Chromosols [CH]

Concept

Soils with strong texture contrast between A horizons and B horizons. The latter are not strongly acid and are not sodic. The soils of this order are among the most widespread soils used for agriculture in Australia, particularly those with red subsoils.

Distribution of Chromosols in Australia.
Soil Profile (View type example photo of Brown Chromosol).

Definition

Soils other than Hydrosols with a clear or abrupt textural B horizon and in which the major part of the upper 0.2 m of the B2 horizon (or the major part of the entire B2 horizon if it is less than 0.2 m thick) is not sodic and not strongly acid. Soils with strongly subplastic upper B2 horizons are also included even if they are sodic.

Comment

In the case of those soils with strongly subplastic B horizons, care needs to be taken to ensure if they qualify for the clear or abrupt textural B horizon. As far as is presently known, such soils appear to be largely confined to the Riverine Plain of south eastern Australia.

Suborders

• The dominant colour class in the major part of the upper 0.2 m of the B2 horizon (or the major part of the entire B2 horizon if it is less than 0.2 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

The Red and Brown suborders account for 80% of the profiles classified.

Great Groups

These will vary somewhat among the various colour class suborders, but it is likely that the subdivision given below will apply to most.

• Soils with a red-brown hardpan either within or directly underlying the B horizon.
Duric [BJ]
• Soils with a petroferric horizon within the solum.
Petroferric [EA]
• Soils with a B horizon that is not calcareous and which directly overlies a calcrete pan.
Petrocalcic [DZ]
• Soils in which the upper 0.2 m of the B2 horizon (or the B2 horizon if it is less than 0.2 m thick) has a strong blocky or polyhedral structure in which average ped size is usually in the range of 5 - 20 mm. There is very weak adhesion between peds (when dry it is very easy to insert a spade into the upper B2 horizon). Salt contents are usually high, resulting in weak dry strength and a bulk density of about 1.3 t m^-3 or less. In some soils the B2 horizon may be weakly subplastic. A common feature (but not diagnostic) of the overlying A horizons is the presence of a band of vesicular pores near the surface or on the underside of any surface flake.
Pedaric [BK]
• Soils in which the major part of the B2 horizon is strongly subplastic.
Subplastic [ET]
• Soils with an exchangeable Ca/Mg ratio of less than 0.1 in the major part of the B2 horizon.
Magnesic [DB]
• Soils in which the major part of the B2 horizon is dystrophic.
Dystrophic [AF]
• Soils in which the major part of the B2 horizon is mesotrophic.
Mesotrophic [AG]
• Soils in which the major part of the B2 horizon is eutrophic but the B and BC horizons are not calcareous.
Eutrophic [AH]
• 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 class was found to be most common in soils with a calcareous horizon. However, almost half of the Chromosol great groups classified were Eutrophic. The Duric and Pedaric soils are virtually confined to the arid zone, the former being particularly widespread in Western Australia and the latter in western Queensland and New South Wales, and in South Australia.

Subgroups

The subgroups listed below may not all be relevant for every great group of every suborder.

• Soils with a peaty horizon.
Peaty [DW]
• Soils with a humose horizon and a conspicuously bleached A2 horizon.
Humose-Bleached [EY]
• Soils with a humose horizon and the major part of the B2 horizon is mottled.
Humose-Mottled [CM]
• Other soils with a humose horizon.
Humose [CK]
• Soils with a melacic horizon and the major part of the B2 horizon is mottled.
Melacic-Mottled [DI]
• Other soils with a melacic horizon.
Melacic [DG]
• Soils with a melanic horizon and a B horizon in which at least 0.3m has vertic properties.
Melanic-Vertic [DN]
• Soils with a melanic horizon and the major part of the B2 horizon is mottled.
Melanic-Mottled [DM]
• Other soils with a melanic horizon.
Melanic [DK]
• Soils with a conspicuously bleached A2 horizon overlying a B horizon in which at least 0.3m has vertic properties.
Bleached-Vertic [BB]
• Other soils with a B horizon in which at least 0.3m has vertic properties.
Vertic [EX]
• Soils with a gypsic horizon within the B or BC horizon.
Gypsic [BZ]
• Soils with a ferric horizon within the solum, and at least the lower part of the B horizon is sodic.
Ferric-Sodic [HC]
• Soils with a conspicuously bleached A2 horizon and a ferric horizon within the solum.
Bleached-Ferric [AV]
• Other soils with a ferric horizon within the solum.
Ferric [BU]
• Soils with a conspicuously bleached A2 horizon and a manganic horizon within the solum.
Bleached-Manganic [AY]
• Other soils with a manganic horizon within the solum.
Manganic [DC]
• Soils with fine earth effervescence (1M HCl) throughout the solum.
Effervescent [IE]
• Soils with a conspicuously bleached A2 horizon and a B horizon in which at least the lower part is sodic.
Bleached-Sodic [BA]
• Soils in which the major part of the B2 horizon is mottled, and at least the lower part of the B horizon is sodic.
Mottled-Sodic [HB]
• Other soils with a B horizon in which at least the lower part is sodic.
Sodic [EO]
• Soils with a conspicuously bleached A2 horizon and the major part of the B2 horizon is mottled.
Bleached-Mottled [AZ]
• Other soils with a conspicuously bleached A2 horizon.
Bleached [AT]
• Soils with a reticulite horizon below the B2 horizon.
Reticulate [EF]
• Other soils in which the major part of the B2 horizon is mottled.
Mottled [DQ]
• Other soils in which the major part of the B2 horizon is whole coloured.
Haplic [CD]

Comment

Forty percent of the profiles classified so far have a Haplic subgroup. This would suggest that the class may need to be further subdivided, but it is difficult to find suitable criteria to base this on. The presence of a pale (unbleached) A2 horizon could be used, but the significance of this is uncertain. A subdivision could be made between soils with clear or abrupt textural changes if this was thought to be of importance. Similarly, a distinction between structured and massive B2 horizons could be made. Possible changes such as these can easily be introduced if evidence is produced to justify their use.

Family Criteria

A horizon thickness

Gravel of the surface and A1 horizon

A1 horizon texture

B horizon maximum texture¹

Soil depth

¹ This refers to the most clayey field texture category.

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