Soil Development in Volcanic Ash

抄録

　Volcanoes are revered and feared for their awesome and devastating eruptions that often obliterate terrestrial ecosystems and bury landscapes with volcanic ash. Yet from these ashes of devastation arise some of the most productive soils in the world with the capacity to sustain high human population densities. This paper presents an overview of soil developmental processes occurring in soils derived from volcanic materials. We examine the genesis of six volcanic soils placed in a developmental sequence that involves climate and time. Climate, in conjunction with its influence on vegetation, and time of exposure to weathering are the two primary factors regulating soil development pathways in volcanic materials. Soils formed in volcanic ejecta have many distinctive morphological, physical and chemical properties that are rarely found in soils derived from other parent materials. These distinctive properties are largely due to the formation of noncrystalline materials (i.e., allophane, imogolite, ferrihydrite, Al/Fe-humus complexes) and the accumulation of organic carbon, the two dominant pedogenic processes occurring in volcanic soils. Formation of noncrystalline materials is directly related to the properties of volcanic ejecta as a parent material, namely the rapid weathering of the glassy particles. Andisols generally form rapidly in humid climates and alter to other soil orders as soil age and the degree of weathering increase. In regions with intermittent deposition of volcanic ash, each addition of new material rejuvenates soil developmental processes so that Andisols may be maintained as a relatively stable soil condition.