• Vegetation: variety of vegetation types
  • Climate: all soil temperature regimes, except pergelic
  • Soil moisture regime: all soil moisture regimes
  • Major soil property: andic soil properties (low bulk density, oxalate extractable aluminum and iron, short-range-order minerals compounds - amorphous material, high phosphate sorption capacity) related to volcanic origin of materials.
  • Diagnostic horizons: cambic
  • Epipedon: histic, melanic
  • Major processes: weathering, humification, melanization, leaching, P-fixation

Andisols- Environmental Conditions
Climate: Andisols form in all soil moisture and all soil temperature regimes, except pergelic. Formation of Andisols in arid regions is limited because of slow weathering of volcanic parent materials.  
Vegetation: Andisols develop under a variety of vegetation types ranging from coniferous and deciduous forest, tundra, to shrubs.  
Relief: Andisols are found on any topography, however, often they occur on steep slopes formed by volcanic activity.  
Parent Material: The vast majority of Andisols formed from pyroclastic deposits (volcanic ejecta) such as ash, pumice, cinders, and lava. Volcanic terrains have a greater variety of rock-types than other surface environment on earth. These terrains include lavas, pyroclastic deposits (from explosions), and deposits from a wide range of sedimentary processes that occur in volcanic terrains. The nature of volcanic material ejected from a volcano varies greatly in time and space and determines the size of particles, composition of materials, and depth of volcanic material deposited. Rapid cooling of the molten materials upon ejection prevents crystallization of minerals with long range atomic order, and the resulting product is vitric material or volcanic glass, which are dominated by amorphous, short-range-order minerals.  
Time: Because volcanoclastic material is more weatherable than crystalline materials Andisols do not need very long time periods to form.    


Andisols - Processes
Volcanic ash is chemically/mineralogically distinct from most other soil parent materials. It is composed largely of vitric or glassy materials containing varying amounts of Al and Si. Volcanic glass lacks a well-defined crystal structure (i.e., amorphous) and is quite soluble. Environmental conditions, notably vegetation and soil moisture regime together with chemical composition (Al:Si ratio, base status, pH etc.) strongly influence weathering pathways of volcanic glass.

Allophane and imogolite are common early-stage residual weathering products of volcanic glass and both have poorly-ordered structures. Allophane forms inside glass fragments where Si concentration and pH are high and has a characteristic spherule shape. Imogolite tends to form on the exterior of glass fragments under conditions of lower pH and Si concentration, and has a characteristic thread-like morphology. Both allophane and imogolite may complex with organic matter. In some instances, where organic matter is rapidly accumulating, neither allophane or imogolite form in large amounts. Instead, opaline silica and Al-humus complexes are formed, which appear to inhibit allophane and imogolite formation.  

Allophane, imogolite and humus complexes are generally transformed under leaching conditions. In Si-rich environments, halloysite formation is favored, under more basic conditions gibbsite is favored. In non-allophanic ashes 2:1 clays occur although their pathways of formation are not well-defined. Soil moisture regimes influence transformation rates - crystalline clay formation is favored under regimes that include dry seasons (e.g., ustic and drier) and moist regimes (udic) favor persistence of amorphous complexes.  

The weathering products such as Al, Fe, and non-crystalline aluminosilicates stabilize humic substances and render them recalcitrant to decomposition, i.e., humic acids are accumulated (humification). Al, Fe-humus complexes are only sparingly soluble and therefore they accumulate at the surface, forming dark thick surface horizon especially under grass vegetation and humid climate (histic or melanic epipedons). The formation of Al, Fe-humus complexes is associated with a change in soil color (black color -organic matter), which is called melanization. Leaching of base cations is associated with the free drainage of many Andisols, i.e., percolating water leaches the cations out of the soil.  

A characteristic of Andisols is their tendency to fix phosphate in a plant-unavailable form. The highest P fixation is found in those Andisols that are fine textured and have relatively high Al/Si ratios. The phosphate is apparently bound by the aluminum via an anion exchange for hydroxyl.    


