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Vertosols [VE]

Concept

Clay soils with shrink-swell properties that exhibit strong cracking when dry and at depth have slickensides and/or lenticular structural aggregates. Although many soils exhibit gilgai microrelief, this feature is not used in their definition. Australia has the greatest area and diversity of cracking clay soils of any country in the world.

map
Distribution of Vertosols in Australia.
Soil Profile (View type example photo of Black Vertosol).

Definition

Soils with the following:

  1. A clay field texture or 35% or more clay throughout the solum except for thin, surface crusty horizons 0.03 m or less thick and.
  2. When dry, open cracks occur at some time in most years1. These are at least 5 mm wide and extend upward to the surface or to the base of any plough layer, self-mulching horizon, or thin, surface crusty horizon; and
  3. Slickensides and/or lenticular peds occur at some depth in the solum. See Comment below.

Comment

In some clay soils it may be difficult to decide if sufficient cracks are present, or at the time of inspection the soil may be too moist to exhibit cracking. Also, in arid zone clay soils which commonly have high salt contents, the soil structure may be so fine and strong granular, or 'puffy', that it is difficult to decide if cracks are present or not. In such soils it is also obviously difficult to discern slickensides or lenticular peds. In yet other clay soils (up to 50% clay or more) cracks may develop but slickensides and lenticular peds are apparently not present.

Because cracking, slickensides and lenticular peds are essentially used as evidence to indicate shrink-swell behaviour, it is desirable that surrogate measurements be available if the morphological evidence is lacking or cannot be determined. See Vertic properties.

Suborders

  • Soils with stagnant water on the soil surface and/or saturation of some part of the upper 0.5 m more or less continuously for prolonged periods in most years. The length of a 'prolonged period' is probably of the order of 2-3 continuous months. Evidence of wetness may be indicated by the presence of mottling and gley colours (chroma of 2 or less).
    • Aquic [AM]
  • The dominant colour class in the major part of the upper 0.5 m of the solum (or the major part of the entire solum if it is less than 0.5 m thick) is red.
    • Red [AA]
  • The dominant colour class is brown.
    • Brown [AB]
  • The dominant colour class is yellow.
    • Yellow [AC]
  • The dominant colour class is grey.
    • Grey [AD]
  • The dominant colour class is black.
    • Black [AE]

Comment

Of the soils entered in the data base, the most common class was Black (40%) which is probably a reflection of the agricultural importance of these soils.

Great Groups

These may not all apply to each suborder, in particular our knowledge of the Aquic suborder is limited.

  • Soils with a surface that is moderately to strongly self-mulching; when the soil is dry the self-mulching layer should be at least 10 mm thick. Initial drying may form a thin (2-3 mm) surface flake which readily disintegrates to a mulch on further drying. This process is accelerated by mechanical disturbance.
    • Self-mulching [EI]
  • Soils with a pedal (stronger than weak grade, blocky or polyhedral) A horizon which is either not or only weakly self-mulching, and there is no surface crusty horizon. Some soils after wetting and drying may form a thin 5-10 mm surface flake which cracks into irregular polygons (plates) 0.03-0.1 m diameter. These may be readily separated and removed from the underlying pedal clay.
    • Epipedal [GS]
  • Soils with a massive or weakly structured surface crusty horizon 0.03 m or less thick, often of lighter texture (lower clay content) than the underlying pedal clay (blocky or polyhedral) which is not self-mulching.
    • Crusty [BH]
  • Soils with a massive or weak blocky (usually > 0.05 m peds) A horizon, and there is no surface crusty horizon.
    • Massive [DF]

Comment

Each of the above soil surface conditions tends to reform despite cultivation or surface trampling. There may be a problem in identifying the self-mulching condition in periods of initial drying, i.e. in assessing the stability of the surface flake which forms following rainfall. If there is doubt as to whether a soil is self-mulching or has only a pedal surface, it is suggested that the latter condition be recorded, i.e. use the self-mulching great group only for those soils where the condition is not in doubt. It may be difficult to determine the surface condition if a dense grass sward is present. In this situation it will be necessary to look for a patch of bare ground, or even to kill the grass with herbicide and return at a later date. Note also that large soil units bounded by cracks are not considered to be coarse peds. It is usually necessary to examine these soils in the moist state to determine their degree of pedality.

Fifty four percent of the soils classified were judged to be Self-mulching, with 35% classed as Epipedal.

Subgroups

It is thought that the following subgroups will be required for most of the suborders and great groups. Note that some of the differentiating criteria are not mutually exclusive, and thus sometimes it has been a subjective decision as to which attributes have priority in the key.

  • Soils with a seasonal saline water table present in the upper 0.5 m of the profile (water conductivity >2 dSm-1 ). Salt efflorescence may occur on the surface soil when dry.
    • Salic [EG]
  • Soils in which sulfuric materials occur within the upper 1.5 m of the profile.
    • Sulfuric [EV]
  • Soils in which sulfidic materials occur within the upper 1.5 m of the profile.
    • Sulfidic [EU]
  • Soils with a red-brown hardpan either within or directly underlying the B horizon.
    • Duric [BJ]
  • Soils in which the B horizon directly overlies a calcrete pan.
    • Petrocalcic [DZ]
  • Soils in which the upper 0.1 m of the solum is sodic and a gypsic horizon is present within the B or BC horizon.
    • Episodic-Gypsic [GQ]
  • Other soils with a gypsic horizon within the B or BC horizon.
    • Gypsic [BZ]
  • Soils in which the upper 0.1 m of the solum is sodic and the major part of the upper 0.5 m of the solum is strongly acid.
    • Episodic-Epiacidic [EP]
  • Soils in which the upper 0.1 m of the solum is sodic and the major part of the solum below 0.5 m is strongly acid.
    • Episodic-Endoacidic [GG]
  • Soils in which the upper 0.1 m of the solum is sodic and the major part of the upper 0.5 m of the solum is calcareous.
    • Episodic-Epicalcareous [GH]
  • Soils in which the upper 0.1 m of the solum is sodic and the major part of the solum below 0.5 m is calcareous.
    • Episodic-Endocalcareous [GI]
  • Other soils in which the upper 0.1 m of the solum is sodic.
    • Episodic [BN]
  • Soils in which some subsurface horizon within the upper 0.5 m of the solum has an ESP of 15 or greater and the major part of the upper 0.5 m of the solum is strongly acid.
    • Epihypersodic-Epiacidic [CU]
  • Soils in which some subsurface horizon within the upper 0.5 m of the solum has an ESP of 15 or greater and the major part of the solum below 0.5 m is strongly acid.
    • Epihypersodic-Endoacidic [GN]
  • Soils in which the major part of the upper 0.5 m of the solum is strongly acid and mottled.
    • Epiacidic-Mottled [GK]
  • Other soils in which the major part of the upper 0.5 m of the solum is strongly acid.
    • Epiacidic [GA]
  • Soils in which the major part of the upper 0.5 m of the solum is calcareous and the major part of the solum below 0.5 m is strongly acid.
    • Epicalcareous-Endoacidic [GJ]
  • Soils in which the major part of the upper 0.5 m of the solum is calcareous and some subsurface horizon within this depth has an ESP of 15 or greater.
    • Epicalcareous-Epihypersodic [FM]
  • Soils in which some subsurface horizon within the upper 0.5 m of the solum has an ESP of 15 or greater and the major part of the solum below 0.5 m is calcareous.
    • Epihypersodic-Endocalcareous [GO]
  • Other soils in which some subsurface horizon within the upper 0.5 m of the solum has an ESP of 15 or greater.
    • Epihypersodic [BR]
  • Soils in which the major part of the upper 0.5 m of the solum is calcareous and an ESP of 15 or greater occurs in some subhorizon of the solum below 0.5 m.
    • Epicalcareous-Endohypersodic [GB]
  • Other soils in which the major part of the upper 0.5 m of the solum is calcareous.
    • Epicalcareous [FY]
  • Soils in which the major part of the solum below 0.5 m is strongly acid and mottled.
    • Endoacidic-Mottled [GL]
  • Other soils in which the major part of the solum below 0.5 m is strongly acid.
    • Endoacidic [BL]
  • Soils in which the major part of the solum below 0.5 m is calcareous and some subhorizon of the solum below 0.5 m has an ESP of 15 or greater.
    • Endocalcareous-Endohypersodic [GM]
  • Other soils in which some subhorizon of the solum below 0.5 m has an ESP of 15 or greater.
    • Endohypersodic [BP]
  • Soils in which the major part of the solum below 0.5 m is calcareous and the major part of the upper 0.5 m of the solum is mottled.
    • Endocalcareous-Mottled [HE]
  • Other soils in which the major part of the solum below 0.5 m is calcareous.
    • Endocalcareous [FZ]
  • Soils in which the major part of the B horizon has an exchangeable Ca/Mg ratio of less than 0.1.
    • Magnesic [DB]
  • Soils with a conspicuously bleached A2 horizon.
    • Bleached [AT]
  • Other soils in which the major part of the upper 0.5 m of the solum is mottled.
    • Mottled [DQ]
  • Other soils in which the major part of the upper 0.5 m of the solum is whole coloured.
    • Haplic [CD]

Comment

It should be noted that all the Endoacidic soils classified are also Endohypersodic, with some also being Epihypersodic. Additionally, some Epicalcareous-Epihypersodic soils are Endoacidic at depth. It is not possible to cater for all these combinations.

The most common subgroup recorded was Haplic, although this accounted for only 26% of the subgroups classified. It was dominantly associated (64%) with the Black Vertosols.

Family Criteria

Because of the uniform clayey nature of these soils and their usual lack of distinct horizonation, several of the usual family criteria are not appropriate for Vertosols. Field texture in these soils may not be a reliable guide to actual clay content (see McDonald et al. 1990 p.121), and it may also be difficult to achieve consistent results between operators. Hence it is thought more appropriate to provide for a subdivision of actual clay content as determined by laboratory analysis. The classes used are similar to those used for clayey particle-size classes in Soil Taxonomy. Other criteria used are gravel content of surface and A1 horizon and soil depth.

Gravel of surface and A1 horizon

Non-gravelly [E] : < 2%
Slightly gravelly [F] : 2 - < 10%
Gravelly [G] : 10 - < 20%
Moderately gravelly [H] : 20 - 50%
Very gravelly [I] : > 50%

Clay content of upper 0.1 m (excluding any surface crusty horizon)

Fine [Q] : < 45% clay
Medium fine [R] : 45 - 60% clay
Very fine [S] : > 60% clay

B horizon maximum clay content

Fine [Q] : < 45% clay
Medium fine [R] : 45 - 60% clay
Very fine [S] : > 60% clay

Soil depth

Very shallow [T] : < 0.25 m
Shallow [U] : 0.25 - < 0.5 m
Moderate [V] : 0.5 - < 1.0 m
Deep [W] : 1.0 - < 1.5 m
Very deep [X] : 1.5 - 5 m
Giant [Y] : > 5 m

1 Note that there is no crack frequency criterion as in the Factual Key.

[ AN ] [ OR ] [ PO ] [ VE ] [ HY ] [ KU ] [ SO ] [ CH ] [ CA ] [ FE ] [ DE ] [ KA ] [ RU ] [ TE ]