Fulv L. fulvus, dull brownish yellow Full Dark brown color, presence of organic carbon
Glac L. glacialis, icy Glacier Ice lenses or wedges
Gyps L. gypsum, gypsum Gypsum Presence of gypsic horizon
Gloss Gr. glossa, tongue Glossary Presence of a glossic horizon
Hal Gr. hals, salt Halibut Salty
Hapl Gr. haplous, simple Haploid Minimum horizon development
Hem G. hemi, half Hemisphere Intermediate stage of decomposition
Hist Gr. histos, tissue History Presence of organic materials
Hum L. humus, earth Humus Presence of organic matter
Hydr Gr. hydo, water Hydrophobia Presence of water
Kand, kan Modified from kandite Can 1:1 layer silicate clays
Luv Gr. louo, to wash Ablution Illuvial
Melan Gr. melasanos, black Me + Land Black, presence of organic carbon
Moll L. mollis, soft Mollusk Presence of a mollic epipedon
Natr Modified from natrium, sodium Date Presence of natric horizon
Pale Gr. paleos, old Paleontology Excessive development
Petr Gr. comb. form of petra, rock Petrified A cemented horizon
Plac Gr. base of plax, flat stone Placard Presence of thin pan
Plagg Modified from Ger. plaggen, sod Awe Presence of plaggen epipedon
Plinth Gr. plinthos, brick In Presence of plinthite
Psamm Gr. psammos, sand Sam Sandy texture
Quartz Ger. quarz, quartz Quarter High quartz content
Rhod Gr. base of rhodon, rose Rhododendron Dark red color
Sal L. base of sal, salt Saline Presence of salic horizon
Sapr Gr. saprose, rotten Sap Most decomposed stage
Somb F. sombre, dark Somber Presence of sombric horizon
Sphagn Gr. sphagnos, bog Sphagnum Presence of Sphagnum
Sulf L. sulfur, sulfur Sulfur Presence of sulfides or their oxidation products
Torr L. torridus, hot and dry Torrid Torric moisture regime
Ud L. udus, humid You Udic moisture regime
Umbr L. umbra, shade Umbrella Presence of umbric epipedon
Ust L. ustus,burnt Combustion Ustic moisture regime
Verm L. base of vermes, worm Vermilion Wormy, or mixed by animals
Vitr L. vitrum, glass It Presence of glass
Xer Gr. xeros, dry Zero Xeric moisture regime