During a survey conducted in April 2015 ten representative soil profiles
were selected: 1 soil profile on Mt. Anak Krakatau (elevation 60 m
a.s.l), 2 soil profiles on Rakata (elevation 2 and 50 m a.s.l), 2 soil
profiles on Panjang (elevation 2 and 48 m a.s.l) and 5 soil profiles on
Sebesi islands (elevation 66, 156, 250, 356, and 455 m a.s.l). The
location of the samplings is given in Figure 1. As the Anak Krakatau,
Rakata, and Panjang islands do not have roads and a national park,
sampling was done in the best possible manner to select a typical
representative soil in the area. At Anak Krakatau, loose materials
covered the entire island, and thus only 1 profile was be excavated. At
Rakata and Panjang islands, the soils are loose sandy materials, and
thus one profile near the coast and one at a higher elevation were
excavated. Sebesi island is more developed with vegetation, and
therefore the sampling locations represent a transect of elevation.
The soil morphological features were observed from soil profiles in the
field, and soils were sampled by collecting approximately 3 kg from each
soil horizon and transferred into polyethylene plastic bags for
analyses. Undisturbed soil samples were obtained using a standard ring
sampler with a known volume to determine soil bulk density. The samples
were then brought to the laboratory, air-dried, ground homogenously, and
sieved to a size fraction smaller than 2 mm.
The particle sizes of ash and soil samples were determined by sieving
and pipette method (Staff, 1996). Silt- and clay-sized fractions were
measured after sedimentation, according to Stokes law. Soil pH was
measured in H2O and 1 M KCl at a solution ratio of 1:5
after 30 min of equilibration. The Walkley and Black wet oxidation
method was used to determine organic C content, and total N was
determined by the Kjeldahl method (Tan, 2005). The available phosphate
anion was analyzed colorimetrically using a visible-light
spectrophotometer (Tan, 2005). P retention was determined following the
method described by Blakemore, Searle, and Daly (1987). Exchangeable
cations and cation exchange capacity (CEC) were determined by 1 N
NH4OAc, pH 7.0, extraction; the leachate was used to
determine the exchangeable base cations, which were measured by atomic
absorption spectrophotometry (AAS). Saturation of individual base
cations was calculated by dividing the content of individual
exchangeable cations by the effective CEC and expressed as a percentage
(Anda, 2012). Extraction with acid ammonium oxalate for the dissolution
of non-crystalline materials and metal-organic complexes was carried out
as outlined by Van Reeuwijk (2002). Contents of acid-ammonium
oxalate-extractable Si, Al, and Fe (Sio,
Alo, and Feo) were measured by AAS.
The elemental composition of the samples was determined using a benchtop
X-ray fluorescent spectrometer (XRF) PANalytical Epsilon 3, with
120-second readings with three replicates.