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Chapter 2: Key to the reference soil groups
Key to the reference soil groups of the world reference base for soil resources
For describing and defining the reference soil groups of the World Reference Base for Soil Resources, use is made of soil characteristics, properties and horizons which are combined to define soils and their relationships.
Soil characteristics are single parameters which are observable or measurable in the field or laboratory, or can be analysed using microscope techniques. They include such characteristics as colour, texture and structure of the soil, features of biological activity, arrangement of voids and pedogenic concentrations (mottles, cutans nodules, etc.) as well as analytical determinations (soil reaction, particle-size distribution, cation exchange capacity, exchangeable cations, amount and nature of soluble salts, etc.).
Soil properties are combinations ("assemblages") of soil characteristics which are known to occur in soils and which are considered to be indicative of present or past soil-forming processes (e.g. vertic properties, which are a combination of heavy texture, smectitic mineralogy, slickensides, hard consistence when dry, sticky when wet, shrinking when dry and swelling when wet).
Soil horizons are three-dimensional pedological bodies which are more or less parallel to the earth's surface. Each horizon contains one or more property, occurring over a certain depth, which characterizes it. The thickness varies from a few centimetres to several metres; most commonly it is about a few decimetres. The upper and lower limits ("boundaries") are gradual, clear or abrupt. Laterally, the extension of a soil horizon varies greatly, from a metre to several kilometres. However, a soil horizon is never infinite. Laterally, it disappears or grades into another horizon.
Soils are defined by the vertical combination of horizons, occurring within a defined depth, and by the lateral organization ("sequence") of the soil horizons, or by the lack of them, at a scale reflecting the relief or a land unit.
Key to the reference soil groups of the world reference base for soil resources
Soils having a histic or folic horizon,
1. either a. 10 cm or more thick from the soil surface to a lithic or paralithic contact;
or b. 40 cm or more thick and starting within 30 cm from the soil surface; and2. lacking an andic or vitric horizon starting within 30 cm from the soil surface.
HISTOSOLS (HS)
Other soils having one or more cryic horizons within 100 cm from the soil surface.
CRYOSOLS (CR)
Other soils having either
1. a hortic, irragric, plaggic or terric horizon 50 cm or more thick; or
2. an anthraquic horizon and an underlying hydragric horizon with a combined thickness of 50 cm or more.
ANTHROSOLS (AT)
Other soils, which are either
1. Imitated in depth by continuous hard rock within 25 cm from the soil surface; or
2. overlying material with a calcium carbonate equivalent of more than 40 percent within 25 cm from the soil surface; or
3. containing less than 10 percent (by weight) fine earth to a depth of 75 cm or more from the soil surface; and
4. having no diagnostic horizons other than a mollic, ochric, umbric, yermic or vertic horizon.
LEPTOSOLS (LP)
Other soils having
1. a vertic horizon within 100 cm from the soil surface; and
2. after the upper 20 cm have been mixed, 30 percent or more clay in all horizons to a depth of 100 cm or more, or to a contrasting layer (lithic or paralithic contact, petrocalcic, petroduric or petrogypsic horizons, sedimentary discontinuity, etc.) between 50 and 100 cm; and
3. cracks2 which open and close periodically.
2 A crack is a separation between gross polyhedrons. If the surface is strongly self-mulching, i.e. a mass of granules ("grumic"), or if the soil is cultivated while cracks are open, the cracks may be filled mainly by granular materials from the soil surface but they are open in the sense that the polyhedrons are separated. A crack is regarded as open if it controls the infiltration and percolation of water in dry, clayey soil (Soil Survey Staff, 1996). If the soil is irrigated the upper 50 cm has a coefficient of linear extensibility (COLE) of 0.06 or more throughout.
VERTISOLS (VR)
Other soils having
1. fluvic soil material starting within 25 cm from the soil surface and continuing to a depth of at least 50 cm from the soil surface; and
2. no diagnostic horizons other than a histic, mollic, ochric, takyric, umbric, yermic, salic or sulfuric horizon.
FLUVISOLS (FL)
Other soils having
1. a salic horizon starting within 50 cm from the soil surface; and
2. no diagnostic horizons other than a histic, mollic, ochric, takyric, yermic, calcic, cambic, duric, gypsic or vertic horizon.
SOLONCHAKS (SC)
Other soils having
1. gleyic properties within 50 cm from the soil surface; and
2. no diagnostic horizons other than a anthraquic, histic, mollic, ochric, takyric, umbric, andic, calcic, cambic, gypsic, plinthic, salic, sulfuric or vitric horizon within 100 cm from the soil surface.
GLEYSOLS (GL)
Other soils having
1. either a vitric or an andic horizon starting within 25 cm from the soil surface; and
2. having no diagnostic horizons (unless buried deeper than 50 cm) other than a histic, fulvic, melanic, mollic, umbric, ochric, duric or cambic horizon.
ANDOSOLS (AN)
Other soils having a spodic horizon starting within 200 cm from the soil surface, underlying an albic, histic, umbric or ochric horizon, or an anthropedogenic horizon less than 50 cm thick.
PODZOLS (PZ)
Other soils having either
1. a petroplinthic horizon starting within 50 cm from the soil surface; or
2. a plinthic horizon starting within 50 cm from the soil surface; or
3. a plinthic horizon starting within 100 cm from the soil surface when underlying either an albic horizon or a horizon with stagnic properties.PLINTHOSOLS (PT)
Other soils
1. having a ferralic horizon at some depth between 25 and 200 cm from the soil surface; and
2. lacking a nitic horizon within 100 cm from the soil surface; and
3. lacking a layer which fulfils the requirements of an argic horizon and which has in the upper 30 cm, 10 percent or more water-dispersible clay (unless the soil material has geric properties or more than 1.4 percent organic carbon).
