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Kurosols [KU]

[Pronounced Cure-oh-sols]


Concept

Soils with strong texture contrast between A horizons and strongly acid B horizons. Many of these soils have some unusual subsoil chemical features (high magnesium, sodium and aluminium).

map
Distribution of Kurosols in Australia.
Soil Profile (View type example photo of Brown Kurosol).

Definition

Soils other than Hydrosols 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 strongly acid.

Comment

The relevance of sodicity in strongly acid soils is open to question as in theory the presence of aluminium in such soils should counterbalance the usual deleterious effect of sodium (via dispersion) on soil physical properties. Unpublished data from many localities in Australia imply that for B horizons the critical limits of pHw 5.5 and ESP of 6 to distinguish dispersive and non-dispersive soils seems to generally work in practice, although as might be expected, some soils do not behave as predicted. For this reason, sodicity is also used in Kurosols, but at a lower hierarchical level, to cater for those soils which have an ESP > 6 and may disperse in spite of having a pHw less than 5.5. The role of the high exchangeable magnesium in many Kurosols is largely unknown.

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

These will vary somewhat among the various colour class suborders, but it is likely that the subdivisions given below will apply to most.

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

A feature of the soils classified is the common occurrence of high subsoil exchangeable magnesium with or without sodium. In spite of an upper B2t horizon that is strongly acid, Mesotrophic great groups are more common than the Dystrophic forms. This is often related to relatively high magnesium values.

Subgroups

The subgroups listed will not all be relevant for every great group. eg. Sodic classes will not be required for the Natric 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

A significant proportion of the soils classified to date have mottled B2t horizons which suggests a trend to poorer internal drainage in the Kurosols compared to Chromosols.

Family Criteria

A1 horizon thickness

Thin [A] : <0.1 m
Moderately deep [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] : 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)

B horizon maximum texture 1

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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