Andic properties (WRB)

Andic properties is one of the diagnostic properties used, in the WRB system, to discriminate some soils from others.

Description

The term "Andic properties" (from Japanese an, dark, and do, soil) refers to the result of moderate weathering of mainly pyroclastic deposits. However, some soils develop andic properties from non-volcanic materials (e.g. loess, argillite and ferralitic weathering products). The presence of short-range-order minerals and/or organo-metallic complexes is characteristic for andic properties. These minerals and complexes are commonly part of the weathering sequence in pyroclastic deposits (tephric soil material > vitric properties > andic properties).

Andic properties may be found at the soil surface or in the subsurface, commonly occurring as layers. Many surface layers with andic properties contain a high amount of organic matter (more than 5 percent), are commonly very dark coloured (Munsell value and chroma, moist, are 3 or less), have a fluffy macrostructure and, in some places, a smeary consistence. They have a low bulk density and commonly have a silt loam or finer texture. Andic surface layers rich in organic matter may be very thick, having a thickness of 50 cm or more (pachic characteristic) in some soils. Andic subsurface layers are generally somewhat lighter coloured.

Andic layers may have different characteristics, depending on the type of the dominant weathering process acting upon the soil material. They may exhibit thixotropy, i.e. the soil material changes, under pressure or by rubbing, from a plastic solid into a liquefied stage and back into the solid condition. In perhumid climates, humus-rich andic layers may contain more than twice the water content of samples that have been oven-dried and rewetted (hydric characteristic).

Two major types of andic properties are recognized :

• one in which allophane and similar minerals are predominant (the sil-andic type), giving a strongly acid to neutral soil reaction
• one in which Al complexed by organic acids prevails (the alu-andic type)giving an extremely acid to acid reaction

Criteria

Andic properties require:

• an Al_ox + ½Fe_ox value of 2.0 percent or more

and :

• a bulk density of 0.90 kg dm^-3 or less

and :

• a phosphate retention of 85 percent or more

and :

• less than 25 percent (by mass) organic carbon.

Andic properties may be divided into sil-andic and alu-andic properties. Sil-andic properties show an acid-oxalate (pH 3) extractable silica (Si_ox) content of 0.6 percent or more or an Al_py/Al_ox of less than 0.5; alu-andic properties show a Si_ox content of less than 0.6 percent and an Al_py/Al_ox of 0.5 or more. Transitional alu-sil-andic properties that show a Si_ox content between 0.6 and 0.9 percent and an Al_py/Al_ox between 0.3 and 0.5 may occur (Poulenard and Herbillon, 2000).

Field identification

Andic properties may be identified using the sodium fluoride field test of Fieldes and Perrott (1966). A pH in NaF of more than 9.5 indicates allophane and/or organo-aluminium complexes. The test is indicative for most layers with andic properties, except for those very rich in organic matter. However, the same reaction occurs in spodic horizons and in certain acid clays that are rich in Al-interlayered clay minerals. Uncultivated, organic matter-rich surface layers with sil-andic properties typically have a pH (H₂O) of 4.5 or higher, while uncultivated surface layers with alu-andic properties and rich in organic matter typically have a pH (H₂O) of less than 4.5. Generally, pH (H₂O) in sil-andic subsoil layers is more than 5.0.

Relationships with some diagnostic horizons and properties

Vitric properties are distinguished from andic properties by a lesser degree of weathering. This is typically evidenced by a lower amount of non-crystalline or paracrystalline pedogenetic minerals, as characterized by a moderate amount of acid oxalate (pH 3) extractable Al and Fe in layers with vitric properties (Al_ox + ½Fe_ox = 0.4–2.0 percent), by a higher bulk density (BD > 0.9 kg dm^-3), or by a lower phosphate retention (25 – <85 percent). Histic or folic horizons with less than 25 percent organic carbon may have andic properties. In organic layers with 25 percent or more organic carbon, andic properties are not considered. Spodic horizons, which also contain complexes of sesquioxides and organic substances, can exhibit andic properties as well.

See also

• The FAO reference text, (2007 version)