4  Key to the Reference Soil Groups with lists of principal and supplementary qualifiers

Modified

January 10, 2024

Important

Before using the key, please read the ‘Rules for naming soils’ (Chapter 2)

Overview of Key to Reference Soil Groups

Histosols Solonchaks Nitisols Gypsisols Cambisols
Anthrosols Gleysols Ferralsols Calcisols Fluvisols
Technosols Andosols Chernozems Retisols Arenosols
Cryosols Podzols Kastanozems Acrisols Regosols
Leptosols Plinthosols Phaeozems Lixisols
Solonetz Planosols Umbrisols Alisols
Vertisols Stagnosols Durisols Luvisols

Histosols (HS)

Soils having one or more of the following:

  1. organic material starting ≤ 40 cm from the soil surface and having within 100 cm of the soil surface a combined thickness of:
    1. ≥ 40 cm if < 75% (by volume, related to the fine earth plus all dead plant residues) consists of moss fibres; or
    2. ≥ 60 cm; or
  2. organic material starting at the soil surface, having a thickness of ≥ 10 cm and directly overlying ice, continuous rock or technic hard material; or
  3. a layer of coarse fragments that, together with overlying organic material, if present, starts at the soil surface and has a thickness of
    1. ≥ 10 cm if overlying continuous rock or technic hard material; or
    2. ≥ 40 cm;

      and the major part of the interstices between the coarse fragments is filled with organic material and the remaining interstices, if present, are void.

Anthrosols (AT)

Other soils having:

  1. a hortic, irragric, plaggic or terric horizon, ≥ 50 cm thick; or
  2. an anthraquic horizon and an underlying hydragric horizon with a combined thickness of ≥ 50 cm; or
  3. a pretic horizon, the layers of which have a combined thickness of ≥ 50 cm, within 100 cm of the mineral soil surface.

Technosols (TC)

Other soils:

  1. with all of the following:
    1. one or both of the following:
      1. having ≥ 20% (by volume, weighted average, related to the whole soil) artefacts in the upper 100 cm from the soil surface or to a limiting layer, whichever is shallower; or
      2. having a layer, ≥ 10 cm thick and starting ≤ 50 cm from the soil surface, with ≥ 80% (by volume, weighted average, related to the whole soil) artefacts; and
    2. not having a layer containing artefacts that qualifies as an argic, duric, ferralic, ferric, fragic, hydragric, natric, nitic, petrocalcic, petroduric, petrogypsic, petroplinthic, pisoplinthic, plinthic, spodic or vertic horizon starting ≤ 100 cm from the soil surface, unless buried; and
    3. not having a limiting layer, unless consisting of artefacts, starting ≤ 10 cm from the soil surface; or
  2. having a continuous, very slowly permeable to impermeable, constructed geomembrane of any thickness or technic hard material starting ≤ 100 cm from the soil surface.
Note

Technosols may bury other soils, which can be mentioned behind the Technosol classification using the word ‘over’ in between (see Chapter 2.4). Alternatively, buried diagnostic horizons or buried layers with a diagnostic property can be indicated with the Thapto- specifier followed by a qualifier. The soil material above a geomembrane or technic hard material may also be characterized by qualifiers. If the thickness or depth criteria of these qualifiers are not met, the Supra- specifier can be used (see Chapter 2.3.2).

Cryosols (CR)

Other soils having:

  1. a cryic horizon starting ≤ 100 cm from the soil surface; or
  2. a cryic horizon starting ≤ 200 cm from the soil surface and
    evidence of cryogenic alteration (cryoturbation, frost heave, cryogenic sorting, thermal cracking, ice segregation, patterned ground, etc.) in some layer within 100 cm of the soil surface.

Leptosols (LP)

Other soils having:

  1. one of the following:
    1. continuous rock starting ≤ 25 cm from the soil surface; or
    2. < 20% (by volume, related to the whole soil) fine earth plus dead plant residues of any size1, averaged over a depth of 75 cm from the soil surface or to continuous rock, whichever is shallower; and
  2. no duric, petrocalcic, petroduric, petrogypsic, pisoplinthic or spodic horizon.

1 The volume occupied neither by fine earth not by dead plant residues is occupied by coarse fragments, remnants of broken-up cemented layers ≥ 2 mm, or interstices.

Solonetz (SN)

Other soils having a natric horizon starting ≤ 100 cm from the mineral soil surface.

Vertisols (VR)

Other soils having:

  1. a vertic horizon starting ≤ 100 cm from the mineral soil surface; and
  2. ≥ 30% clay between the mineral soil surface and the vertic horizon throughout; and
  3. shrink-swell cracks that start:
    1. at the mineral soil surface; or
    2. at the base of a plough layer; or
    3. directly below a layer with strong granular structure or strong angular or subangular blocky structure with an aggregate size of ≤ 1 cm (self-mulching surface); or
    4. directly below a surface crust; and

      extend to the vertic horizon.

