Andisols
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In USA soil taxonomy, Andisols are soils formed in volcanic ash and defined as soils containing high proportions of glass and amorphous colloidal materials, including allophane, imogolite, and ferrihydrite. In the FAO soil classification, Andisols are 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 can usually support intensive cropping, with areas used for wet rice in Java supporting some of the densest populations in the world. Other Andisol areas support crops of fruit, maize, tea, coffee or tobacco.
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.
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.
Andisols are divided into the following suborders:
- Aquands - Andisols with a water table at or near the surface for much of the year.
- Cryands - Andisols of cold climates.
- Torrands - Andisols of very dry climates.
- Ustands - Andisols of semiarid and subhumid climates.
- Udands - Andisols of humid climates.
- Xerands - temperate Andisols with very dry summers and moist winters.
- Vitrands - relatively young Andisols that are coarse-textured and dominated by glass.
