Variability of soil characteristics around an active volcano, Mt. Mayon in the Philippines with special reference to the distance and direction from the crater

This study investigated soil characteristics around Mt. Mayon, an active stratovolcano in the Philippines, which might depend on the meteorological condition and distribution of volcanic material deposition varying with distance and direction from the crater. Soil profiles were prepared at four and six sites in different directions, on a circle of 6 km and 15 km away from the crater, respectively. Soil profile morphology was described and the soil samples collected from each horizon were analyzed for physicochemical properties including total element, oxalate, and dithionite-citrate extractable Si, Al, and Fe, and phosphate adsorption coefficient, as well as clay mineralogical composition by XRD analysis. In general, regarding the distance, the soils showed less weathered status at 6 km and moderately weathered status at 15 km. In terms of direction, the soils at 6 km were moist and shallow in the northeast and southeast sites but dry and deep in the northwest and southwest sites. Meanwhile, despite all being deep, the soils at 15 km were moist in the northeast and east sites and dry in the other sites. While all soils showed less acidic nature and high levels of basic cations irrespective of distance and direction, phosphate adsorption coefficient was varied widely depending on the contents of oxalate-extractable Al and dithionite-citrate extractable Fe, for which the soils containing the higher amount of the oxalate-extractable Al showed the higher values of the coefficient. Consequently, the soils at 6 km were classified into Aquandic Humudepts (northeast), Vitrandic Eutrudepts (southeast and southwest), and Typic Udivitrands (northwest) while the soils at 15 km were Vitrandic Eutrudepts (northeast), Duric Epiaquands (east), and Typic Hapludalfs (southeast, southwest, northwest, and San Miguel Island). Such differences in soil formation and characteristics around Mt. Mayon seemed to be ascribable to the different geographical setting such as decrease in the supply of new volcanic material depending on the distance from the crater and the impacts of precipitation and typhoons, of which amount and intensity are higher in the east side than the west side because of the east side facing to the Pacific Ocean.