Solonchaks (SC)

Other soils:

  1. having a salic horizon starting ≤ 50 cm from the soil surface; and
  2. not having a thionic horizon starting ≤ 50 cm from the soil surface; and
  3. not being permanently submerged by water and not located below the line affected by tidal water (i.e. not located below the line of mean high water springs).

Gleysols (GL)

Other soils having one or more of the following:

  1. a layer, ≥ 25 cm thick and starting ≤ 40 cm from the mineral soil surface, that has
    1. gleyic properties throughout; and
    2. reducing conditions in some parts of every sublayer; or
  2. both of the following:
    1. a mollic or umbric horizon, > 40 cm thick, that has reducing conditions in some parts of every sublayer, from 40 cm below the mineral soil surface to the lower limit of the mollic or umbric horizon; and
    2. directly underneath the mollic/umbric horizon, a layer, ≥ 10 cm thick, that has its lower limit ≥ 65 cm below the mineral soil surface, and that has:
      1. gleyic properties throughout; and
      2. reducing conditions in some parts of every sublayer; or
  3. permanent saturation by water starting ≤ 40 cm from the mineral soil surface.

Andosols (AN)

Other soils having:

  1. one or more layers with andic or vitric properties with a combined thickness of:
    1. ≥ 30 cm, within 100 cm of the soil surface and starting ≤ 25 cm from the soil surface; or
    2. ≥ 60% of the entire thickness of the soil, if a limiting layer starts > 25 and ≤ 50 cm from the soil surface; and
  2. no argic, ferralic, petroplinthic, pisoplinthic, plinthic or spodic horizon starting ≤ 100 cm of the soil surface, unless buried deeper than 50 cm from the mineral soil surface.
Note

Andosols may bury other soils, which can be mentioned behind the Andosol classification using the word ‘over’ in between (see Chapter 2.4). Alternatively, buried diagnostic horizons or buried layers with a diagnostic property can be indicated with the Thapto- specifier followed by a qualifier.

Podzols (PZ)

Other soils having a spodic horizon starting ≤ 200 cm from the mineral soil surface.

Plinthosols (PT)

Other soils having a plinthic, pisoplinthic or petroplinthic horizon starting ≤ 100 cm from the mineral soil surface.

Planosols (PL)

Other soils having an abrupt textural difference ≤ 75 cm from the mineral soil surface and having within the range of 5 cm directly above or below the abrupt textural difference:

  1. stagnic properties, in which the area of reductimorphic features plus the area of oximorphic features is ≥ 50% (weighted average, related to the fine earth plus oximorphic features of any size and any cementation class) of the total area; and
  2. reducing conditions for some time during the year in some parts of the soil volume that has the reductimorphic features.

Stagnosols (ST)

Other soils having:

  1. stagnic properties, in which the area of reductimorphic features plus the area of oximorphic features is ≥ one third (weighted average, related to the fine earth plus oximorphic features of any size and any cementation class) of the area from the mineral soil surface to a depth of 60 cm or to continuous rock, whichever is shallower; and
  2. reducing conditions for some time during the year in some parts of the soil volume that has the reductimorphic features within 60 cm from the mineral soil surface or to continuous rock, whichever is shallower.

Nitisols (NT)

Other soils having:

  1. a nitic horizon starting ≤ 100 cm from the mineral soil surface; and
  2. from the mineral soil surface to the nitic horizon, a clay content that is at least half of the weighted average clay content of the nitic horizon; and
  3. no vertic horizon starting above or at the upper limit of the nitic horizon.

Ferralsols (FR)

Other soils having:

  1. a ferralic horizon starting ≤ 150 cm from the mineral soil surface; and
  2. no argic horizon starting above or at the upper limit of the ferralic horizon, unless the argic horizon has, in its upper 30 cm or throughout, whichever is shallower, one or more of the following:
    1. < 10% water-dispersible clay; or
    2. a ΔpH (pHKCl - pHwater) ≥ 0 (both in 1:1 solution); or
    3. ≥ 1.4% soil organic carbon.

Chernozems (CH)

Other soils having:

  1. a chernic horizon; and
  2. starting ≤ 50 cm below the lower limit of the mollic horizon2 and, if present, above a petrocalcic horizon, a layer with protocalcic properties, ≥ 5 cm thick, or a calcic horizon; and
  3. a base saturation (by 1 M NH4OAc, pH 7)3 of ≥ 50% from the mineral soil surface to the layer with protocalcic properties or to the calcic horizon, throughout.

2 Any chernic horizon also meets the criteria of a mollic horizon. The mollic horizon may extend below the chernic horizon.

3 If the data for base saturation are not available, pH values may be used according to Annex 2 (Chapter 9.13).

Kastanozems (KS)

Other soils having:

  1. a mollic horizon; and
  2. starting ≤ 70 cm of the mineral soil surface and, if present, above a petrocalcic horizon, a layer with protocalcic properties, ≥ 5 cm thick, or a calcic horizon; and
  3. a base saturation (by 1 M NH4OAc, pH 7)4 of ≥ 50% from the mineral soil surface to the layer with protocalcic properties or to the calcic horizon, throughout.

