Mineralogy, Chemical Composition, and Dissolution of Fresh Ash Eruption: New Potential Source of Nutrients

Abstract

Unusually huge amounts of fresh ashfall from the eruption of the Merapi volcano (occurring October–November 2010) resulted in severe deposition problems (1–29-cm thickness) on many agricultural lands. The objectives of the study were (i) to determine the mineralogy and elemental compositions of the fresh ash, (ii) to assess solvent-extracted cations and the composition of pristine ash leachates in water, and (iii) to anticipate the possibility of using fresh ash as a new source of nutrients. Mineralogical composition was determined by a polarization microscope and total elemental composition by x-ray fluorescence. Dissolved elements were measured from the solid phase (extracted by NH4AOc) and water from river stream flow and canals. Results showed that the mineralogical composition was relatively uniform and dominated by volcanic glass (49% on average) followed by labradorite (26%) and augite (13%), with trace amounts of bytownite, hypersthene, hornblende, and opaque. This indicates that the fresh ash contains mainly easily weatherable minerals. Total element contents consisted of macronutrients in the order of Ca >> Na > K > Mg > P > S and micronutrients of Zn, Fe, Mn, Cu, and Co. Findings from study of pristine ash leachates from river and canal waters, representing natural conditions, clearly demonstrated the high concentrations of base cations (Ca >> Na >> Mg > K) and anions (SO4 >> Cl > NO3 >> PO4). Base cations extracted by NH4OAc were in the order Ca >> Mg > Na > K and the base saturation was >100% (114–484%), confirming that cations in rivers and canals are from soluble salts. The high amounts of ions released from fresh volcanic ash indicated its potential as a source of nutrients.