Andisols are soils that have formed in volcanic ash or other volcanic ejecta. 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.

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.

Andisols are divided into 8 suborders: Aquands, Gelands, Cryands, Torrands, Xerands, Vitrands, Ustands, and Udands. Click here for more information about these suborders. Click here to view a map of their distribution in the US.