Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel. 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
Andisols: Soils with Andic Soil Properties
Andisols are soils with properties dominated by short-range-order compounds such as aluminosilicates, ferrihydrite, and organometallic complexes. The vast majority of Andisols formed from volcanic tephra (ash, pumice, cinders) and related volcanic parent materials, although a few have formed from nonvolcanic materials. Formally adopted as the 11th soil order in 1990 (Soil Survey Staff, 1990), Andisols represent many soils known by other names, including Ando soils, Kurobokudo, Andosols, and Volcanic Ash Soils. Although the definition does not precisely match the definitions of these other groups of soils, Andisols are closely related to Andosols, one of the 32 reference soil groups in the World Reference Base for Soil Resources (IUSS Working Group WRB 2006). In the first edition of Soil Taxonomy, Andisols were mostly identified as Andepts and Andaquepts (Inceptisols), a placement that recognized the properties resulting from minimal crystallization and redistribution of weathering products.
SettingThe primary control of Andisol characteristics is volcanic parent material, especially ash. Volcanic ash refers to the
250 Soil Genesis and Classification
(McDaniel et al. 2010). Small areas of The Rift Valley of Africa, as well as Italy, France, Spain, and Romania also have important areas of Andisols.
All soil moisture regimes and all soil temperature regimes are found in Andisols. However, limited water for hydrolytic reactions in extremely arid regions prevents the weathering of volcanic ash, and hence the formation of Andisols. Vegetation is also very diverse, ranging from desert shrubs in arid regions, to dense coniferous forests in humid regions, to tundra in cold regions of higher latitudes and elevations. The asso-ciation of Andisols with volcanic activity also dictates that they are frequently on steep mountain slopes at higher elevations. However, forming from airborne material, they can be found on any terrain, including floodplains where the volcanic material is water deposited.
Pedogenic ProcessesMany of our usual concepts of soil formation have to be modified somewhat when considering the genesis of Andisols. The most noticeable difference is that the youngest and sometimes least-weathered part of the soil is the surface layer or horizon. Many Andisols are formed in unconsolidated volcanic ejecta originating from sequential eruptions of one or more volcanoes. The result is a distinctly layered soil profile with buried soil horizons (Figure 9.1). In quiet periods between eruptions, weathering and other soil-forming processes proceed in a top-down manner, giving rise to a sequence of soil horizons. A subsequent eruption can then cover the land surface with fresh tephra. In cases where tephra accumulates rapidly, existing soil horizons may be deeply buried and effectively isolated from further pedogenesis as soil formation resumes at the new land surface. This scenario is known as retardant upbuilding (Schaetzl and Anderson 2005) and gives rise to soils such as the one shown in Figure 9.1. Alternatively, if subsequent eruptions produce relatively thin additions of new tephra and accumulation rates are low, developmental upbuilding continues (Schaetzl and Anderson 2005). As new tephra is added to the soil profile, it is exposed to pedogenesis before burial. This results in the intermixed tephra deposits having a soil fabric inherited from when the tephra was part of the A or B horizon (McDaniel et al. 2010). In short, understanding Andisol genesis in many instances requires a stratigraphic approach in combination with an appreciation of buried soil horizons and polygenesis.
The nature of ash falls varies greatly both in time and space. Volcanic events can be of short duration or persist as a continuous eruption over several years. The size of particles deposited at a site may one day be coarse and the next fine, depend-ing upon the direction and speed of the wind. Some eruptions completely bury both soil andvegetation, whereas many and perhaps most ash falls only partially bury the vegetation, which continues to grow as the ash accumulates. Materials of contrasting composition are common from the same volcano, even during a single eruption.
