Key to Soil Orders

In the keys to orders, groups or subgroups, any surface mantle of new material, e.g. fresh alluvium, that is less than 30 cm thick, is not considered as part of the soil for assignment to orders, groups or subgroups, except in the Recent Soils or Raw Soils, or Sulphuric Gley or Recent Gley groups of the Gley Soils. A surface mantle of new material can be recognised because it will not meet the requirements of the key for any order or group except those specified above.

If any soil has an overthickened A horizon with its base at 45 cm or more from the mineral soil surface and is not mottled within 30 cm of the mineral surface, the key should be entered directly at Recent Soils.

O - Organic Soils

Soils that have horizons that consist of organic soil material (including soils that have skeletal layers in which the matrix of the gravel consists of organic soil material) that within 60 cm of the soil surface are EITHER

  1. 30 cm or more thick (cumulative) and are entirely formed from peat or other organic soil materials that have accumulated under wet conditions (they are saturated with water for at least 30 consecutive days in most years, or have been artificially drained) (O horizons), OR
  2. 40 cm or more thick and are formed from partly decomposed or well decomposed litter (F and H horizons).

G - Gley Soils

Other soils that have EITHER

  1. a gley profile form in which the reductimorphic horizon (but not including soils with pedal horizons in which the defined greyish colours occur on ped faces but not in 50% or more of the matrix) meets all of the following:
    1. it has a lower boundary 90 cm or more from the mineral soil surface, or to a lithic or paralithic contact if shallower, or rests upon a permeable sandy-skeletal layer with a base that extends to 90 cm or more from the mineral soil surface, or to the base of the B horizon, and
    2. there is no underlying fragipan with matrix of chroma 3 or more dominant, duripan or underlying podzolic-B horizon, and
    3. a distinct topsoil occurs at the surface or is buried with its upper surface within 60 cm of the mineral soil surface, and
    4. there is no horizon with a moderately fluid or very fluid fluidity class within 30 cm of the mineral soil surface, OR
  2. an ironstone pan with an upper surface 30 cm or less from the mineral soil surface, and has sufficient ferrous iron to give a positive reaction to α,α’-dipyridyl at some time of the year.

U - Ultic Soils

Other soils that meet ALL of the following:

  1. pH of less than 5.5 in the major part1 from the base of the A horizon to 60 cm from the mineral soil surface, AND
  2. a B horizon which in the major part:
    1. is pedal with clay or humus coatings present on 10% or more ped faces, and
    2. has slightly firm or stronger soil strength when moist unless the texture is sandy loam or sandy clay loam, and
    3. has less silt than clay, unless there are weathered stones, and
    4. has sandy loam or finer texture, and
    5. has base deeper than 100 cm, AND
  3. do not have
    1. an oxidic or cutanoxidic horizon, nor
    2. a layer or layers of allophanic soil material that totals 35 cm or more thick within 60 cm of the mineral soil surface, nor
    3. stones, other than quartz stones, that are strongly or very strongly indurated, AND
  4. have either an E horizon, or the uppermost subhorizon of the B has colour value of 5 or more.

Z - Podzols

Other soils that have either a podzolic-B horizon, or an ortstein-pan, that has pH less than 5.5 in some part or a humus-pan.

L - Allophanic Soils

Other soils that have a layer or layers of allophanic soil material, that total 35 cm or more thick, and occur within 60 cm of the mineral soil surface.

M - Pumice Soils

Other soils that have BOTH

  1. a layer of vitric soil material extending from the mineral soil surface to 25 cm or more, or 35 cm or more thick occurring within 60 cm of the mineral soil surface, AND
  2. a weathered-B horizon 5 cm or more thick.

E - Melanic Soils

Other soils that have ALL the following

  1. an A horizon that in the major part has both
    1. moist colour value of 3 or less, and
    2. either moderate or strong pedality (with peds that are less than 60 mm in size), is earthy apedal, or has cracks 4 mm or more wide that extend to a depth of 30 cm or more at some time of the year, or is peaty; AND
  2. has either
    1. a visible reaction to 10% HCl in the soil matrix at 60 cm or less from the mineral soil surface, or
    2. has a weathered-B, argillic or cutanic horizon (more than 10 cm thick) in which the major part (to 60 cm from the mineral soil surface or to its base, whichever is less) has pH of 5.9 or more, moderate or strong pedality, and is sticky or very sticky, AND
  3. has in some part of the B horizon to its base, or to 90 cm from the mineral soil surface, whichever is less, either
    1. gravel that is not very highly weathered or completely weathered, or
    2. a subhorizon that is not clayey, or
    3. the uppermost subhorizon of the B has colour value of 5 or more.

