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.