Andisols are soils that have formed in volcanic ash or other volcanic ejecta. Also known as Andosols. Because they are generally quite young, Andisols typically are very fertile except in cases where phosphorus is easily fixed (this sometimes occurs in the tropics). They differ from those of other orders in that they typically are dominated by glass and poorly crystalline colloidal materials such as allophane, imogolite, and ferrihydrite. As a result, Andisols have andic properties - unique chemical and physical properties that include high water-holding capacity and the ability to 'fix' (and make unavailable to plants) large quantities of phosphorus.
Most Andisols occur around the Pacific Ring of Fire, with the largest areas found in central Chile, Ecuador, Colombia, Mexico, the Pacific Northwest, Japan, Java and New Zealand's North Island. Other areas occur in the Great Rift Valley, Italy, Iceland and Hawaii.
Globally, Andisols are the least extensive soil order and only account for ~1% of the ice-free land area. They occupy ~1.7% of the US land area, including some very productive forests in the Pacific Northwest region
Fossil Andisols are known from areas far from present-day volcanic activity and have in some cases been dated as far back as the Precambrian 1.5 billion years ago.
Aquands - Andisols with a water table at or near the surface for much of the year. Occur in lower landscape positions and support forest or grass vegetation. Occur in areas of volcanic influence such as the Pacific Northwest USA. Common to have a dark colored surface horizon that meets requirements for a histic, umbric or mollic epipendon.
Cryands - Andisols of cold climates Cryalfs are moderatly extensive, forming in the western part of N. America, northeastern part Asia above the 49 N. latitude and in some high mountains south of that latitude. Most Cryalfs formed under a coniferous forest vegetation. Cryalfs in the USA generally formed in late-Pleistocend or Holocene deposits.
Torrands - Andisols of very dry climates. These soils are not extensive. Some occur in the western part of North America, some in Hawaii or other Pacific nations. Most of the soils formed under grassy or shrub vegetation. Torrands in the USA generally developed in late-Pleistocene or Holocene deposits.
Xerands -temperate Andisols with very dry summers and moist winters. Most Xerands in the USA are in Oregon, Washington, Idaho, and California. Most formed under coniferous forest vegetation. Some formed under grass or shrub vegetation. Most Xerands in the USA developed in late-Pleistocene or Holocene deposits.
Vitrands - relatively young Andisols that are coarse-textured and dominated by glass. Most Vitrands form near volcanos. In the USA, most of the Vitrands are in Oregon, Washington, and Idaho, forming mainly under coniferous forest vegetations in Holocene deposits.
Ustands - Andisols of semiarid and subhumid climates and relatively small extent. Mostly in Mexico, western USA, Pacific Islands and the eastern part of Africa. Locations in the USA include Hawaii, Arizona, and New Mexico. Most formed under grass, shrub or forest vegetation and in the USA generally developed in the late-Pleistocene or Holocene deposits.
Udands - Andisols of humid climates. These soils are moderatly extensive on the Pacific Rim, including Washington, Oregon and Hawaii in the USA. Most Udands formed under forest vegetation.