The Australian Soil Classification

Third Edition

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Kurosols [KU]

[Pronounced Cure-oh-sols]

Concept

Soils with strong texture contrast between A horizons and strongly acid B horizons. Many of these soils have some unusual subsoil chemical features (high magnesium, sodium and aluminium).

Distribution of Kurosols in Australia.
Soil Profile (View type example photo of Brown Kurosol).

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 B2t horizon (or the major part of the entire B2t horizon if it is less than 0.2 m thick) is strongly acid.

Comment

The relevance of sodicity in strongly acid soils is open to question as in theory the presence of aluminium in such soils should counterbalance the usual deleterious effect of sodium (via dispersion) on soil physical properties. Unpublished data from many localities in Australia imply that for B horizons the critical limits of pH_w 5.5 and ESP of 6 to distinguish dispersive and non-dispersive soils seems to generally work in practice, although as might be expected, some soils do not behave as predicted. For this reason, sodicity is also used in Kurosols, but at a lower hierarchical level, to cater for those soils which have an ESP > 6 and may disperse in spite of having a pH_w less than 5.5. The role of the high exchangeable magnesium in many Kurosols is largely unknown.

Suborders

The dominant colour class in the major part of the upper 0.2 m of the B2t horizon (or the major part of the entire B2t horizon if it is less than 0.2 m thick) is:

• Red.
Red [AA]
• Brown.
Brown [AB]
• Yellow.
Yellow [AC]
• Grey.
Grey [AD]
• Black.
Black [AE]

Great Groups

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

If a diagnostic feature in the key begins more than 1.5 m from the soil surface it may not have a significant impact on the performance of the soil. Refer to diagnostic features in the glossary for guidance on the use of such features in the classification.

• Soils with a petroferric horizon within the solum.
Petroferric [EA]
• Soils with an exchangeable Ca/Mg ratio of less than 0.1 in the major part of the B2t horizon, and the major part of the upper 0.2 m of the B2t horizon is sodic.
Magnesic-Natric [GP]
• Other soils with an exchangeable Ca/Mg ratio of less than 0.1 in the major part of the B2t horizon.
Magnesic [DB]
• Other soils in which the major part of the upper 0.2 m of the B2t horizon is sodic.
Natric [FD]
• Soils in which the major part of the B2t horizon is dystrophic.
Dystrophic [AF]
• Soils in which the major part of the B2t horizon is mesotrophic.
Mesotrophic [AG]
• Soils in which the major part of the B2t horizon is eutrophic.
Eutrophic [AH]

Comment

A feature of the soils classified is the common occurrence of high subsoil exchangeable magnesium with or without sodium. In spite of an upper B2t horizon that is strongly acid, Mesotrophic great groups are more common than the Dystrophic forms. This is often related to relatively high magnesium values.

Subgroups

The subgroups listed will not all be relevant for every great group. eg. Sodic classes will not be required for the Natric great groups.

If a diagnostic feature in the key begins more than 1.5 m from the soil surface it may not have a significant impact on the performance of the soil. Refer to diagnostic features in the glossary for guidance on the use of such features in the classification.

• Soils with a humose horizon and a conspicuously bleached A2 horizon.
Humose-Bleached [EY]
• Other soils with a humose horizon.
Humose [CK]
• Soils with a melacic horizon and a conspicuously bleached A2 horizon.
Melacic-Bleached [EZ]
• Other soils with a melacic horizon.
Melacic [DG]
• Soils with a melanic horizon and a B horizon in which at least 0.3 m has vertic properties.
Melanic-Vertic [DN]
• Other soils with a melanic horizon.
Melanic [DK]
• Soils with a conspicuously bleached A2 horizon and a B horizon in which at least 0.3 m has vertic properties.
Bleached-Vertic [BB]
• Other soils with a B horizon in which at least 0.3 m has vertic properties.
Vertic [EX]
• 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 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 B2t 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 B2t horizon is mottled.
Bleached-Mottled [AZ]
• Other soils with a conspicuously bleached A2 horizon.
Bleached [AT]
• Soils with a reticulate horizon below the B2t horizon.
Reticulate [EF]
• Other soils in which the major part of the B2t horizon is mottled.
Mottled [DQ]
• Other soils in which the major part of the B2t horizon is whole coloured.
Haplic [CD]

Comment

A significant proportion of the soils classified to date have mottled B2t horizons which suggests a trend to poorer internal drainage in the Kurosols compared to Chromosols.

Family Criteria

A1 horizon thickness

Gravel of the surface and A1 horizon

A1 horizon texture

B horizon maximum texture ¹

Soil depth

Water repellence of surface soil

¹ This refers to the most clayey field texture category.

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