Acid sulfate soils are predominantly near-coastal soils and sediments formed in the last 10 000 y that contain sedimentary pyrite (FeS2). The presence of appreciable amounts of pyrite (or other iron sulfides) in these soils confers upon them certain characteristic properties. When the pyrite in these soils (which was formed in a chemically reducing and anaerobic environment) is exposed to oxygen it oxidises, generating considerable quantities of sulfuric acid. This exposure to oxygen can occur naturally, but more commonly it is anthropogenic—a result of drainage/lowering of the water table for reclamation (when these soils are under water), cropping (e.g. sugar cane) or construction (e.g. canal development). In the absence of sufficient acid neutralising capacity, these soils can become highly acidic (pH <4) and export copious quantities of iron and toxic aluminium to water bodies—with
disastrous environmental consequences.
Acid sulfate soils are notoriously variable, both spatially and temporally, with their management critically dependent on chemical analysis. The ultimate goal of chemical analysis is to be able to predict the net acid export from these soils into the environment. However, the extent and rate of pyrite oxidation in ASS and the interaction of the acid generated with acid neutralising components in the soil is an extremely complex chemically, biologically and hydrologically mediated process that defies any accurate prediction. The best that can be expected of chemical analysis is to provide a realistic estimate of the maximum acid generation in soils and use this as the basis for calculating lime amelioration rates that will prevent any net acid export in the future.
Through carefully orchestrated laboratory experiments (and using incubation and leaching column experiments as validation tools) the
existing ASS chemical methods (which had only been developed to rudimentary levels) were evaluated and iteratively improved, culminating in the development of the suspension peroxide oxidation combined acidity and sulfur (SPOCAS) method. This method, combined with hydrochloric acid extraction, represents a comprehensive approach to the chemical analysis of ASS. It allows an estimation of the soil's pyrite sulfur content to be made, as well as quantifying actual and retained acidity. The pyrite content can be used to calculate the soil's 'sulfidic' acidity, by assuming the stoichiometric generation of 4 moles of acidity per mole of pyrite. Where a soil contains acid neutralising components (e.g. carbonate minerals, shell), this is reflected in the TSA (titratable sulfidic acidity) result from SPOCAS, which may be substantially less than the sulfidic acidity predicted from the soil's pyrite content. A diminished TSA result, coupled with appreciable quantities of calcium and magnesium
brought into solution by the peroxide digest of SPOCAS provides strong evidence for the presence of carbonate other calcium or magnesium containing alkaline soil minerals. Where a soil's acid neutralising capacity is greater than the potential acidity generated by the oxidation of sulfides, then a back-titration can be performed to quantify this excess. When ASS are to be disturbed (because of construction or land development), it can be established whether such soils need any further remedial action to be performed on them (e.g. liming) in order to comply with the requirements of regulatory authorities (i.e. local councils, state governments).
The effect of sample size, sample grinding, extraction ratio, volume of peroxide added, length and vigour of digestion, presence of CaCO3 and gypsum, titration in suspension, use of a peroxide decomposition catalyst, and the timing of KCl solution addition on acidity titration and sulfur results
were variously investigated. The final method developed overcame many of the difficulties associated with various previous peroxide-based methods, such as gypsum insolubility, incomplete pyrite oxidation and jarosite precipitation, although the method is still subject to interference from organic sources of acidity and sulfur in highly organic soils.
The methods developed and refined in this study provide a sound basis for the chemical analysis of ASS, allowing a net acid-base account to be constructed and enabling the calculation of appropriate liming amelioration rates that will ensure no net acid export from these soils in the future.