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Sodosols [SO]

[Pronounced So-doh-sols]


Concept

Soils with strong texture contrast between A horizons and sodic B horizons which are not strongly acid. Australia is noteworthy for the extent and diversity of sodic soils (Isbell 1995).

map
Distribution of Sodosols in Australia.
Soil Profile (View type example photo of Grey Sodosol).

Definition

Soils 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 sodic and not strongly acid. Hydrosols and soils with strongly subplastic upper B2t horizons are excluded.

Comment

There is convincing evidence from the Riverine Plain of south eastern Australia that soils with sodic clay B horizons (ESP 25-30) which are strongly subplastic behave very differently in terms of permeability to the more commonly found plastic sodic clay B horizons which are characterised by low to very low saturated hydraulic conductivity (McIntyre 1979). It is on this basis that subplastic sodic soils are excluded from Sodosols, because of their very different land use properties.

Strongly acid sodic soils are also excluded from Sodosols because they usually contain appreciable exchangeable aluminium (KCl extractable) and thus should be unlikely to disperse.

It is usually possible to assess, in the field, the likelihood of a soil possessing a sodic B2t horizon. Such a criterion as the presence of a bleached A2 horizon with an abrupt change to a B2t horizon which has columnar or prismatic structure is a useful but not universal guide. A high pHw value (>8.5) suggests sodicity, but the converse is not true. The soapy nature of the bolus produced in field texturing will also often suggest appreciable sodium (and/or magnesium) on the clay exchange complex.

Increasing experience in many parts of Australia is confirming that the Emerson dispersion test (Emerson 1967) and the modified version of Loveday and Pyle (1973) is a reliable guide to sodicity. It can be carried out as a preliminary test in the field, but should always be repeated under the better controlled conditions of the laboratory. In the initial test at least several fragments (each about 0.2 g or about 4 -5 mm diameter) are immersed in 100 ml of distilled water. The large water:soil ratio is necessary to remove any salt present in the aggregate. Also for this reason Emerson (1991) suggests that the classification for each test be made after 24 hours. For the remoulded test, use a 5 mm cube which can be obtained from the bolus used for determining field texture. This will be approximately in the plastic limit condition. Note that distilled water should be used to prepare the bolus.

Data from Loveday and Pyle (1973) and unpublished data available to the original author Ray Isbell suggest that a dispersive soil will usually indicate sodicity, ie. ESP of 6 or greater. The data of Murphy (1995) indicate that whereas Emerson class 1 and the more strongly dispersive soils of class 2 are a reliable indicator of sodicity, whereas class 3 is a more variable predictor. Emerson classes of 5 or greater or a Loveday and Pyle score of zero strongly suggest soils are non-sodic. There is less evidence that the dispersion tests give a reliable indication of the degree of sodicity; factors such as extent of initial slaking and initial salt content in the aggregates, and the amount of magnesium and amount and form of aluminium on the exchange complex (Emerson 1994), can also influence the rapidity of dispersion. Nevertheless, the data of Loveday and Pyle measuring dispersion after 2 hours and 20 hours showed that the rate of dispersion could be used as a guide to ESP. This relationship does not apply to the subplastic soils of the Riverine Plain where subsoils with an ESP of 25-30 do not disperse unless remoulded (Blackmore 1976). Other anomalous results occur in a small minority of normal plastic soils in which dispersion will not occur even after remoulding in spite of an ESP much greater than 6 and a pHw as high as neutral. In at least some of these non-dispersive soils the typical sodic soil morphology is present, ie. a conspicuously bleached A2 horizon abruptly overlying a B2t horizon with prismatic or columnar structure, suggesting the low hydraulic conductivity expected of a sodic B horizon.

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:

Great Groups

Some Great Group soils are much more common in certain colour Suborders than others. The Duric and Pedaric Great Groups are known only from 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.

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.

Subgroups

Not every Subgroup defined below will be required or be appropriate for each Great Group of each Suborder. Some possible attributes have not been used for various reasons, e.g. bleaching has not been used because the great majority of soils in the class probably are bleached. Structure has not been used because of the diverse range that is encountered in these particular soils.

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.

Comment

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

Family Criteria

A horizon thickness

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

Gravel of 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

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)

B horizon maximum texture1

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
Moderately deep [V] : 0.5 - <1.0 m
Deep [W] : 1.0 - <1.5 m
Very deep [X] : 1.5 - 5 m
Giant [Y] : >5 m

Water repellence of surface soil

Non water repellent [NR] : Water absorbed in 10 seconds or less
Water repellent [WR] : Water takes more than 10 seconds and 2 Molar ethanol takes 10 seconds or less to be absorbed into soil
Strongly water repellent [SR] : 2 Molar ethanol takes more than 10 seconds to be absorbed into soil


1 This refers to the most clayey field texture category.

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