Anomalies in the Production of Australia's soil

J. Chappell

Soil generally is comprised of mineral particles produced by weathering of the underlying bedrock or sediments, and organic compounds derived from vegetation and soil fauna. The rate at which rock converts to soil is considered to be broadly affected by climate, with the properties and availability of soil water, together with the activity of soil microbes, governing the process in detail.

The rate of soil production from rock weathering has recently been determined by measuring concentrations of radionuclides produced in situ by cosmic rays passing through quartz particles (see Annual Report 2000). This work found that the rate of soil production decreases as the soil thickness increases, which is consistent with the concept that soil water availabilty and chemical activity decreases with soil depth. Analyses from soil-mantled granite lanscapes in humid temperate southeastern Australia found that the rate under thin soil reaches a maximum of about 50 metres per million years. However, reconnaissance samples from similar granite terrain the West Australian wheat belt failed to reproduce these results: the maximum production rate was found to be only 2-3 metres per million years, and no depth-dependence was detected. The explanation seems to lie with the impact of Quaternary climatic changes, which led to repeated episodes of aridity, soil loss and the mixing of wind-driven sand with the residual soil. The implication that the West Australian arable soils may have a quite different history from their counterparts in southeastern Australia is now being examined.