Buol_c09.indd 250Buol_c09.indd 250 7/1/2011 12:41:49 PM7/1/2011 12:41:49 PM
9 / Andisols: Soils with Andic Soil Properties 251
Volcanic ash is mineralogically different from most other parent materials. Rapid cooling of the molten materials upon ejection prevents crystallization of min-erals with long-range atomic order, and the resulting product is known as volcanic glass or vitric material (Figure 9.2). Although the bulk chemical composition may be similar to that of other rocks with well-defined mineral constituents (e.g., rhyolitic, andesitic, basaltic), discrete minerals do not exist. As a result, volcanic glass is more weatherable than crystalline materials with the same bulk composition. Volcanic eruptions that produce significant amounts of ash are usually explosive due to the high silica content of the magma and a large component of gases (water, carbon dioxide, hydrogen sulfide). The explosive eruptions consist not only of glass formed by the cooling of magma, but also of fragments of rocks, often hydrothermally altered, from the throat of the volcano. Many of these lithic fragments may be coated by glass. The resulting ash fall, often called tephra deposits or pyroclastic material (approximate synonyms), therefore may be a mixture of crystalline minerals and volcanic glass.
The glassy materials that dominate the coarser fractions of many Andisols weather relatively rapidly to form colloidal materials that possess short-range order. Once
Figure 9.1. Typic Udivitrand from central North Island of New Zealand. Exposure shows sequence of buried soil horizons formed in multiple tephra deposits. Stratigraphy, tephra unit ages (left side of figure), and horizon designations are from Lowe (2008). Scale divisions = 10 cm.
Buol_c09.indd 251Buol_c09.indd 251 7/1/2011 12:41:49 PM7/1/2011 12:41:49 PM
252 Soil Genesis and Classification
thought to be amorphous, these materials comprise very small but structured nanominerals (Hochella 2008). The predominant weathering products include allophane, imogolite, ferrihydrite, and metal-humus complexes. These minerals differ markedly from those of most other mineral soils and confer the unique properties associated with Andisols. Even a fundamental soil property such as texture takes on a new meaning, as many of the weathering products, especially in moister environments, tend to exist as gels rather than discrete clay-size particles. This, coupled with high variable charge, makes complete dispersion of these materials difficult. Not surprising is the fact that laboratory particle-size analyses generally indicate less clay-size material than do field estimates (Ping et al. 1989).
The suite of weathering products found in Andisols depends to a large extent on the leaching regime, the acidity of the weathering environment, the supply of organic acids, and the presence of 2:1 layer silicates. Shoji et al. (1993) and Dahlgren et al. (2004) provide extensive discussions of the weathering environments and processes. Brief descriptions of two of the most common weathering scenarios are presented here.
As glass and other minerals weather via dissolution and hydrolysis by carbonic acid, aluminum and silicon are released. Under conditions where soil pH >5 or when organic matter production is relatively low, aluminum and silicon polymerize and precipitate to form the short-range-order aluminosilicates allophane and imogolite in surface horizons. Iron may also precipitate as ferrihydrite, a short-range-order oxyhydroxide (Bigham et al. 2002). These weathering products are characteristic of allophanic Andisols (Figure 9.3).
Figure 9.2. Scanning electron micrograph of a glass shard from the eruption of Mount Mazama (nowCrater Lake, OR) approximately 7,700 years ago. The vesicles seen in the glass indicate a highly explosive eruption of viscous magma. Today, this glass blankets most of the forested regions of the Pacific Northwest. (Image courtesy of University of Idaho.)
Buol_c09.indd 252Buol_c09.indd 252 7/1/2011 12:41:50 PM7/1/2011 12:41:50 PM
9 / Andisols: Soils with Andic Soil Properties 253
Although once considered amorphous, allophane consists of hollow spheres with diameters of 3.55 nm (Parfitt 2009) (Figure 9.4A). Imogolite has a somewhat simi-lar chemical composition but appears as long thread-like tubes with inner and outer diameters of 1 and 2 nm, respectively (Churchman 2000) (Figure 9.4B). In some cases, poorly crystalline smectites appear to be the initial weathering products, which subsequently are desilicated to form allophane and imogolite (Southard and Southard 1989). Over time, dehydration and structural rearrangement of allophane and imogolite may lead to halloysite formation, especially if desilication is not too severe. Halloysite formation is favored under conditions of lower rainfall, Si-rich parent materials, and restricted drainage (Lowe 1986; Churchman 2000). Progressive desil-ication may ultimately produce gibbsite in some Andisols, although the horizons that contain gibbsite may not have andic properties. Seasonal dryness, as in ustic and xeric soil moisture regimes, seems to speed the formation of the crystalline clays, while perudic soil moisture regimes favor the persistence of the short-range-order compounds.
