An Overview of Andisols

An Overview of Andisols Lilly Sanovia Oglala Lakota College November, 2011

What Is An Andisol? • USDA soil taxonomy classifies an Andisol as a soil formed on pyroclastic parent materials. • Andisolscontain a significant percentage of volcanic glass and colloidal materials. • In the FAO soil classification, Andisols are called Andosols.

Occurrence • Andisols are found over less than 0.8% of the Earth’s surface. • 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 and imogolite Source: http://www.mindat.org/min-139.html Source: http://www.mindat.org/photo-143668.html

Parent Material • Parent materials include volcanic ash and other material ejected during volcanic explosion (pyroclastics) • 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 • Weathering of pyroclastics forms short-range-order compounds • 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. Source: http://webmineral.com/specimens/photos/Ferrihydrite.jpg

Time • 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 environments. • As a result, Andisols in arid environments tend to be older than Andisols in wetter environments.

Humification • Aluminosilicates stabilize humic acids, causing them to accumulate (humification). • This process causes the soil to darken on the surface (melanization). • These complexes of humic acids and aluminosilicates are not very soluble • 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% organic carbon. • 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

Epipedon • The epipedon is the top layer of a soil, commonly used for classification. • Andisols may have a histicepipedon (thin horizon of peat or muck) • Or a melanicepipedon(thick, black horizon) Source: http://soils.cals.uidaho.edu/soilorders/i/And_04b.jpg

AndisolSuborders • 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 volcanic glass) • Ustands – Form in sub-humid environments • Udants – Form in humid environments. These are the most common Andisols.

Andisol Distribution Map

References • 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: http://soils.cals.uidaho.edu/soilorders/andisols.htm • University of Wisconsin – Madison, Department of Soil Science: http://www.soils.wisc.edu/courses/SS325/soilorders.htm

Acknowledgements • Dr. HannanLaGarry • Oglala Lakota College, Math and Science Department

An Overview of Andisols