Geochemical fingerprinting of volcanic soils used for wetland rice in West Sumatra, Indonesia

Published in Geoderma Regional 10 (2017) 48-63.

Toward the end of 2016, Prof. Dian Fiantis (Department of Soil Science, Faculty of Agriculture, Universitas Andalas, Kampus Limau Manis) spent around 3 months in our soil science research group. With her in her kit bag, were a number of soil samples collected from various sites around a few different volcanoes situated in West Sumatra, Indonesia. Together with Budiman Minansy, Dian wanted to determine whether soils from different volcanoes had unique and identifiable geochemical fingerprints. We set about determining this using X-ray fluorescence technology and other soil analysis approaches to determine the concentration of the mineral elements of these soils.

This paper is the outcome of some of Dian’s research. There are more research outputs to come! Prof. Fiantis is a credit to her university, and was a joy to have around our research group in 2016.

Cultivation of paddy (rice) in volcanic soils is commonly practiced in West Sumatra, Indonesia. This study aims to provide a detailed geochemical fingerprinting of topsoils of paddy fields derived from volcanic materials in the vicinity of mount (Mt) Marapi, Mt. Sago, Mt. Singgalang, Mt. Tandikek and Caldera Maninjau in West Sumatra, Indonesia as a function of different geochronology of volcanic parent materials. Seventy-nine topsoil samples were collected along an altitudinal gradient ranging from 44 m in the Maninjau area to 1220 m above sea level (a.s.l.) at Mt. Singgalang. In addition to conventional physical and chemical analysis, geochemical analysis was carried out using a portable X-ray fluorescent spectrometer (XRF) and organic matter composition was analyzed using mid-infrared Fourier transform infrared spectroscopy (FTIR). The chemical composition of the volcanic paddy soils in this area is controlled by the origin of parent materials and weathering processes. Soils of Mt. Sago have lower soil pH (5.46) and smaller cation exchange capacity (CEC = 16.5 cmolc kg-1) compared to soils from the other three mountains. On the other hand, soils of Mt. Marapi have higher pH (6.05) and larger CEC (19.8 cmolc kg-1). Linear discriminant analysis revealed that the major geochemical elements in volcanic paddy soils can be ascribed to the different volcanic origin. The results of Mahalanobis distance statistics clearly separated soils of Mt. Sago with the other four soils. Soils from Mt. Marapi were also dissimilar with the other three soils, while soils from Mt. Singgalang-Tandikek and Maninjau were more related. Clear differentiation among weathering indices was also observed. Soils of Mt. Sago again showed higher weathering stages when evaluated using indices with immobile elements (Al2O3, Fe2O3, TiO2 and Zr). The following sequence of the degree of weathering can be concluded: Sago > Maninjau > Marapi > Singgalang-Tandikek. Soil analysis using FTIR revealed that labile aliphatic (CeH) compounds were the dominant organic matter fractions in these soils with abundances between 64 and 77%. Soils with total C less than 2% tend to be dominated by aromatic fractions, while soils greater than 2% C are dominated by the more labile aliphatic fractions. In conclusion, although the soils have been cultivated with paddy for hundreds of years, they still retain distinct geochemical signatures that can be revealed using a portable XRF.