Andisols - Properties
Andisols are dominated by short-range-order compounds (e.g. allophane, imogolite), including organo-metallic complexes, ferrihydrite, and aluminosilicates, that formed largely in situ.A typical soil profile show a thick, dark-colored, greasy mineral horizon (e.g. melanic epipedon), a weakly developed cambic subsurface horizon (Bw), and relatively unaltered volcanic or volcanoclastic parent material (C). Histic or melanic epipedons are common in Andisols. A melanic epipedon has to be 30-cm or thicker with a black color and a histic epipedon requires more than 12 % to 18 % organic carbon, depending on clay content. Typically, Fe-Al-humus complexes are found in the A horizon, whereas short-range-order minerals are found in the Bw horizon. In general, the pH-functional cation exchange capacity (CEC) is high, due to a high surface area of the mineral and organic compounds in Andisols.  The %-base saturation is often low because of high percolation and leaching of cations in many Andisols. Physical soil properties of Andisols comprise a low bulk density, high macroporosity with rapid drainage at low soil moisture tensions, and weak mechanical strength. When they are dry Andisols are highly susceptible to wind erosion.    


Andisols - Classification
To qualify for an Andisol a soil have to have andic soil properties in 60 % or more of the thickness of soil material within 60 cm of the mineral soil surface, or on the top of an organic layer with andic properties. Andic soil materials contain less than 25 % organic carbon (by weight) and, in the fine-earth fraction (> 2 mm), meet one or both of the following:

  • Al plus 1/2 Fe extractable % (by ammonium oxalate - amorphous phases) totals 2% or more
  • A bulk density, measured at 33 kPa water of 0.9 g/cm 3 or less,
  • Phosphate retention of 85% or more.

In cases where the particle size is composed of 30% or more particles in the 0.02 to 2.00 mm fraction, the limits listed above are modified to account for less of an active amorphous component in the soil and thus lower limits on P-adsorption and amounts of amorphous Al/Fe.  

There are 7 different suborders in the Andisol order distinguished by soil moisture regime, water holding capacity, or organic matter content:
Aquands: Aquands are Andisols that have a histic epipedon or have aquic conditions which result in redoximorphic features. Aquands occur locally in depressions and along floodplains where water tables are at or near the soil surface for at least part of the year.  

Cryands: They are defined as Andisols with cryic soil temperature regimes. These soils are the Andisols of high latitude (e.g. Alaska, Kamchatka) and high altitude (e.g. Sierra Nevada in the U.S.).   Torrands: They are defined as Andisols with aridic soil moisture regimes. Vegetation is mostly desert shrubs.  

Xerands: They are defined as Andisols with xeric soil moisture regimes.  

Vitrands: They are Andisols that have a low water-holding capacity. Vitrands are restricted to ustic and udic soil moisture regimes.

Ustands: They are defined as Andisols with ustic soil moisture regimes. These are the Andisols of the intertropical regions that experience seasonal precipitation distribution.  

Udants: They are defined as Andisols with udic soil moisture regimes (most extensive Andisols).  

Shallow Andisols that have a lithic contact within 50 cm either of the mineral soil surface, or of the top of an organic layer with andic soil properties, whichever is shallower are denoted 'Lithic' (e.g. Lithic Cryaquands, Lithic Haploxerands).   Andisols with very low base status (that have extractable bases plus KCl-extractable Al3+ totaling less than 2.0 cmol(+)/kg in the fine-earth fraction) are named 'Acrudoxic' (e.g. Acrudoxic Placudands), low base status soils that have more than 2.0 cmol(+)/kg Al3+ (by KCl) in the fine-earth fraction are named 'Alic' (e.g. Alic Epiaquands), and Andisols that have extractable bases plus KCl-extractable Al3+ totaling less than 15.0 cmol(+)/kg are labeled 'Dystric' (e.g. Dystric Haplustand), whereas Andisols with high base status (that have a sum of extractable bases of more than 25.0 cmol(+)/kg in the fine-earth fraction) are named 'Eutric' (e.g. Eutric Placudands).  

