Environmental Chemistry Letters
High‑resolution mapping of acid sulfate soils in Northern Australia
through predictive models
Elisabeth N. Bui
Received: 10 January 2018 / Accepted: 14 May 2018
© UK Crown 2018
Acid sulfate soils can form when pyrite-rich marshes are drained or tidal inﬂuence decreases, naturally or anthropogenically.
Often they are vegetated by mangroves, serve as important aquatic bird habitats, and harbor unique microbial biodiversity.
They limit coastal development potential. The current best map of Australian acid sulfate soils is based on expert interpre-
tation of soil surveys, most at 1:100,000 scale. In the current study, we attempted the ﬁrst predictive spatial modeling to
map acid sulfate soils and soil sulfur concentration on a 90-m grid. We use 34 national environmental 90-m grids and 614
observations on the presence/absence of acid sulfate soils to train a machine learning algorithm, random forests, to predict
the location of acid sulfate soils across three regions of Northern Australia. We use the same environmental data and 1315
measurements of total elemental sulfur at 60–80 cm depth across Australia to train another random forest model to predict
the spatial pattern of sulfur concentrations in the three regions. The model for acid sulfate soils is better than that for sulfur
but both related maps showed that high sulfur concentrations generally coincide with the predicted presence of acid sulfate
soils. This coincidence between the predicted occurrence of acid sulfate soils and sulfur concentrations conﬁrms the modeling
approach presented here was reliable and could be applied to map acid sulfate soils at high resolution globally.
Keywords Acid sulfate soil · Australia · Mapping · Predictive model · Random forest
Most organisms are adapted to living in a narrow range of
pH values. Most bacteria prefer pH around 7.0 (Lauber et al.
2009). Most cultivated plants prefer slightly acidic soils (pH
5.5–6.5) (Islam et al. 1980). Aquatic organisms prefer pH in
the range of 6.5–9 (Boyd and Tucker 1998). Acidity origi-
nating from acid sulfate soils poses serious problems for
shrimp farming in ponds along coastal zones (Gosavi et al.
2004; Mahmood and Saikat 1995). Most coastal subaqueous
soils contain sulﬁdic materials that, if dredged and deposited
in upland disposal areas, become exposed to oxidizing con-
ditions and give rise to active acid sulfate soils with sulfuric
horizons (Fanning et al. 2017). Dissolution and leaching of
heavy metals into the environment during the formation of
acid sulfate soils can result in ﬁsh kills, metal accumulation
in crops and cows’ milk; and under chronic exposure, organ
impairment, neurological and cognitive disorders, and death
(Abdu et al. 2017; Fältmarsch et al. 2008).
Active acid sulfate soils are mineral or organic soils that
have a pH
≤ 3.5, as measured in 1:1 soil/solution, in near
surface horizons; or they have the potential to become ultra-
acidic (with pH
≤ 3.5) when they contain sulﬁde miner-
als, such as pyrite, that can be oxidized to produce sulfuric
acid; or they have experienced such conditions in the past as
evidenced by the presence of jarosite (Fanning 2002). Acid
sulfate soils are generally more prevalent in saline and brack-
ish tidal swamps, marshes, and mangroves along low-lying
coastlines where sedimentation keeps pace with sea level
rise (Dent and Pons 1995), but such soils can occur inland
when sulﬁdic material is exposed to the air and is oxidized
(Fitzpatrick et al. 1996; Hall et al. 2006). Acid sulfate soils
harbor unique microbial communities (Abdu et al. 2017;
Gomes et al. 2011; Ling et al. 2015).
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s1031 1-018-0753-4) contains
supplementary material, which is available to authorized users.
* Elisabeth N. Bui
CSIRO Land and Water, GPO Box 1700, Canberra,
ACT 2601, Australia