A second mineral weathering scenario occurs under more acidic conditions when soil pH is
254 Soil Genesis and Classification
the aluminum-humus complexes accumulate in surface horizons. Note the similarity of this process to that discussed in Chapter 17 regarding Spodosols. These Andisols are referred to as nonallophanic because they are dominated by organic rather than inorganic short-range-order compounds (Figure 9.5). If 2:1 layer silicate minerals arepresent in the parent material, the aluminum may also precipitate as hydroxy- interlayer islands. Under these conditions, desilication is a dominant process in the surface and upper subsurface horizons. Opaline silica accumulations are common, especially in semiarid climates.
Andisols generally have higher organic matter contents than do other mineral soils in similar environments. Many researchers have demonstrated positive correla-tions between organic matter and allophane, imogolite, and ferrihydrite. Interactions between these components appear to result in organic matter stabilization (Dahlgren etal. 2004). Organic matter may sorb to these short-range-order minerals, thereby decreasing mineralization rates. Iron and aluminum are also able to bind with humic substances, creating complexes that are very resistant to degradation and leaching (Hiradate 2004). Active aluminum associated with Al-humus complexes can be toxicto many microorganisms, thereby inhibiting decomposition and increasing the residence time of the organic fraction.
Figure 9.4. Micrographs of (A) allophane spherules and (B) imogolite threads from an Icelandic Andisol (Haplocryand). (Courtesy Geoderma and modified from Wada et al. 1992, Clay minerals of four soils formed in eolian and tephra materials in Iceland, Geoderma 52, p. 359, with permission from Elsevier)
Buol_c09.indd 254Buol_c09.indd 254 7/1/2011 12:41:51 PM7/1/2011 12:41:51 PM
9 / Andisols: Soils with Andic Soil Properties 255
Andisols may be pedogenically linked with soils of other orders. Soils formed in fresh, unweathered volcanic ejecta have not undergone sufficient weathering to develop andic properties and are classified as Entisols. Dahlgren et al. (1997) estimated that approximately 200300 years of weathering are required for silicic tephra to attain andic soil properties under cryic-udic conditions. Other pedogenic linkages may also occur. In cool, moist environments, Fe and Al derived from weathering of volcanic ash can form organometallic complexes that may then be translocated to form spodic horizons. Spodosols in tephra deposits have been reported in Japan (Shoji et al. 1988), New Zealand (Parfitt and Saigusa 1985), the Pacific Northwest region of the United States (Dahlgren and Ugolini 1991; McDaniel et al. 1993), and Alaska (Ping et al. 1989). Under warm, moist conditions and with-out significant additions of fresh volcanic ejecta, Andisols may eventually develop intosoils of other orders as meta-stable weathering products are transformed into crystalline minerals. As an example, both Ultisols and Oxisols have formed from Andisols under humid, tropical conditions in Costa Rica (Martini 1976; Nieuwenhuyse etal. 2000) and Hawaii (Chadwick et al. 1999). Transformation of Andisols to Inceptisols and Alfisols has also been reported in the literature (Dahlgren et al. 2004).
Selected properties of representative Andisols are presented in Table 9.1. The Typic Vitrixerand has formed in 7,700-year-old tephra under xeric/frigid regimes, and represents a relatively early stage of Andisol development. In contrast, the Acrudoxic
Figure 9.5. Melanudand from Costa Rica. Soil has a melanic epipedon and is an example of a nonallophanic Andisol in which organo-metal compounds dominate the colloidal fraction. Scale divisions are decimeters. For color detail, please see color plate section.
Buol_c09.indd 255Buol_c09.indd 255 7/1/2011 12:41:52 PM7/1/2011 12:41:52 PM