3.2.3 Cation exchange capacity (CEC) and exchangeable base cations
CEC values for Anak Krakatau and higher elevation profiles at Rakata,
Sebesi, and Panjang all displayed a low and uniform value (mean value of
1.61 cmolc kg-1). As soils developed,
the CEC values showed a decrease with soil depth (Figure 7). Soils of
Anak Krakatau had low CEC while soils of Sebesi and Rakata had higher
CEC values, 13.19 and 9.76 cmolc kg-1,
respectively. The highest CEC value in soils from Sebesi and Rakata
could be ascribed to the presence of organic matter and clay content
with a Pearson correlation value between CEC and OC of 0.72, and 0.31
with clay content.
The sum of exchangeable base cations varied from 0.3 to 13.67
cmolc kg-1 and soils of Sebesi and
Rakata were significantly different from those of Panjang and Anak
Krakatau (Figure S2 in Supplementary materials). The values decreased in
the following order Sebesi > Rakata > Panjang
> Anak Krakatau. Among the base cations, Ca and Mg
dominated, especially in the soils of Sebesi and Rakata. In Anak
Krakatau the values were low, less than 0.04 and 0.02
cmolc kg-1. This finding indicates
that Ca and Mg present on the exchange complex was initially low, as
observed in younger soils of Anak Krakatau compared to the soils of
Sebesi and Rakata. The soils on Panjang island occasionally received
volcanic ash from the Anak Krakatau.
The exchange complex of the studied soils was half-saturated with
exchangeable base cations, and base saturation decreased in the
following order
Rakata>Panjang>Sebesi>Anak
Krakatau. Base saturation values ranged from 37 to 57% in soils of Anak
Krakatau < Sebesi < Panjang < Rakata. The
base status of soils on Krakatau islands varied, as shown by the low to
the very high sum of exchangeable base cations (from 0.18 to 51.28
cmolc kg-1). Soils from Anak Krakatau
showed lower values than the soils of Rakata, Panjang, and Sertung
(Higashi et al., 1987). They also reported that the exchange complexes
were nearly saturated to over-saturated with exchangeable base cations
and decreased in the order
Rakata>Panjang>Sertung> Anak
Krakatau.