Mount Sinabung was active again in 2010 after 400 years of dormancy and it is still erupting to date. The eruptions produced volcanic ash which blanket the soil surface. The distribution of volcanic materials was monitored by using Landsat Satellite which covers an area of 30, 320 Ha in 2018, then in 2019, it changed to 1, 371 Ha. After eruptions for 10 years, we believed soil characteristics are changed. The objective of this study was to characterize soil properties in the vicinity of Mt. Sinabung. There were 34 soil samples taken at a depth of 0-20 cm with a total research area of 4, 517.25 Ha. The samples were analyze for chemical properties included pH (H2O), total N, organic carbon, exchangeable base cations, and cation exchange capacity (CEC). The results showed that the pH (H2O) ranged from very acidic to neutral (4.14-6.52). The total N low to high category (0.13-0.60%). Organic carbon low to very high (1.73-13.05%). The exchangeable base cations have a high concentration with values of K (1.60-2.98 cmol kg−1), Na (3.72-7.45 cmol kg−1) and Mg (5, 79-12.15 cmol kg−1) respectively. Cation exchange capacity (CEC) showed high to very high category values (11.78-97.71 cmol kg−1). Volcanic ash provides many benefits to soil properties. This indicates that volcanic ash enrich soils by providing nutrients to the soils aftermath of volcanic eruptions.

Volcanic activity produces pyroclastic deposits when erupted and cover the surrounding area. The minerals contained in these deposits are the source of plant nutrients. The volcanic deposits weathered, release nutrients to the environment, and improve soil chemical properties. The eruption of Mt. Sinabung in 2018 covered an area of 30, 320 ha, while in 2019 was 1, 371 ha. The study aims to investigate the status of nutrient content and the volcanic ash weathering level in 2020. There were 16 samples taken from ash deposits at various depths, with a total area of 1, 585.31 ha. Samples were analyzed to determine the total elemental composition using X-ray fluorescence (XRF) spectrometer, nutrient reserves, and weathering indices. The results showed that the total elemental composition of SiO2 is 51.51-67.51% classified as mafic (basalt) to felsic (dacite) materials, Al2O3; 15.54-23.41%, Fe2O3; 2.84-10.02% and CaO; 3.94-6.46%. Mount Sinabung’s volcanic ash has a nutrient reserve capacity of MgO, CaO, P2O5, K2O, and SO3, respectively with the amount of 37, 384.17 kg/ha, 235, 794.99 kg/ha, 34, 293.12 kg/ha, 72, 357.39 kg/ha, and 70, 352.22 kg/ha. The weathering indices of volcanic ash of 2020 were determined with a value of 2.76-4.19 for Ruxton ratio and Product of Weathering Index (PWI) of 67.39-76.13, indicates the weathering rate of silicates from volcanic ash are still at initial stage and are still in the fresh condition.

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