Alkaline shallow duplex soil

Sand or loam topsoil over alkaline clay subsoil at < 30 centimetres (cm).

Identifying properties

  • Sandy or loamy surface texture
  • Abrupt change to clayey subsoil at less than 30cm
  • Topsoil is grey, yellow or red
  • Alkaline pH subsoil
  • Subsoil often calcareous
  • Subsoil commonly has poor rooting conditions and slow water permeability

Map of distribution

Map of distribution

Key soil issues

Issues Description
Water storage Often have very low soil water storage. Profiles with better structured subsoils can have up to moderate storage
Fertility These soils typically have moderate natural fertility
Alkalinity Usually moderately alkaline subsoils, with a few strongly alkaline
Acidity and aluminium toxicity Moderately acid sandy to neutral topsoils are common
Salinity Slightly soil salinity is common, with a few soils ranging to high levels of salinity
Hardpan and gravel Hardpans are rare. Small amounts of ironstone are common in the profiles
Shallow soil depth These soils are usually deep, but moderately shallow crop rooting is common on poor clays (sodic, alkaline or saline). Some structured clays can have deep rooting conditions
Boron toxicity Boron toxicity is likely in these soils especially if they are derived from marine sediments in the south east
Soil structure Surface soils are loose (sands) or earthy (loams). Poorly structured subsurface clays can be brought to the surface during cultivation in profiles with thin topsoil
Perched water tables Perched water tables are common but vary seasonally
Water repellence The sandy surfaced topsoils are often moderately water repellent
Hard setting topsoil Hard setting surfaces are common for the loamy duplexes. Sandy duplexes are often firm
Erodibility Soils with loose sandy surfaces are highly prone to wind erosion on exposed flats and rises. Slaking topsoils and dispersive subsoils can lead to water erosion on long slopes
Workability These soils usually have good workability though sodic subsoils may cause some problems where the topsoil is thin. Many of the loamy duplex soils can not be worked when they are too wet, or are very hard when dry

West Australian (WA) soil groups included in this category

WA soil group

(Ref no.)

Brief description Common names


(millimetres per minute (mm/m))

Alkaline grey shallow sandy duplex (402) Grey sandy over alkaline sandy clay loam to clay at less than 30cm Mallee soil, Scadden soil, Moot soil Very low to moderate
Alkaline grey shallow loamy duplex (502) Grey loam over alkaline clay at < 60cm Moort soil, Grey clay Very low to moderately low
Alkaline red shallow loamy duplex (503) Red loam over alkaline clay at < 30cm Salmon Gums soil, York gum soil, Red-brown earth Very low to moderate

**Extremely low: < 30mm/m

Very low: 30 - 50mm/m

Low: 50 - 70mm/m

Moderately low: 70 - 100mm/m

Moderate: 100 - 130mm/m

High: >130mm/m

Bold indicates the dominant or most common values 

Contact information

David Hall
+61 (0) 8 9083 1142
Page last updated: Monday, 17 February 2014 - 3:38pm