FERRALSOLS (FR)
Other soils having a natric horizon within 100 cm from the soil surface.
SOLONETZ (SN)
Other soils having
1. an eluvial horizon, the lower boundary of which is marked, within 100 cm from the soil surface, by an abrupt textural change associated with stagnic properties above that boundary; and
2. no albeluvic tonguing.
PLANOSOLS (PL)
Other soils having
1. a mollic horizon with a moist chrome of 2 or less if the texture is finer than sandy loam, or less than 3.5 if the texture is sandy loam or coarser, both to a depth of at least 20 cm, or having these chromes directly below any plough layer; and
2. concentrations of secondary carbonates starting within 50 cm of the lower limit of the Ah horizon but within 200 cm from the soil surface; and
3. no petrocalcic horizon between 25 and 100 cm from the soil surface; and
4. no secondary gypsum; and
5. no uncoated silt and sand grains on structural ped surfaces.
CHERNOZEMS (CH)
Other soils having
1. a mollic horizon with a moist chrome of more than 2 to a depth of at least 20 cm, or having this chrome directly below any plough layer; and
2. concentrations of secondary carbonates within 100 cm from the soil surface; and
3. no diagnostic horizons other than an argic, calcic, cambic, gypsic or vertic horizon.
KASTANOZEMS (KS)
Other soils having
1. a mollic horizon; and
2. a base saturation (by 1 M NH4OAc) of 50 percent or more and a calcium carbonate-free soil matrix at least to a depth of 100 cm from the soil surface, or to a contrasting layer (lithic or paralithic contact, petrocalcic horizon) between 25 and 100 cm; and
3. no diagnostic horizons other than an albic, argic, cambic or vertic horizon, or a petrocalcic horizon3 in the substratum.
3 A petrocalcic horizon may be present locally (e.g. tile "Tosca" in Argentina). Such petrocalcic horizons are considered to be polygenetic and may best be handled for classification purposes at phase level (e.g. Luvic Phaeozem, Tosca phase).
PHAEOZEMS (PH)
Other soils having
1. either a gypsic or petrogypsic horizon within 100 cm from the soil surface, or 15 percent (by volume) or more gypsum, which has accumulated under hydromorphic conditions, averaged over a depth of 100 cm; and
2. no diagnostic horizons other than an ochric or cambic horizon, an argic horizon permeated with gypsum or calcium carbonate, a vertic horizon, or a calcic or petrocalcic horizon underlying the gypsic horizon.
GYPSISOLS (GY)
Other soils having a duric or petroduric horizon within 100 cm from the soil surface.
DURISOLS (DU)
Other soils having
1. a calcic or petrocalcic horizon within 100 cm of the surface; and
2. no diagnostic horizons other than an ochric or cambic horizon, an argic horizon which is calcareous, a vertic horizon, or a gypsic horizon underlying a petrocalcic horizon.
CALCISOLS (CL)
Other soils having an argic horizon within 100 cm from the soil surface with an irregular upper boundary resulting from albeluvic tonguing into the argic horizon.
ALBELUVISOLS (AB)
Other soils having
1. an argic horizon, which has a cation exchange capacity (by 1 M NH4OAc) of 24 cmolc kg-1 clay or more, either starting within 100 cm from the soil surface, or within 200 cm from the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout; and
2. alic properties in the major part between 25 and 100 cm from the soil surface; and
3. no diagnostic horizons other than an ochric, umbric, albic, andic, ferric, nitic, plinthic or vertic horizon.
ALISOLS (AL)
Other soils having
1. a nitic horizon starting within 100 cm from the soil surface; and
2. gradual to diffuse horizon boundaries between the surface and the underlying horizons; and
3. no ferric, plinthic or vertic horizon within 100 cm from the soil surface.
NITISOLS (NT)
Other soils having
1. an argic horizon, which has a cation exchange capacity (by 1 M NH4OAc) of less than 24 cmolc kg-1 clay in some part, either starting within 100 cm from the soil surface, or within 200 cm from the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout, and
2. a base saturation (by 1M NH4OAc) of less than 50 percent in the major part between 25 and 100 cm.
ACRISOLS (AC)
Other soils having an argic horizon with a cation exchange capacity (by 1 M NH4OAc) equal to or more than 24 cmolc kg-1 clay throughout.
LUVISOLS (LV)
Other soils having an argic horizon.
LIXISOLS (LX)
Other soils having
1. an umbric horizon; and
2. no diagnostic horizons other than an anthropedogenic horizon less than 50 cm thick, or an albic or cambic horizon.
UMBRISOLS (UM)
Other soils having either
1. a cambic horizon; or
2. a mollic horizon overlying a subsoil which has a base saturation (by 1 M NH4OAc) of less than 50 percent in some part within 100 cm from the soil surface; or
3. one of the following diagnostic horizons within the specified depth from the soil surface:
a. an andic, vertic or vitric horizon starting between 25 and 100 cm;
b. a plinthic, petroplinthic or salic horizon starting between 50 and 100 cm, in the absence of loamy sand or coarser textures above these horizons.
CAMBISOLS (CM)
Other soils having
1. a texture which is loamy sand or coarser either to a depth of at least 100 cm from the soil surface, or to a plinthic, petroplinthic or salic horizon between 50 and 100 cm from the soil surface; and
2. less than 35 percent (by volume) of rock fragments or other coarse fragments within 100 cm from the soil surface; and
3. no diagnostic horizons other than an ochric, yermic or albic horizon, or a plinthic, petroplinthic or salic horizon below 50 cm from the soil surface, or an argic or spodic horizon below 200 cm depth.
ARENOSOLS (AR)
Other soils.
REGOSOLS (RG)