4 If the data for base saturation are not available, pH values may be used according to Annex 2 (Chapter 9.13).

Phaeozems (PH)

Other soils having:

  1. a mollic horizon; and
  2. a base saturation (by 1 M NH4OAc, pH 7)5 of ≥ 50% throughout to a depth of 100 cm from the mineral soil surface or to a limiting layer, whichever is shallower.

5 If the data for base saturation are not available, pH values may be used according to Annex 2 (Chapter 9.13).

Umbrisols (UM)

Other soils having an umbric or mollic or hortic horizon.

Durisols (DU)

Other soils having a petroduric or duric horizon starting ≤ 100 cm from the mineral soil surface.

Gypsisols (GY)

Other soils having:

  1. a gypsic or petrogypsic horizon starting ≤ 100 cm from the mineral soil surface; and
  2. no argic horizon starting above or at the upper limit of the gypsic or petrogypsic horizon, unless the argic horizon contains secondary gypsum or secondary carbonates, throughout.

Calcisols (CL)

Other soils having:

  1. a calcic or petrocalcic horizon starting ≤ 100 cm from the mineral soil surface; and
  2. no argic horizon starting above or at the upper limit of the calcic or petrocalcic horizon unless the argic horizon contains secondary carbonates, throughout.

Retisols (RT)

Other soils having an argic horizon starting ≤ 100 cm from the mineral soil surface and having retic properties at its upper boundary.

Acrisols (AC)

Other soils having:

  1. an argic horizon starting ≤ 100 cm from the mineral soil surface; and
  2. a CEC (by 1 M NH4OAc, pH 7) of < 24 cmolc kg-1 clay in some subhorizon of the argic horizon within 150 cm of the mineral soil surface; and
  3. exchangeable Al > exchangeable (Ca+Mg+K+Na)6 in half or more of:
    1. the depth range between 50 and 100 cm of the mineral soil surface; or
    2. the lower half of the mineral soil above a limiting layer starting ≤ 100 cm from the mineral soil surface, whichever is shallower.

6 Exchangeable cations are given in cmolc kg-1. If these data are not available, pH values may be used according to Annex 2 (Chapter 9.13).

Lixisols (LX)

Other soils having:

  1. an argic horizon starting ≤ 100 cm from the mineral soil surface; and
  2. a CEC (by 1 M NH4OAc, pH 7) of < 24 cmolc kg-1 clay in some subhorizon of the argic horizon within 150 cm of the mineral soil surface.

Alisols (AL)

Other soils having:

  1. an argic horizon starting ≤ 100 cm from the mineral soil surface; and
  2. exchangeable Al > exchangeable (Ca+Mg+K+Na)7 in half or more of:
    1. the depth range between 50 and 100 cm of the mineral soil surface; or
    2. the lower half of the mineral soil above a limiting layer starting ≤ 100 cm from the mineral soil surface whichever is shallower.

7 Exchangeable cations are given in cmolc kg-1. If these data are not available, pH values may be used according to Annex 2 (Chapter 9.13).

Luvisols (LV)

Other soils having an argic horizon starting ≤ 100 cm from the mineral soil surface.

Cambisols (CM)

Other soils having:

  1. a cambic horizon
    1. a. starting ≤ 50 cm from the mineral soil surface; and
    2. having its lower limit ≥ 25 cm from the mineral soil surface; or
  2. an anthraquic, hydragric, irragric, plaggic, pretic or terric horizon; or
  3. a fragic, thionic or vertic horizon starting ≤ 100 cm from the mineral soil surface; or
  4. a tsitelic horizon with a texture class of sandy loam or finer, starting ≤ 50 cm from the mineral soil surface; or
  5. one or more layers with andic or vitric properties with a combined thickness of ≥ 15 cm within 100 cm of the soil surface.

Fluvisols (FL)

Other soils having fluvic material:

  1. ≥ 25 cm thick and starting ≤ 25 cm from the mineral soil surface; or
  2. from the lower limit of a plough layer, ≤ 40 cm thick, to a depth of ≥ 50 cm from the mineral soil surface.

Arenosols (AR)

Other soils having within 100 cm of the mineral soil surface:

  1. a weighted average texture class of loamy sand or sand; and
  2. layers of finer texture, if present, with a combined thickness of < 15 cm; and
  3. layers with ≥ 40% (by volume, related to the whole soil) coarse fragments, if present, with a combined thickness of < 15 cm.
Note

Arenosols may bury other soils, which can be mentioned behind the Arenosol classification using the word ‘over’ in between (see Chapter 2.4). Alternatively, buried diagnostic horizons or buried layers with a diagnostic property can be indicated with the Thapto- specifier followed by a qualifier. Arenosols may have diagnostic horizons at depths of > 100 cm. These can be indicated with the Bathy- specifier followed by a qualifier, e.g. Bathyacric (> 100 cm), Bathyspodic (> 200 cm).

Regosols (RG)

Other soils: