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Calcarosols [CA]

Concept

As the name suggests, the soils in this order are usually calcareous throughout the profile, often highly so. They constitute one of the most widespread and important groups of soils in southern Australia.

map
Distribution of Calcarosols in Australia.
Soil Profile (View type example photo of Supracalcic Calcarosol).

Definition

Soils that are calcareous throughout the solum - or calcareous at least directly below the A1 or Ap horizon, or a depth of 0.2 m (whichever is shallower). Carbonate accumulations must be judged to be pedogenic1 (either current or relict), and the soils do not have clear or abrupt textural B horizons. Hydrosols, Organosols and Vertosols are excluded.

Comment

A difficulty may arise in separating those Calcarosols that are not calcareous throughout from calcareous Kandosols and from Tenosols containing pedogenic carbonate. However, in the latter two soils it is usual for the carbonate to occur in the lower part of the B horizon, and not immediately below the A horizon. Even so, transitional cases will arise where it becomes a matter of judgement in to which order the particular soil is best placed. Similar transitions might occur between shallow Calcarosols and Arenic Rudosols overlying a layer of calcrete or limestone. Again, Calcareous Arenic Rudosols will occur where recent aeolian calcareous material has been deposited.

In dune landscapes, where these soils frequently occur, it is common to find evidence of post-European settlement deflation and layering of soil profiles caused by wind erosion and consequent deposition. Unless the surface depositional material is 0.3 m or more thick, it is ignored in the classification and treated as a phase. (see 'What do we classify?').

Suborders

  • Soils that dominantly consist of fine fragments of shells and other aquatic skeletons (identifiable under a 10 x hand lens). The pedogenic carbonate occurs as soft white films coating the fragments or as discrete accumulations.
    • Shelly [EL]
  • Soils that dominantly consist of gypsum crystals which are sand-sized or finer.
    • Hypergypsic [FJ]
  • Soils in which the carbonate is evident only as a slight to moderate effervescence (1M HCl), and/or contain less than 2% soft finely divided carbonate, and have less than 20% hard carbonate nodules or concretions.
    • Hypocalcic [CV]
  • Soils with a calcareous horizon containing more than 50% of hard calcrete fragments and/or carbonate nodules or concretions and/or carbonate-coated gravel.
    • Lithocalcic [DA]
  • Soils with a calcareous horizon containing 20-50% of hard calcrete fragments and/or carbonate nodules or concretions and/or carbonate-coated gravel.
    • Supracalcic [FB]
  • Soils with a calcareous horizon containing more than 20% of mainly soft, finely divided carbonate, and 0-20% of hard calcrete fragments and/or carbonate nodules or concretions, and/or carbonate-coated gravel.
    • Hypercalcic [CQ]
  • Other soils with a calcareous horizon. (See carbonate classes).
    • Calcic [BD]

Comment

The calcareous classes above approximately correspond to those of Wetherby and Oades (1975) as follows: Hypocalcic - Class IV, Lithocalcic - Class III B and IIIC, Supracalcic - Class III B, Hypercalcic - Class III A, Calcic - Class 1 and IIIA.

In the Lithocalcic and Supracalcic classes the coarse fragments may be >0.2 m in size and soft carbonate may or may not be present.

Of the profiles classified, the Calcic and Hypercalcic suborders are the most common.

Great Groups

Shelly and Hypergypsic Calcarosols

More details of these soils are required for further subdivision.

Other Calcarosols

Not all great groups will be relevant for every suborder, for example, Rendic will be required only for the Hypercalcic suborder.

  • Soils that directly overlie a red-brown hardpan.
    • Duric [BJ]
  • Soils that directly overlie a calcrete pan.
    • Petrocalcic [DZ]
  • Soils in which the A horizon directly overlies a Bk horizon consisting almost entirely of soft carbonate (>80%).
    • Rendic [EE]
  • Soils with an argic horizon within the B horizon.
    • Argic [AP]
  • Soils in which the major part of the B horizon has a grade of structure that is stronger than weak.
    • Pedal [DY]
  • Soils that directly overlie hard rock.
    • Lithic [CZ]
  • Soils which directly overlie partially weathered or decomposed rock or saprolite.
    • Paralithic [DU]
  • Soils that directly overlie marl.
    • Marly [DD]
  • Soils that directly overlie unconsolidated mineral materials.
    • Regolithic [GF]

Subgroups

The following subgroups will not necessarily be applicable to all great groups of each suborder, and not all subgroups are mutually exclusive. The Supravescent and Hypervescent classes may also be Epihypersodic or Endohypersodic. However, the high content or absence of carbonate in the upper 0.1m is thought to have more influence on land use than high sodicity. A number of soils has been recorded as having a conspicuously bleached A2 horizon. In many cases, however, this may be a reflection of high contents of soft carbonate, hence this feature has not been used as a class differentia.

  • Soils with a melanic horizon overlying a B horizon in which at least 0.3m has vertic properties.
    • Melanic-Vertic [DN]
  • Other soils with a melanic horizon.
    • Melanic [DK]
  • Other soils with a B horizon in which at least 0.3m has vertic properties.
    • Vertic [EX]
  • Soils in which the B horizon is strongly subplastic and the B or BC horizon contains a gypsic horizon.
    • Gypsic-Subplastic [FL]
  • Other soils with a strongly subplastic B horizon.
    • Subplastic [ET]
  • Other soils with a gypsic horizon within the B or BC horizon.
    • Gypsic [BZ]
  • Soils that are not calcareous in the A1 or Ap horizon, or to a depth of 0.2m if the A1 horizon is only weakly developed.
    • Epibasic [IB]
  • Soils in which the upper 0.1m of the profile consists of dominantly soft, finely divided carbonate (visual estimate), and/or contains more than 40%2 (by analysis) of soft, finely divided carbonate.
    • Supravescent [HK]
  • Soils in which the upper 0.1m of the profile consists of more than 20% (visual estimate) of soft, finely divided carbonate, and/or has a strong effervescence with 1M HC1, and/or contains more than 8%2 (by analysis) of soft, finely divided carbonate.
    • Hypervescent [CP]
  • Soils in which at least some subhorizon within the upper 0.5 m of the profile has an ESP of 15 or greater.
    • Epihypersodic [BR]
  • Soils in which an ESP of 15 or greater occurs in some subhorizon below 0.5 m.
    • Endohypersodic [BP]
  • Other soils.
    • Ceteric [IC]

Family Criteria

Use of the term A horizon may be inappropriate for some of these soils because of either minimal development due to an arid environment, or common surface deflation or accumulation caused by wind. Hence it is thought better to use the term surface soil for texture and to delete the thickness criteria. In general, surface soil in this context will probably be in the range of 0.1-0.2 m in thickness. For the Calcarosols, a criterion is used to indicate the thickness above the upper boundary of the Bk horizon when present.

Thickness of soil above upper boundary of Bk horizon (when present)

Thin [A] : <0.1 m
Medium [B] : 0.1 - < 0.3 m
Thick [C] : 0.3 - 0.6 m
Very thick [D] : >0.6 m

Gravel of the 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%

Surface soil texture

Peaty [J] : see Peaty horizon
Sandy [K] : S-LS-CS (up to 10% clay)
Loamy [L] : SL-L (10-20% clay)
Clay loamy [M] : SCL-CL (20-35% clay)
Silty [N] : ZL-ZCL (25-35% clay and silt 25% or more)
Clayey [O] : LC-MC-HC (more than 35% clay)

B horizon maximum texture3

Sandy [K] : S-LS-CS (up to 10% clay)
Loamy [L] : SL-L (10-20% clay)
Clay loamy [M] : SCL-CL (20-35% clay)
Silty [N] : ZL-ZCL (25-35% clay and silt 25% or more)
Clayey [O] : LC-MC-HC (more than 35% 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 The carbonate is a result of soil-forming processes, in contrast to fragments of calcareous rock such as limestone. See also calcrete.

2 Based on numerous fine earth analyses by Primary Industries, South Australia.

3 This refers to the most clayey field texture category.

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