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The Monitor - Issue 5 - A dirty job...

Jody Bruce and David Osborne

David Osborne from DPI&F QLD and Jody Bruce from CSIRO thinking about how things are so different in Queensland

Map of regolith showing areas with strong weathering

Map of regolith showing areas with strong weathering (from Australian soils and landscapes: An Illustrated compendium by Neil McKenzie et al., CSIRO Publishing 2004)

road cutting

Soil profile in south-east Queensland—the soils here are less than a metre deep but the rock below is so decomposed that no fresh rocks could be found. Tree roots can easily penetrate this zone

Much of the work thus far has focused on the soils in south-eastern Australia, in the regions of erosion and depositional soils, where soil development can be largely explained by features such as position in landscape, climate, glaciations and geology. Weathering plays a role in soil formation in these regions but, as we move into areas such as south-east Queensland and south-west Western Australia (WA), deeply weathered landscapes are far more common.

These landscapes are generally formed from the deeply weathered mantle from the Tertiary period and the type of soil depends on the degree and rate at which the mantle has been stripped. Soils may be very deep where there has been little movement of material off the site. In areas with some relief, erosion combined with weathering and leaching may be so extensive that only the most resistant minerals are left. In south-east Queensland the remaining soils may actually be shallow, but with deep regolith.

Inverted landscapes (often seen in WA) develop where hard duricrusts (from iron, silica, aluminium or calcium carbonate) are formed in low parts of the landscape and, over time, the surrounding landscapes are eroded away. This exposes different zones in the profile, providing new material for soil formation. Lower zones of the profile often contain soluble salts, which will again influence the soil formed. These landscapes provide a challenge for us and require us to think differently about how we model soil properties in these regions. Expert knowledge from those working in these regions will be essential for developing reasonable working models as we move away from the traditional south-eastern Australian context.

Jody Bruce

Tel: 03 62375664