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Kurosols [KU]


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).

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


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 B2 horizon (or the major part of the entire B2 horizon if it is less than 0.2 m thick) is strongly acid.


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 Unpublished data from many localities in Australia imply that for B horizons the critical limits of pH 5.5 and ESP of 6 to distinguish dispersive and non-dispersive soils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 pH less than 5.5. The role of the high exchangeable magnesium in many Kurosols is largely unknown.


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.


A feature of the soils classified is the common occurrence of high subsoil exchangeable magnesium with or without sodium. Thus 40% of soils have Magnesic or Magnesic-Natric great groups and 41% have Natric or Magnesic-Natric great groups. In spite of an upper B2 horizon that is strongly acid, Mesotrophic great groups are more common (22%) than the Dystrophic forms (9%). This is also often related to relatively high magnesium values.


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


Fifty percent of the soils classified to date have mottled B2 horizons, compared with only 23% for the Chromosols. This suggests a trend to poorer internal drainage in the Kurosols.

Family Criteria

A 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 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)

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
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 texturecategory.

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