Identifying acid sulfate soils
Accurate and early identification is integral to developing sound strategies to manage acid sulfate soils (ASS) ‘hot spots’. When confirmed by laboratory analysis, it is an excellent basis on which to develop and adopt best management practices for sites underlain by ASS. It is also most likely to reduce costs associated with unnecessary disturbance.
Potential acid sulfate soils
Potential acid sulfate soils (PASS) are soils containing iron sulfides (commonly pyrite) which have the potential to produce sulfuric acid if they are drained or excavated. Preliminary identification can be carried out using the field test for peroxide oxidised pH (pHFOX), and confirmed by laboratory analyses using the suspension peroxide oxidation combined acidity and sulfate method (SPOCAS), or the chromium reducible sulfur method (SCR).
Actual acid sulfate soils
Actual acid sulfate soils (AASS) have already undergone oxidation to produce acid, resulting in a soil pH of less than 4. They also often exhibit a yellow and/or red mottling in the soil profile. If these soils still contain sulfides, they have the potential to produce more acid if allowed to oxidise further. This would be identified by a further decrease in pH reflected in the pHFOX. AASS should also be confirmed by laboratory analyses.
Collective evidence
Generally, the more indicators there are at a site, the more confidence there can be in a positive identification of ASS. Some indicators are more conclusive than others, with jarosite being one of the most conclusive.
Though factors other than ASS may be the sole or partial cause of individual indicators and environmental effects, ASS should always be considered in coastal areas below five metres AHD, and each site should be assessed on an individual basis.
Laboratory analysis is the most accurate indicator of ASS, and results can be used to determine the net acidity (existing plus potential acidity), from which liming rates can be calculated.
Last updated: 23 February 2009