Role of Sodicity in the Australian Soil Classification

S.C. Boucher
School of Geography and Environmental Science
Monash University, Victoria 3800

The Australian Soil Classification (Isbell, 1996) has a particular application to the problem of soil sodicity especially as it relates to dispersion. The rationale for these classifications was discussed by Isbell (1995). Soil sodicity is relevant in eight of the scheme's 14 orders (Isbell, 1995). Most obvious is the Sodosol which is introduced at the order level and applies to texture contrast soil profiles where the pH in a 1:5 soil:water suspension is at least 5.5 and the Exchangeable Sodium Percentage (ESP) is six or more. At the next (i.e. great group) level, the classes are subdivided into subnatric (i.e. ESP 6-14), mesonatric (i.e. ESP 15-25) and (i.e. ESP >25) categories.

A Kurosol is a texture contrast soil where the pH in a 1:5 soil:water suspension is less than 5.5 and, at the great group level, is considered natric where the ESP in the upper B horizon is at least six.

A Hydrosol applies to soils that are seasonally saturated for a prolonged period and is termed Sodosolic at the great group level. At the subgroup level, the soil is termed sodic where the ESP is at least six in the lower B horizon.

A Chromosol is a texture contrast soil where the pH in a 1:5 soil:water suspension is greater than 5.5 and is normally considered non-sodic. However, a sodic characteristic may be found at the subgroup level where the ESP in the lower B horizon is at least six.

Both a Dermosol and Kandosol lack a strong texture contrast in the soil profile, although the B horizon of the former is structured but is massive in the latter order, and the same sodic classification used for the Chromosol applies.

A strong texture contrast is also absent in a Calcarosol, which is also calcareous throughout the profile. The epihypersodic subgroup applies to soils where the ESP is at least 15 in the upper 0.5 m, whilst the term endohypersodic is used for the ESP is at least 15 below a depth of 0.5 m.

A Vertosol contains at least 35 % clay, cracks extensively when dry, and contains slickensides. Three subgroups are defined as episodic (i.e. the ESP is at least six in the upper 0.1 m) and the epihypersodic at least 15 and endohypersodic classes used for a Calcarosol also apply.

References

Isbell, R.F., 1995: The use of sodicity in Australian soil classification systems. In Naidu, R., Sumner, M.E. and Rengasamy, P. (eds) `Australian Sodic Soils - Distribution, Properties and Management'. CSIRO Australia, East Melbourne, 41-45.

Isbell, R.F., 1996: `The Australian Soil Classification'. CSIRO Australia, Collin