3.1. Morphological soil properties
All surface soils in Anak Krakatau, Rakata, and Panjang are characterized by black (10 YR 2/1) to dark reddish-brown (5 YR 3/3) colours, and they are darker than those from Sebesi. The topsoil colour changed from darker to a lighter colour with depth. The dark soil surface originated from the volcanic materials of Anak Krakatau. It was estimated that the thickness of dark volcanic ejecta was 0.5-1 m when deposited on Rakata and Panjang islands. After 100 years, the colour of the surface soil in Rakata and Panjang was dark brown to very dark brown (10 YR 3/2-3/3) to dull yellowish brown to yellowish-orange (10 YR 5/4-6/3) in the subsoils (Hardjowigeno, 1992; Akio Shinagawa et al., 1986).
Soils of Anak Krakatau were mainly sand (sand content 75-80%), soils of Panjang were sand to sandy loam, Rakata soils were sandy loam to loam, while some of the subsoils of Sebesi already have a clay texture (Figure 4). The coarser texture of the soils at Anak Krakatau, Rakata, and Panjang are related to the initial stage conditions of the volcanic deposits of Anak Krakatau. The 1883 volcanic deposits were sandy and contained fine particles of pumice (R. Whittaker et al., 1992). Soil texture of the upper horizon of the Rakata soils was described as gravelly silt loam or sandy loam and gravelly sandy loam in the subsoil
All soils in Anak Krakatau, Rakata, and Panjang were structureless. Soils of Sebesi had developed a granular, weak, and fine soil structure. These results suggest that no clear structural development occurred in the Rakata and Panjang soils after 132 years (from 1883 to 2015) and after 85 years (from 1930 to 2015) in the Anak Krakatau soils. While in Sebesi, early development of soil structure seems to be related to the incorporation of organic matter. Similar findings were reported by (Hardjowigeno, 1992; A Shinagawa et al., 1992).
Based on the field observations and chemical analysis (Table 1), the soils of Anak Krakatau, Rakata, and Panjang were allocated to the family level of the US Soil Taxonomy: sandy, glassy, isohyperthermic, Vitrandic, Udorthent. This is similar to the one observed (Akio Shinagawa et al., 1986), who classified the soils of Anak Krakatau and Rakata as Typic Troporthents.
The profiles at Sebesi already showed some development of andic soil properties (Al + ½ Fe > 2%, high P retention, see Table 1) and allocated to medial, amorphic, isohyperthermic Typic Hapludand.