Podosols [PO]
Concept
Soils with B horizons dominated by the accumulation of compounds of organic matter, aluminium and/or iron. These soils are recognised world-wide, and Australia is particularly noted for its 'giant' forms.
Distribution of Podosols in Australia.
Soil Profile (View type example photo
of Aeric Podosol).
Definition
Soils which possess either a Bs horizon (visible dominance of iron compounds), a Bhs horizon (organic-aluminium and iron compounds), or a Bh horizon (organic-aluminium compounds). These horizons may occur singly in a profile or in combination (see Podosol diagnostic horizons).
Comment
Extensive revisions of the classification of Spodosols and podzols have recently taken place in the USA (with world-wide input) and New Zealand. It is clear that there is considerable diversity of opinion on the desirability, nature and efficacy of chemical criteria to define Spodosols/podzols. For this reason the present proposal for Australian 'podzols' has deliberately avoided the use of chemical diagnostic criteria. It is realised that problems may arise in distinguishing a Bs horizon from a tenic B horizon; the diagnostic feature of the former is the presence of discontinuous patches or a thin band of darker organic accumulations.
Suborders
These are based on soil and site drainage conditions. The intention is to separate soils with no short-term saturation in the B horizon, those with short-term saturation of the B horizon, and those that are saturated for long periods in the B horizon.
- Soils with free drainage, ie. rapidly drained. There is no restriction to through drainage in the B horizon or within the substrate. There is no perching of water within the B horizon or saturation due to a high ground water table. The B horizons are weakly coherent and porous. They are often brightly coloured, and lack evidence of seasonal reduction. Aeric [AL]
- Soils with short-term saturation in the B horizon. The saturation may be caused by impedance within the B horizon, perching of water by substrate material, or by seasonally high groundwater tables. The duration of saturation may range from several days to several weeks but is insufficient to reduce and remove significant amounts of the accumulated iron. However, there may be a greater accumulation of organic compounds and less iron in the zone of maximum saturation. Semiaquic [EJ]
- Soils with long-term saturation in the B horizon and sulfuric material(at least 15 cm thick) is present in the A or B horizons. Aquic-Sulfuric [JD]
- Soils with long-term saturation in the B horizon and i>hypersulfidic material is present in the A or B horizons. Aquic-Hypersulfidic [JE]
- Soils with long-term saturation in the B horizons. The saturation may be caused as in the semiaquic soils but the duration is of the order of months. The period of saturation is sufficient to reduce most iron compounds and move them out of the B horizon, hence Bh horizons are usually prominent. Aquic [AM]
Great Groups
Classes are based on observable B horizon characteristics reflecting the dominance of organic or iron compounds and their distribution in the accumulation zone. The organic accumulations can usually be recognised by their dark colours and the iron compounds by generally bright colours. Aluminium is always present, usually complexed by organic matter and therefore not usually visible, except in some horizons where large amounts of amorphous aluminium and silica (imogolite-allophane complexes) may induce a yellowish brown colouration. Yellowish brown bands in poorly-drained B horizons should not be interpreted as evidence of iron compounds without chemical verification.
Aeric Podosols
- Soils with a pipey B horizon. Pipey [EB]
- Soils with only a Bs horizon. Sesquic [EK]
- Soils with only a Bhs horizon. Humosesquic [CO]
- Soils with a Bhs/Bs horizon. Humosesquic/Sesquic [IG]
Semiaquic Podosols
- Soils with a pipey B horizon. Pipey [EB]
- Soils with only a Bs horizon. Sesquic [EK]
- Soils with only a Bhs horizon. Humosesquic [CO]
- Soils with only a Bh horizon. Humic [CG]
- Soils with a Bh/Bs horizon. Humic/Sesquic [CJ]
- Soils with a Bh/Bhs horizon. Humic/Humosesquic [CI]
- Soils with a Bh/Basi horizon. Humic/Alsilic [IH]
Aquic-Sulfuric, Aquic-Hypersulfidic and Aquic Podosols
- Soils with only a Bh horizon. Humic [CG]
- Soils with a Bh/Basi horizon. Humic/Alsilic [IH]
Comment
In the soils classified the most common class in the Aeric suborder is the Sesquic great group, but Humic is most common in the other two suborders.
Subgroups
Each class listed below may not be relevant for every great group of each suborder.
- Soils with a peaty horizon and a strongly coherent B horizon. Peaty-Parapanic [DX]
- Soils with a peaty horizon and a thin ironpan which occurs within or directly underlying the B horizon. Peaty-Placic [GD]
- Other soils with a peaty horizon. Peaty [DW]
- Soils with a humose horizon and a strongly coherent B horizon. Humose-Parapanic [CN]
- Other soils with a humose horizon. Humose [CK]
- Soils with a melacic horizon and a strongly coherent B horizon. Melacic-Parapanic [DJ]
- Other soils with a melacic horizon. Melacic [DG]
- Soils with a melanic horizon. Melanic [DK]
- Soils with a densipan present in the A2 horizon and a thin ironpan which occurs within or directly underlying the B horizon. Densic-Placic [HN]
- Other soils with a densipan present in the A2 horizon. Densic [BI]
- Other soils with a thin ironpan which occurs within or directly underlying the B horizon. Placic [EC]
- Soils with a hard siliceous pan directly underlying the B horizon. Silpanic [EM]
- Soils with a ferric horizon within the B horizon. Ferric [BU]
- Other soils with a strongly coherent B horizon. Parapanic [DV]
- Soils with a weakly coherent B horizon. Fragic [BY]
Comment
The term 'parapanic' is meant to imply 'pan-like'. Note also that the A1 horizons of many Podosols may have a distinct surface layer of lighter coloured sand with clean quartz grains and discrete lumps of organic matter and charcoal, giving the layer a speckled appearance. Below this layer, a dark A1 horizon may occur. Because the great majority of Australian Podosols have a bleached A2 horizon, this attribute is not used in the classification. Similarly, the great majority have a B horizon pH of less than 5.5, hence acidic subgroups have not been used.
Family Criteria
A1 horizon thickness
| 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 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) |
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 (> 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.