Soil moisture regime is used to distinguish Andisols at the great group and subgroup level: xeric (e.g. Xeric Vitricryands), ustic (e.g. Ustivitrands), udic (e.g. Udivitrands), aquic (e.g. Aquic Ustivitrands), and 'oxyaquic', i.e., soils that are saturated with water, in one or more layers within 100 cm of the mineral soil surface, for 1 month or more per year in 6 or more out of 10 years (e.g. Oxyaquic Vitricryands). Andisols with episaturation, i.e., the soil is saturated with water in one or more layers within 200 cm of the mineral soil surface and also has one or more unsaturated layers with an upper boundary above 200 cm depth, below the saturated layer(s) (a perched water table) are denoted by 'Epi' (e.g. Epiaquands).  

Epipedons are used to classify 'Melanic' and 'Histic' Andisols (e.g. Melanaquands, Histic Cryaquands). Andisols, which show a layer 10 cm or more thick with characteristics of a mollic epipedon and more than 3 % organic carbon are named 'Thaptic' (e.g. Thaptic Cryaquands). Andisols, which have more than 6.0 percent organic carbon and colors of a mollic epipedon throughout a layer 50 cm or more thick within 60 cm either of the mineral soil surface, or of the top of an organic layer with andic soil properties, whichever is shallower are named 'Pachic' (e.g. Pachic Melanoxerands). Generally, Pachic is term to identify a thickened mollic epipedon.  

Water retention characteristics are used to classify Andisols at the great group and subgroup level. Andisols that have a 1500-kPa water retention of less than 15 % on air-dried samples and of less than 30 % on undried samples dominant in the upper 60 cm are named 'Vitric' (e.g. Vitraquands, Vitric Haplocryands). Andisols that have, undried, a 1500-kPa water retention of 70 % or more throughout a layer 35 cm or more thick within 100 cm either of the mineral soil surface, or of the top of an organic layer with andic soil properties, whichever is shallower are named 'Hydric' (e.g. Hydrocryands, Hydric Melanaquands).  

Diagnostic horizons are used to classify 'Petrocalcic', i.e., an indurated calcic horizon (e.g. Petrocalcic Vitritorrands), 'Calcic', i.e., a horizon with secondary accumulation of carbonates (e.g. Calcic Vitritorrands), 'Alfic', i.e., the presence of an argillic or kandic horizon (e.g. Alfic Vitrixerands), 'Ultic', i.e., the presence of an argillic or kandic horizon plus a base saturation (by sum of cations) of less than 35 percent throughout its upper 50 cm (e.g. Ultic Haploxerands), 'Oxic', i.e., an horizon with sandy loam or finer and a high content of low-charge 1:1 clays (e.g. Oxic Haplustands), 'Placic', i.e., a 2 to 10-mm thick dark reddish brown to black iron or manganese pan (e.g. Placaquands), or presence of a duripan, i.e., a horizon cemented by illuvial silica (e.g. Duric Placaquands).    


Andisols - Distinguishing Characteristics
The geographic distribution of Andisols is closely related to volcanoes that are active or have been active during the Holocene. Soils formed on older volcanic deposits are dominated by crystalline aluminosilicates or the material is mixed with other parent material, therefore, the criteria to qualify for Andisols are not given. Andisols are limited to soils formed on volcanic materials that have weathered enough to produce short-range-order organo-metallic and aluminosilicate compounds, but that have not weathered to the point where crystalline materials predominate or where significant transformations has occurred.  

Soils from a variety of soil orders may be found on volcanic terrains, but Andisols are almost exclusively confined to the pyroclastic materials. Soils developed in pyroclastic and other fragmental volcanic materials occupy only about 0.8% of the earth's surface. However, because of their very distinct characteristics, they are recognized as a separate soil order in soil taxonomy.  

Most Andisols are formed from specific parent material (volcanic ejecta). Few soil orders, except Histosols, have such a specific range of parent materials and depositional environments.

The separation between Spodosols and Andisols is difficult, because short-range order aluminosilicates and organo-metallic complexes occur in the B horizons of soils of both orders. A distinguishing characteristic is the transformations in situ and lack of intensive illuviation of these compounds in Andisols.