The diversity of volcanic soils: focusing on the function of aluminum–humus complexes

Andosols (or Andisols) are the typical soils developed from volcanoclastic materials. They possess several distinctive properties that are rarely found in other groups of soils. These properties are largely due to the dominance of short-range ordered minerals (allophane, imogolite, and ferrihydrite) and/or mineral–humus complexes (Al/Fe–humus complexes). In this review, I survey the diversity of volcanic soils mainly from the viewpoint of the function of Al–humus complexes. Specific topics include the role of Al–humus complexes in organic carbon accumulation, Al solubility and kinetics, Al phytotoxicity to plants, and the suppression of soil-borne diseases. It is shown that Al–humus complexes are heavily involved in the diversity of these properties of volcanic soils.