Soil Properties Affecting the Reductive Capacity of Volcanic Ash Soils in Korea

Volcanic ash soils or Andisols have distinct chemical and mineralogical properties. The unique chemical properties of Andisols are due to their Al-rich elemental composition, the highly reactive nature of their colloidal fractions, and their large surface area. The soils that developed from volcanic ash on Jeju Island, Korea, were classified as typical Andisols. The soils had an acidic pH, high water content, high organic matter, and clay-silty texture. The crystalline minerals in the samples were mainly ferromagnesian minerals, such as olivine and pyroxene, and iron oxides, such as magnetite and hematite derived from basaltic materials. A large amount of gibbsite was found in the subsurface horizon as a secondary product of the migration of excess Al. In addition, we found that considerable amounts of poorly ordered minerals like allophane and ferrihydrite were present in the Jeju soils. The SiO2 contents were lower than those of other soil orders, while the Al2O3 and Fe2O3 contents were higher. These results reflect some of the important chemical properties of Andisols. The chromium (VI/III) redox couple was used in the reductive capacity measurement. The mean reductive capacity of the Jeju soils was 6.53 mg/L reduced Cr(VI), which is 5.1 times higher than that of non-volcanic ash soils from inland Korea. The reductive capacity of the inland soils was correlated with the total carbon content. Such a high capacity for the reduction of soluble Cr(VI) must also be due to the relatively high carbon contents of the Jeju soils. Nevertheless, despite having 20 times higher total carbon contents, there was no correlation between the reductive capacity of the Jeju soils and the carbon content. These results imply that the reductive capacity of Jeju soils is not only controlled by the carbon content, but is also affected by other soil properties. Correlations of the reductive capacity with major elements showed that Al and Fe were closely connected to the reductive capacity of Jeju soils. In addition to the carbon content, other factors controlling the reductive capacity of Jeju soils may have a close relationship with the properties of andic soils, which contain considerable amounts of allophane and ferrihydrite. Andic soils offer distinct advantages regarding a high reductive capacity for Cr(VI), such as a very large surface area and unique adsorption characteristics. The adsorption of anions by Andisols results from chemisorption on `active' Al and Fe occurring in various forms, such as allophane, organometallic complexes, and ferrihydrite. In addition, since Andisols have a predominantly positive surface charge at soil pH, the soils can retain mobile anions, such as phosphate, nitrate, and chromate. The positive net charge is derived from the large amount of organic C and allophane. Therefore, the high reactive capacity of the Jeju soils suggests that the combined effects of a large number of electron donors, such as organic matter and Fe(II), and the adsorption characteristics of Andisols are complex and should be considered simultaneously when estimating the reductive capacity of andic soils.