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Hydrosols [HY]

Concept

This order is designed to accommodate a range of seasonally or permanently wet soils and thus there is some diversity within the order. The key criterion is saturation of the greater part of the profile for prolonged periods (2-3 months) in most years. The soils may or may not experience reducing conditions for all or part of the period of saturation, and thus manifestations of reduction and oxidation such as 'gley' colours and ochrous mottles may or may not be present.

Saturation by a water table may not necessarily be caused by low soil permeability. Often site drainage will be the most important factor, while in other well-known cases tidal influence is dominant. The relevant Field Handbook drainage classes are very poorly and poorly drained.

Several major classes of soils are excluded because it is considered their other profile characteristics are of greater significance than wetness. These are the Organosols, Podosols and Vertosols. Although some Hydrosols are dominated by organic materials (see Intertidal Hydrosols below), it is thought that because of their unique nature (ie. largely consisting of mangrove debris that is regularly inundated by saline tidal waters), it is more appropriate to classify them as organic Hydrosols rather than as a class of Organosols that occur in a mangrove environment.

map
Distribution of Hydrosols in Australia.
Soil Profile (View type example photo of Redoxic Hydrosol).

Definition

Soils other than Organosols, Podosols and Vertosols in which the greater part of the profile is saturated for at least 2-3 months in most years.

Comment

The approach taken in this concept of 'wet' soils differs from more traditional usage in that reducing conditions are not emphasised. The rationale for the present approach is based on the assumption that saturation affects soil properties irrespective of whether or not reducing conditions are present. Obvious examples are those relating to certain physical and engineering properties, which result in limitations to the use of a soil eg. trafficability, etc.

A further reason for not making reducing conditions mandatory is the well-known difficulty in identifying such conditions, which are often of a temporal nature and sporadic in spatial distribution. It is widely recognised that the traditional use of 'gley' colours and particular kinds of mottling are not universally indicative of a saturated condition or its duration. In particular, mottles or other segregations can be relict. Another problem in identification of reducing conditions, is the experience that various indicator dyes such as a,a-dipyridyl (Childs & Clayden 1986) may be unreliable.

It will also be apparent that the concept adopted for Hydrosols will normally exclude the pseudo (or surface water) gley class commonly distinguished in several European classification schemes. These soils have a perched water table usually caused by a slowly permeable horizon or layer within the soil profile.

A difficult question arises as to how artificially drained soils are best treated. In many cases drainage will merely lower the water table to depths which permit the successful growth of particular plants. Such depths may still be relatively shallow eg. 0.5 - 1 m, and capillary rise may result in wet soil conditions to variable heights above the new water table. Additionally, the topographic situation and/or the climatic environment may mean that drainage merely reduces the period of saturation. If drainage is such that saturation no longer occurs for appropriate periods in the relevant parts of the profile, the soil can strictly no longer be considered a Hydrosol. Possibly a 'drained' phase may be appropriate in some such circumstances.

In the case of irrigated soils, the main example is when more or less permanent flood irrigation is employed to grow rice. This would be expected to change, for example, a Chromosol to a Chromosolic Hydrosol. It should be noted that the definition of the order is deliberately somewhat equivocal in that the duration and frequency of saturation of a precise section of the profile are not specifically defined. Lack of water table data is one reason, but it is also thought that a degree of flexibility is required for the definition. It is recognised that the extent of soil wetness can seldom be assessed by a single inspection of a particular site. However, in the author's experience judicious questioning of people with local knowledge, together with the soil scientist's assessment of soil and site drainage and climatic environment, can usually achieve a satisfactory resolution of the problem.

Suborders

Comment

The features used in the definitions of the first seven suborders differ from those used elsewhere in the classification in that the classes are essentially based on the frequency of tidal or freshwater inundation and the nature of the soil surface. This is thought to be an appropriate approach as the key feature of Hydrosols is their wetness. The references in parenthesis to vegetation in the definitions are merely indicating accessory properties of the class which may aid identification of what are essentially wetness criteria, for example, the presence of certain mangrove species will normally indicate a regular frequency of tidal wetting. Although it may be difficult to identify the Extratidal suborder by the low frequency of tidal inundation, there is usually a distinct boundary between this zone and the often bare, salt-encrusted Supratidal zone. This boundary is commonly marked by a low (0.1 - 0.2m) 'scarp'. The three tidal-affected suborders are largely based on data from Cook and Mayo (1977).

