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Rudosols [RU]


Concept

This order is designed to accommodate soils that have negligible pedologic organisation. They are usually young soils in the sense that soil forming factors have had little time to pedologically modify parent rocks or sediments. The component soils can obviously vary widely in terms of texture and depth; many are stratified and some are highly saline. Data on some of them are very limited.


Distribution of Rudosols in Australia.
Soil Profile (View type example photo of Stratic Rudosol).

Definition

Soil with negligible (rudimentary) pedologic organisation apart from (a) minimal development of an Al horizon or (b) the presence of less than 10% of B horizon material (including pedogenic carbonate) in fissures in the parent rock or saprolite. The soils are apedal or only weakly structured in the A1 horizon and show no pedological colour changes apart from the darkening of an A1 horizon. There is little or no texture or colour change with depth unless stratified or buried soils are present.

Comment

By definition, these soils will grade to Tenosols, so before deciding on the order check the Tenosol definition as it will often be a matter of judgement as to which order a particular soil is best placed. Hydrosols are excluded on the basis that these will normally show some pedological development eg. mottling. A particular problem with the definition and subdivision of Rudosols is the difficulty in distinguishing between soil and 'non-soil', (see also 'What do we classify?'). In many instances they are classified on the basis of the nature of AC or C horizon materials or other substrates because these are the dominating features of the profile. It also follows that there may be difficulties in deciding if a soil is best classified as an Anthroposol or be regarded as an Ďanthropicí Rudosol because the little-altered soil parent material may be human-made.

Suborders

Comment

The Hypergypsic soils normally occur as gypsum lunettes and the Hypersalic soils are most common in many of the saline playas of the arid interior of the continent. Shelly soils are widespread as coastal and near coastal dunes in southern and south-western Australia, while the Carbic soils have so far only been recorded in the Sydney basin, NSW. The Arenic suborder mainly caters for the widespread siliceous dunes and sandsheets of arid Australia, and for some usually recently deposited fluvial sands and very young coastal sands such as foredunes. The Lutic soils include loamy or clayey aeolian forms common on some of the many lunettes in southern Australia, as well as other coherent soils formed on sandy, loamy or clayey fluvial deposits, or easily weathered rocks. The Stratic and Clastic soils are most common on alluvial terraces, plains and fans.

The most commonly recorded suborder is the Leptic class (44% of the soils classified to date) and these are mostly shallow profiles overlying hard or weathered rock.

Great Groups

No great groups are presently proposed for the Hypergypsic, Shelly, Carbic, Arenic, Lutic or Stratic suborders as data are very limited.

Hypersalic Rudosols

Clastic Rudosols

Leptic Rudosols

Comment

The Tephric soils in the Clastic suborder are known only from some of the Pliocene to Holocene Newer Volcanics in south western Victoria. They have not weathered sufficiently for them to be recognised as possessing andic properties, or to meet the requirements for the Tenosols.

In the Petrocalcic great group of the Leptic suborder the calcrete occurs as a substrate material that may or may not be the parent material of the soil.

Subgroups

No subgroups are yet proposed for the Shelly and Carbic suborders. The following subgroups will be used for the remaining suborders where relevant.

Comment

In the Calcareous subgroups the carbonate present is usually not pedogenic, but is in effect part of the parent material, either of aeolian or residual origin. However, in Calcarosol landscapes it is inevitable that some young alluvial deposits may contain transported pedogenic carbonate nodules, and hence soils developed on such deposits should be regarded as Rudosols rather than Calcarosols. The presence of sedimentary layering will usually be diagnostic. Small amounts of pedogenic carbonate (<10%) may occur in fissures in the parent rock or saprolite of some Leptic Rudosols.

A similar situation arises in the case of the Calcareous Hypergypsic soils. These usually occur on lunettes, and thus the carbonate is also of aeolian origin. Whether or not pedogenic accumulations also occur will probably depend on the age of the lunettes. If so, the soils will be more appropriately classified as Hypergypsic Calcarosols. The distinction between Shelly Rudosols and Shelly Calcarosols will similarly be based on the absence or presence of pedogenic carbonate.

Family Criteria

There are obvious problems in applying the usual family criteria to Rudosols. By definition, A horizons have minimal development and hence may be difficult to recognise, and in some classes the texture is set by definition, e.g. the Arenic suborder is sandy. It has been decided not to use A horizon thickness, and not refer to any classes for subsoil texture. By definition, subsoil texture must be the same as surface soil unless the profile is stratified, in which case the situation is usually too complex to manage satisfactorily. Similarly, the term soil depth is used only in the case of the Leptic suborder, where only two classes will be required. Elsewhere the term becomes meaningless. Gravel content of surface soil can be usefully used for several suborders. In general, surface soil in this context will probably be in the range of 0.1-0.2 m in thickness.

Gravel of surface soil (visual estimate)

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

A1 horizon texture

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 (> 35% clay)

Soil depth

Very shallow [T] : < 0.25 m
Shallow [U] : 0.25 - < 0.5 m

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