Soil classification: the soil orders
How soils are classified: the twelve orders of Soil Taxonomy, the World Reference Base groups, diagnostic horizons, and how to read a soil name.
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Soils differ as much as climates do. A black, cracking clay that swells in the rains, a bright red, deeply weathered tropical earth, a grey, sandy soil under pine forest and a peat bog are all "soil", but each needs very different management. Soil classification groups soils by their properties so that knowledge gained about one can be applied to others like it.
Two international systems dominate: the United States Department of Agriculture's Soil Taxonomy, with its twelve orders, and the World Reference Base for Soil Resources (WRB) of the International Union of Soil Sciences, used for international correlation and by FAO. Exams ask for the orders, their key features and the meaning of their names.
How modern classification works
Early classifications, beginning with V. V. Dokuchaev in Russia in the 1880s, grouped soils by the environment that formed them, such as climate and vegetation. Modern systems classify soils by measurable properties of the profile, especially diagnostic horizons: layers defined by precise criteria.
Some important diagnostic horizons (Soil Taxonomy names):
| Horizon | Type | What defines it |
|---|---|---|
| Mollic epipedon | surface | thick, dark, rich in organic matter, high base saturation (grasslands) |
| Ochric epipedon | surface | light-coloured or thin; too little organic matter to be mollic |
| Argillic horizon | subsurface | clay that has moved down from above and accumulated |
| Cambic horizon | subsurface | weakly altered; some structure or colour change, little accumulation |
| Spodic horizon | subsurface | accumulated humus with aluminium and often iron, washed from above |
| Oxic horizon | subsurface | highly weathered: oxides, kaolinite, very low CEC |
| Calcic, gypsic, salic horizons | subsurface | accumulated calcium carbonate, gypsum or soluble salts |
Soil moisture and temperature regimes (for example aridic for dry soils, udic for humid, ustic for seasonally dry, aquic for saturated) are also used.
The twelve orders of Soil Taxonomy
| Order | Formative element | Key features | Typical settings |
|---|---|---|---|
| Entisols | -ent | little or no horizon development | young floodplain alluvium, dunes, steep slopes |
| Inceptisols | -ept | weakly developed (cambic) subsoil | young landscapes, mountains, many river plains |
| Mollisols | -oll | thick, dark, fertile surface (mollic epipedon), high bases | grasslands: prairies, steppes, pampas |
| Alfisols | -alf | clay-enriched (argillic) subsoil, moderate to high bases | temperate and subhumid forests and savannas |
| Ultisols | -ult | clay-enriched subsoil, low bases, acid | warm humid regions, older landscapes |
| Oxisols | -ox | very weathered; iron and aluminium oxides, kaolinite, low CEC | humid tropics: Amazon basin, central Africa |
| Vertisols | -ert | high in swelling clays; deep cracks when dry, slickensides | basalt and clay plains with wet and dry seasons |
| Aridisols | -id | dry most of the year; accumulations of lime, gypsum, salts | deserts and semi-deserts |
| Andisols | -and | formed in volcanic ash; light, porous, high organic matter | volcanic regions around the Pacific, East Africa |
| Spodosols | -od | acid, often sandy; spodic horizon of humus, Al and Fe | cool humid conifer forests, tropical sandy heaths |
| Histosols | -ist | organic soils (peat, muck) | bogs, fens, wetlands |
| Gelisols | -el | permafrost within about 2 m | arctic and subarctic, high mountains |
The categories below the order are suborder, great group, subgroup, family and series. Each adds a syllable or word, so a Soil Taxonomy name can be read like a short description.
Reading a soil name
Take the subgroup name Typic Haplustalf:
- -alf: the order is Alfisols: a clay-enriched subsoil with moderate to high base saturation.
- ust-: the suborder is Ustalfs, with an ustic moisture regime: moist in the growing season but dry for long periods, typical of semi-arid and subhumid climates.
- Hapl- (from Greek haplous, simple): the great group Haplustalfs, with no special extra features.
- Typic: the subgroup that represents the central concept of the great group.
Another example: Aquic Hapludert is a Vertisol (-ert) with a humid udic regime (-ud-), simple (Hapl-), but with some wetness features (Aquic).
