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Ferrosols [FE]

[Pronounced Ferro-sols]


Concept

Soils with B2 horizons which are high in free iron oxide and lacking a strong texture-contrast between the A and B horizons.

map
Distribution of Ferrosols in Australia.
Soil Profile (View type example photo of Red Ferrosol).

Definition

Soils other than Vertosols, Hydrosols, and Calcarosols that:

  1. Have B2 horizons in which the major part has a free iron oxide content greater than 5% Fe in the fine earth fraction (<2 mm), and
  2. Do not have a clear or abrupt textural B horizon or a B2 horizon in which at least 0.3 m has vertic properties.

Comment

These soils are almost entirely formed on either mafic or ultramafic igneous rocks, their metamorphic equivalents, or alluvium derived therefrom. Although these soils do not occupy large areas in Australia, they are widely recognised and often intensively used because of their favourable physical properties. The most common forms have B2 horizons with a strong grade of pedality; such horizons typically have polyhedral compound peds up to 10-15 mm, usually with smooth and often shiny faces, which break down readily to primary peds about 5 mm or less in size. However forms also occur with a very fine granular structure which may appear massive in place. Horizons are usually high in clay and subplastic.

Suborders

The dominant colour class in the major part of the upper 0.5 m of the B2 horizon (or the major part of the entire B2 horizon if it is less than 0.5 m thick) is:

Great Groups

It is thought that the great group classes listed below will be appropriate for each colour suborder. Red and Brown are by far the most common colour classes. Of the great groups listed below, the Calcareous and Magnesic classes are relatively uncommon.

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

It is thought that the following subgroups will cater for most situations, although obviously some will not be relevant for particular 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.

Comment

The Haplic subgroup is the most common in the Ferrosols classified to date, followed by Acidic, with the remaining subgroups fairly evenly distributed. All Haplic soils have been further examined, but apart from possibly using structure there seem to be few other differentiae that could be used for further subdivision.

Family Criteria

A1 horizon thickness plus the thickness of organic horizons (O2, P1 or P2) if present

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 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 or texture of organic horizons (O2, P1 or P2) if present

Peaty [J] : Dominated by organic materials
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

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
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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