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.