Bubble Bubble ...: Dryland Salinity -- the Notorious Consequence of Land Cover Change in Australia -- Has Two Potentially Devastating Understudies: Acid Sulfate Soils and Soil Acidification. Wendy Pyper and Steve Davidson Profile Research on These Lesser-Known Threats to Our Agricultural Heartlands

Article excerpt

Rising groundwater has brought more than the threat of salinity to inland Australia. In many parts of the country, the combination of water-logging, agriculture, and a rich geology of iron, sulfur and heavy metals, has led to the formation of acid sulfate soils.

Characterised by black bogs, slimy red ooze and scalded, infertile patches of earth, these soils have been reported in the Western Australian wheatbelt, South Australia, south-west Victoria, the Yass valley in New South Wales and south-east Queensland.

While these reports suggest that the distribution of acid sulfate soils across the country is widespread, the true extent, nature and severity of the problem are still unknown. This is because the discovery of inland acid sulfate soils is relatively recent, although the problem has been studied in coastal areas since the 1980s.

For farmers, this lack of knowledge means an inability to identify, prevent or manage acid sulfate soils on their land, resulting in erosion, acid and heavy metal run-off, and reduced water quality and crop yield.

CSIRO Land and Water pedologist, Dr Rob Fitzpatrick, is helping to reverse this trend through his work on acid sulfate soils in western Victoria and South Australia.

After a decade of research, Fitzpatrick and his CSIRO colleagues -- through the CRC for Soil and Land Management -- have developed manuals for identifying and managing saline and acid sulfate soils in the Dundas Tablelands and Adelaide's Mount Lofty Ranges. These manuals have been successfully field tested by community groups and agriculture advisers.

Fitzpatrick is also championing a national strategy for the Australia-wide mapping of acid sulfate soils.

`Little can be done in terms of long-term management strategies until the extent and location of acid sulfate soils is known at both catchment and property levels,' Fitzpatrick says. `This requires risk mapping on a national basis, followed by property assessments and environmental hazard assessments at a catchment level.'

Fitzpatrick and a University of South Australia colleague, David Bruce, have begun a more detailed mapping assessment of the Mount Lofty Ranges using radar data on soils, geology, terrain and moisture, (see story below), and plan to extend this effort into Western Australia and New South Wales.

Acid soil basics

Fitzpatrick says inland acid sulfate soils are commonly associated with dryland salinity, which is caused by rising saline groundwater.

In several parts of Australia, this groundwater is rich in sulfate, which' seeps up through the soil along with other ions including sodium, magnesium, arsenic, iodide and chloride, forming various mineral precipitates within and on top of the soil. If the soil is waterlogged, anaerobic bacteria use the sulfate to help degrade organic matter. This process produces an iron sulfur compound called pyrite.

Pyritic enriched soils or sediments are called `potential' acid sulfate soils because they have all the ingredients necessary to create environmental havoc.

Havoc is wrought when cattle, drainage works, or other disruptive forces breathe air on waterlogged soils and the pyrite within. When this happens, pyrite is oxidised to sulfuric acid and various iron sulfate-rich minerals. `Actual' acid sulfate soils form, and a chain of destruction follows.

`If waterlogged 'potential acid sulfate soils' are exposed to the air, sulfuric acid forms and the soil pH can drop from neutral (pH 7) to below 4,' Fitzpatrick says. `This dissolves the soil, causing trace elements, salt and metal ions such as iron and aluminium to be spewed out onto the soil surface and into stream waters.'

As the soil structure declines, it becomes clogged with clay and mineral precipitates, losing its permeability. This prevents the groundwater below from escaping and forces it to move sideways or upslope. …