Unique Properties of Volcanic Ash Soils

Abstract

　Various kinds of soils can be formed from volcanic ash depending on the individual set of soil forming factors at different sites. Among these soils, Andisols or Andosols show unique proper-ties mostly due to abundant noncrystalline materials such as allophane, imogolite, Al-humus complexes, ferrihydrite and so on. Highly porous structures made of aggregated noncrystalline Andisol materials have a light and fluffy nature, accommodating large amounts of both plant-available and hygroscopic water. They show many other unique physical properties different from other soils. Moreover, Andisols have unique chemical properties, including aluminum-rich elemental composition, large amount of humus accumulation in A horizons, variable charge characteristics, high phosphate retention capacity, high affinity for multi-valent cations, high KCl-extractable and water-soluble Al in nonallophanic Andisols at an acidic to weakly acidic pH range, and so on. Finally, topics related to biological activity in volcanic ash soils are discussed regarding P supply from apatite and K supply from biotite for crops in fresh volcanic ash, effects of KCl-extractable and water-soluble Al on soil-borne diseases, adsorption characteristics of viruses on Andisols and P tropistic root growth of Brassica plants in P-deficient Andisols.