The World Reference Base
The WRB, now in its fourth edition (2022), recognises 32 Reference Soil Groups, many with names inherited from older FAO and Russian traditions, such as Ferralsols, Acrisols, Luvisols, Chernozems, Vertisols, Podzols, Andosols, Histosols, Leptosols, Fluvisols, Cambisols, Gleysols, Solonchaks and Solonetz. Qualifiers added to the group name describe further properties (for example Haplic Luvisol, Calcic Vertisol).
There is no exact one-to-one match between systems, but approximate correspondences help:
| Soil Taxonomy | WRB (approximate) |
|---|---|
| Oxisols | Ferralsols |
| Ultisols | Acrisols, Alisols |
| Alfisols | Luvisols, Lixisols |
| Mollisols | Chernozems, Kastanozems, Phaeozems |
| Spodosols | Podzols |
| Vertisols | Vertisols |
| Andisols | Andosols |
| Histosols | Histosols |
| Aridisols | Calcisols, Gypsisols, Solonchaks, and others |
| Entisols | Fluvisols, Arenosols, Leptosols, Regosols |
| Inceptisols | Cambisols, Gleysols (some), and others |
What the orders mean for farming
- Mollisols (Chernozems) of the world's grasslands, from the North American prairies to the Ukrainian and Russian steppes and the Argentine pampas, are among the most productive soils on Earth.
- Oxisols and Ultisols of the humid tropics are deep and well drained but naturally infertile, acid, low in CEC and high in phosphorus fixation; they need lime, phosphorus, organic matter and careful nutrient management, and become very productive with them, as in the Brazilian cerrado.
- Vertisols hold large amounts of water and nutrients but are hard to work: sticky when wet, hard and cracked when dry, with a narrow window for tillage; they suit cotton, sorghum, chickpea and other crops that use stored moisture.
- Andisols are fertile and easy to work but fix phosphorus strongly.
- Aridisols need irrigation and careful salinity management.
- Histosols, when drained for farming, subside and release large amounts of carbon dioxide as the peat oxidises.
Key points
- Modern systems classify soils by measurable profile properties, especially diagnostic horizons and moisture and temperature regimes.
- Soil Taxonomy has 12 orders: Entisols, Inceptisols, Mollisols, Alfisols, Ultisols, Oxisols, Vertisols, Aridisols, Andisols, Spodosols, Histosols, Gelisols.
- Each order has a formative element (-ent, -ept, -oll, -alf, -ult, -ox, -ert, -id, -and, -od, -ist, -el) used in all lower categories.
- Categories: order, suborder, great group, subgroup, family, series.
- WRB (2022) has 32 Reference Soil Groups; approximate equivalents include Oxisols–Ferralsols and Mollisols–Chernozems.
- Orders guide management: fertile Mollisols, infertile but improvable Oxisols and Ultisols, difficult but productive Vertisols.
What the exams ask
- School-leaving and entrance papers: names of the soil orders and their key features, and examples such as black cracking clays (Vertisols) and laterite-like tropical soils (Oxisols).
- Degree and research-fellowship entrances: diagnostic horizons, formative elements, reading a Soil Taxonomy name, moisture regimes, and correspondence with WRB groups.
- Soil survey and professional examinations: classifying a described profile, and management implications of each order.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why are there two main systems?
Soil Taxonomy was developed for soil survey in the United States and spread widely; the WRB was developed internationally to provide a common language for correlation between national systems. Many countries also keep their own national classifications.
Are "laterite" and Oxisol the same?
Not exactly. "Laterite" is an older term for iron-rich material that hardens irreversibly on exposure; it occurs in some Oxisols and Ultisols. Modern classifications use Oxisols (Ferralsols) and specific horizons such as plinthite instead.
Which order covers the most land?
Estimates vary by mapping source, but the young and weakly developed soils (Entisols and Inceptisols) and the dry Aridisols together cover a large share of the ice-free land surface. Mollisols, Oxisols and Ultisols cover much of the world's productive or potentially productive land.
Sources
- Soil Survey Staff. Keys to Soil Taxonomy, 13th edition. USDA Natural Resources Conservation Service, 2022.
- Soil Survey Staff. Soil Taxonomy, 2nd edition. USDA Agriculture Handbook 436, 1999.
- IUSS Working Group WRB. World Reference Base for Soil Resources, 4th edition. International Union of Soil Sciences, 2022.
- Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 3.
- Buol, S. W. et al. Soil Genesis and Classification, 6th edition. Wiley-Blackwell, 2011.
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