For the purposes of soil classification, hydrosols located within Intermittently Closed and Open Lakes and Lagoons (ICOLLS) are considered to be tidal.

Where inundation at the perimeter of larger water bodies is cyclical as a result of seiches or from variability in rainfall and runoff, it is often difficult to decide if currently submerged soils are either Subaqueous, Redoxic or Oxyaquic. Their classification will depend on conditions at the time of sampling and knowledge of the frequency and duration of any drying on exposure. With some submerged soils, evidence of prior surface soil aeration such as the development of mottles, soil structure or very low pH suggests that the soils are not Subaqueous, i.e. subject to permanent saturation. If the period of exposure is insufficient to allow substantial soil aeration and there is no evidence of oxidation and drying, then the soils are considered to be Subaqueous.

In the Salic suborder the salinisation may or may not be human-induced. Saline water tables may arise as a result of a sequence of wetter-than-average years, or they may result from activities such as a tree clearing and/or unwise irrigation practices. With time it is likely that some of the human-induced saline soils will tend to those of the Hypersalic suborder, as evidenced in some of the saline valleys of the Western Australian wheat belt. It also follows that a wide range of soils is likely to occur in the Salic suborder.

In the Redoxic and Oxyaquic suborders water tables are normally non-saline. However exceptions may occur where these soils are underlain by sulfuric and/or sulfidic materials, as described by Walker (1972). It should be noted that the use of mottling as a diagnostic criterion in the former suborder does not necessarily imply that oxidising and reducing conditions are currently occurring in the soil in most years.

Of the Hydrosol profiles available for classification, 62% were Redoxic and 29% were Oxyaquic.

Great Groups

Because of lack of data in the first seven suborders, further studies may lead to additional great groups.

Subtidal, Intertidal and Subaqueous Hydrosols

Conventional horizon nomenclature is inapplicable to these soils, hence the use of arbitrary depth limits.

Supratidal Hydrosols

Hypersalic Hydrosols

Extratidal and Salic Hydrosols

The provision of great groups for these suborders is incomplete because of lack of data. High salt contents usually tend to obliterate the original morphology, but where this can still be identified, great groups may be established on this basis.

Redoxic and Oxyaquic Hydrosols

The following great groups will not all be relevant for each of these two suborders. For example, the Rudosolic great group will not be required for the Redoxic suborder.

Subgroups

No subgroups for the Subtidal, Subaqueous, Supratidal, Extratidal and Hypersalic Hydrosols are formally proposed at present because of insufficient data.

Subgroups of Intertidal Hydrosols

The following three subgroups will only be applicable to the Histic-Hypersulfidic and Histic-Hyposulfidic great groups.

Subgroups of Salic Hydrosols

The following three subgroups have been identified for several great groups in the Salic suborder. Other possibly relevant subgroups may be found listed below in the subgroups for the Redoxic and Oxyaquic suborders.

Subgroups of Redoxic and Oxyaquic Hydrosols

It is thought that the following subgroups will cater for most situations, although obviously some will not be relevant for particular great groups. As examples, the Acidic subgroups will not be required for the Kurosolic great groups, nor the Sodic and Natric classes for the Sodosolic great group. Although presently not listed, a Petrocalcic subgroup may be required for the Calcarosolic great group. If so, a definition is available above. Some additional subgroups will be required for the Sulfuric and Sulfidic great groups as knowledge of these soils increases eg. a possible subdivision could be based on the nature of the soil profile above the sulfuric or sulfidic materials which commonly occur as a D horizon. "In the case of the Rudosolic and Tenosolic great groups, the most appropriate subgroups will be those used for the relevant suborders and great groups of Rudosols and Tenosols."

Family Criteria

The classes below are primarily for use in the Redoxic and Oxyaquic suborders, and possibly the Extratidal and Salic suborders. The criteria may be partly applicable to the Supratidal and Hypersalic suborders eg. using the terms surface soil and maximum subsoil texture. The different A horizon thickness criteria for great groups with a Clear or abrupt textural B horizon.allows alignment with their adjacent but drier equivalent soil orders.

A horizon thickness (for Chromosolic, Kurosolic and Sodosolic great groups)

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

A1 horizon thickness (for all other great groups)

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%

A1 horizon 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 (greater than 35% clay)

B horizon maximum texture1

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 (greater 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 This refers to the most clayey field texture category.

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