AN OVERVIEW OF ANDISOLS
OGLALA LAKOTA COLLEGE
WHAT IS AN ANDISOL?
• USDA soil taxonomy classifies an Andisol as a soil
formed on pyroclastic parent materials.
• Andisols contain a significant percentage of
volcanic glass and colloidal materials.
• In the FAO soil classification, Andisols are called
• Andisols are found over less than 0.8% of the Earth’s
• Occur most commonly in the Pacific Ring of Fire
due to the volcanism present there.
• Largest occurrences in Chile, Ecuador, Columbia,
Mexico, Japan and Java.
TWO IMPORTANT PROPERTIES
• The presence of extractable aluminum and iron
• Short-range-order clay minerals such as allophane
Source: http://www.mindat.org/min-139.html Source: http://www.mindat.org/photo-143668.html
• Parent materials include volcanic ash and other
material ejected during volcanic explosion
• Volcanic ash consists of glassy materials with
detectable amounts of aluminum and silica
• Volcanic ash is highly soluble and amorphous
(no well-defined crystalline structure)
• Weathering of pyroclastics forms short-range-
• The most detectable of these in an Andisol
are allophane and imogolite
• Weathering of these parent materials also
produces other non-crystalline
aluminosilicates and ferrihydrite.
• Weathering of pyroclastic parent materials
occurs faster than most other parent materials.
• As a result, many Andisols are relatively young
• Weathering is much slower in arid
• As a result, Andisols in arid environments tend
to be older than Andisols in wetter
• Aluminosilicates stabilize humic
acids, causing them to
• This process causes the soil to
darken on the surface
• These complexes of humic acids
and aluminosilicates are not very
• As a result, they tend to
accumulate on the soil surface,
causing a thick, dark horizon
CLASSIFICATION OF ANDISOLS
• Andisols are classified as having less than 25%
• Must also meet one or more of these conditions:
• Extractable percentage of aluminum/iron of 2% or more
• A bulk density of 0.9 g/cm3 or less.
• Phosphate retention 85% or more.
• The E horizon, if present, will consist of volcanic ash
• The B horizon contains SiO2
• Beaded if there is an E horizon
• As silcrete if there is an E horizon
• The epipedon is the top
layer of a soil, commonly
used for classification.
• Andisols may have a histic
epipedon (thin horizon of
peat or muck)
• Or a melanic epipedon
(thick, black horizon)
• Aquands – Form in low-lying areas where the water
table is at or near the soil surface
• Cryands – Form in very cold environments
• Torrands – Form in arid environments
• Xerands – Form in very dry environments
• Vitrands – Young, coarse-textured Andisols with low
water-holding capacity (due to high presence of
• Ustands – Form in sub-humid environments
• Udants – Form in humid environments. These are the
most common Andisols.
ANDISOL DISTRIBUTION MAP
• Soils of the Past: An Introduction to Paleopedology,
Gregory J. Retallack, Blackwell Science Ltd., 2001
• The Twelve Soil Orders, University of Idaho, College
of Agricultural and Life Sciences, accessed on
November 16, 2011:
• University of Wisconsin – Madison, Department of
• Dr. Hannan LaGarry
• Oglala Lakota College, Math and Science