Andisols- Environmental Conditions
Andisols - Processes
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 - Classification
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:
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
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.