S - Semiarid Soils

Other soils that have ALL of the following

  1. no fragipan, AND
  2. a weathered-B horizon 10 cm or more thick, or a calcareous horizon (with evidence of pedogenic carbonate), AND
  3. no primary calcium carbonate minerals in the sand fraction of the B horizon, AND
  4. either
    1. a calcareous horizon, or a horizon with pH of 7.5 or more, within 90 cm of the soil surface (or within 150 cm if the particle-size class is dominantly sandy or sandy-skeletal), or
    2. P retention of 15% or less in the major part of the B horizon to 60 cm depth, and any worm mixed horizon at the base of the A horizon is less than 5 cm thick.

X - Oxidic Soils

Other soils that have an oxidic horizon more than 30 cm thick with an upper boundary between 20 and 60 cm from the mineral soil surface.

N - Granular Soils

Other soils that have BOTH

  1. a B horizon which to its base, or to 90 cm from the mineral soil surface, whichever is less, is both
    1. clayey throughout; and
    2. if any coarse-fragments of rock material occur they are either very highly weathered or completely altered, AND
  2. a cutanoxidic horizon, or a moderately or strongly pedal cutanic or argillic horizon occurs that has either
    1. an overlying or overlapping reductimorphic horizon within 15 cm of the base of the A horizon, or 30 cm of the mineral soil surface; or
    2. polyhedral peds 20 mm or less in size in most of the B horizon within 60 cm of the mineral soil surface.

P - Pallic Soils

Other soils that have textures of loamy fine sand or finer in some part from the base of the A horizon to 60 cm from the mineral soil surface, and have EITHER

  1. a reductimorphic horizon or E horizon, that overlies a fragipan, duripan or argillic horizon; OR
  2. both
    1. between the base of the A horizon to 60 cm from the mineral soil surface the matrix throughout is either
      1. hue 10YR or yellower with chroma 3 or less and value 4 or more (all colours moist), or
      2. hue 10YR or yellower with chroma 4 or 5 or chroma 3 with value 3, or hue 2.5Y or yellower with chroma 6 or more (all colours moist, see Figure 4.1); and P retention is less than 30% in the uppermost subhorizon (10 cm or more thick) of the B horizon, and
    2. a B horizon that is 10 cm or more thick, and has either
      1. a fragipan, duripan or brittle-B horizon, or
      2. a weathered-B horizon that has in part a moderately or strongly pedal subhorizon with blocky, polyhedral or prismatic peds and slightly firm or greater soil or ped strength, or
      3. argillic horizon, calcareous horizon, or a cutanic horizon that has sodic features either within or immediately beneath it, or
      4. eluvial features including an E horizon, skeletans on B horizon peds, or skeletans as apparent segregations of relatively higher colour value in B or BC horizons (“two-tone”).

B - Brown Soils

Other soils that have BOTH

  1. a weathered-B, argillic or cutanic horizon 10 cm or more thick with a lower boundary at 30 cm or more from the mineral soil surface, AND
  2. in the B horizon, a subhorizon that has in the matrix either
    1. hue 7.5YR or redder, or hue 10YR and chroma 6 or more; or
    2. hue 10YR or yellower with chroma 4 or 5 or chroma 3 with value 3, or hue 2.5Y or yellower with chroma 6 or more (all colours moist, see Figure 4.1); and P retention is 30% or more, or the reactive-aluminium test is at least moderate, in the uppermost subhorizon (10 cm or more thick) of the B horizon, or
    3. the texture is sand or loamy sand in the B horizon to its base, or to 60 cm from the mineral soil surface, whichever is less.

A - Anthropic Soils

Other soils that have been formed by the direct action of people by either truncation, drastic mixing or by deposition of material 30 cm or more thick.

R - Recent Soils

Other soils that BOTH

  1. have a distinct topsoil at the surface or buried with its upper surface within 60 cm of the mineral soil surface, or a weathered-B horizon; AND
  2. do not have any horizon with a moderately fluid or very fluid fluidity class within 30 cm of the mineral soil surface.

W - Raw Soils

Other soils.

Figure 4.1: Colour criteria, and colours where P retention and the reactive-aluminium test is used to differentiate Brown Soils from Pallic Soils and Recent Soils. See part 2(a) of the Pallic Soils, and part 2(a) and 2(b) of the Brown Soils in the Key to Orders.

  1. “the major part” means that the requirement must be met over more than half the specified thickness (in this case from the base of the A horizon to 60 cm from the mineral soil surface). Analyses or estimates should be used from subhorizons that subdivide the thickness, or if the subhorizons are not recognised, then from subsamples of the relevant horizons.↩︎