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.