ANDOSOL: CHARACTERISTICS, DEVELOPMENT AND TYPES - GEOGRAFÍA - 2024

An andosol is soil that forms on ash, rocks, glass, and other materials of volcanic origin. It is also called volcanic ash soil and is black in color. It has a high content of organic matter and has a high capacity for water retention, as well as for cation exchange.

Very rarely these soils can be formed on non-volcanic materials, such as argillites and loes, among others. Worldwide, they cover an area of ​​approximately 100 million hectares: they are mainly in volcanic areas with humid and perhumid climates.

Andosol soil profile. Taken and edited from Rockwurm, from Wikimedia Commons Can be found from the Arctic to the tropics. They are not found in areas with long dry seasons and are rare in arid areas. Due to their high content of organic matter, they are widely used for agriculture; in this case, its main limitations are its high phosphate-binding capacity and that they are usually on steep slopes.

characteristics

- Has an AC or ABC horizon profile; the first of these is generally the darkest.

- It has a high natural productivity: the content of organic matter is in the range of 8 to 30% on the surface horizon.

- Its texture is sandy loam, very porous, so it has good drainage.

- Sometimes it has high water saturation and is quite resistant to erosion caused by it.

- It presents minerals such as allophane, imogolite, ferrihydrite and complexes formed by organic material and aluminum or iron.

- It meteorizes easily, causing amorphous mixtures of silicate and aluminum.

- Its apparent density is low.

- It generally has low base saturation values.

Development

The formation of andosol is related to a rapid chemical transformation of volcanic rocks into soil. Likewise, it also depends on the porosity and permeability of the soil, and the presence of organic matter.

The humus is somewhat protected from its alteration by biological agents thanks to the formation of complexes with aluminum; This allows the concentration of organic matter on the soil surface.

Part of the aluminum present in the soil -which does not form complexes with organic matter- can precipitate together with silica, giving rise to allophanes and imogolite.

Soil porosity will increase over time due to loss of percolation and stabilization of soil material by organic matter, allophane, imogolite, and ferrihydrite. The amount and disposition of clays in this type of soil will also change as it ages.

Volcanic soil and Tinajo volcano, Canary Islands. Photograph of Montserrat Labiaga Ferrer. Taken and edited from flickr.com/photos/montse

Types

There are numerous classifications of soils, including andosols. The following classification is based on what is established by the Food Organization of the United Nations (FAO):

Vitric

It is characterized by having in its first meter a horizon in which glass and other minerals of volcanic origin predominate.

Additionally, it must not have a horizon from the moderate decomposition of rocks of volcanic origin (andic horizon) above it.

Eutrisilic

It has a horizon with a silica content of 0.6% and a Ca + Mg + K + Na content (sum of bases) not less than 25 cmolc / kg in the first 0.3 m of the soil.

Silico

It has a horizon with a silica content of 0.6%, or with a less than 0.5 ratio of aluminum extracted with pyrophosphate with respect to that extracted using oxalate.

Gleic

It has gray, green, blue colorations and is saturated in water for most of the year. If these characteristics are present in the first 0.5 m from the ground, it is called epigleic; if they appear between 0.5 or 1 m, it is called endogleic.

Melanic

It has a very thick dark horizon. Its organic matter content is high, mainly due to the decomposition of grass roots. It presents abundant allophane and complexes formed by aluminum and organic compounds.

Fulvic

It has a horizon similar to melanic, but the organic matter does not come from the decomposition of the roots of grasses.

Hydric

Of the first 100 cm, at least 35 cm have a water saturation of 100% at a pressure of 1500 kPa in sediment samples that have not been subjected to drying.

Pachico

It has a well structured and dark horizon. Its organic matter content is medium to high, saturated (molic) or unsaturated with bases (umbric). It has a thickness greater than 0.50 m.

Historical

It has an abundant and poorly oxygenated horizon of organic matter. According to the depth of this horizon and its composition, three modalities are established:

Fibrihistic

It is located in the first 0.40 m of soil and presents more than 66% of the organic material constituted by plant remains that can be identified.

Saprihístico

It differs from the previous one because less than 17% of the organic material comes from still recognizable plant remains.

Taptohist

It is located between 0.40 and 1 m deep.

Molico

It has a well-defined, dark and basic horizon, with a medium or high content of organic matter.

Duric

The first 100 cm of soil presents nodules compacted with silica and microcrystalline forms of this same material.

Umbric

It is similar to molic, but it is not saturated with bases.

Luvic

The soil has a surface or subsurface horizon richer in clay than the next horizon. Additionally, its base saturation is greater than 50% up to the first meter of depth.

Plaquic

It presents a horizon with a large amount of iron oxides and organic matter at a depth greater than 0.50 m, followed by a very thin sub-horizon joined by a complex of organic matter and aluminum. Iron can be present or absent.

Leptic

It is characterized by presenting a continuous and hard rock layer at a depth ranging from 0.25 to 0.5 m (epileptic) or 0.5 to 1.0 m (endoleptic).

Acroxic

In these soils the concentration of exchangeable bases and aluminum extracted with 1M potassium chloride is less than 2 cmol (c) / kg, in one or more subhorizons in the first meter of depth.

Vetic

A soil is vetic if the sum of the interchangeable bases and hydrogen is less than 6 cmol (c) / kg of clay in some subhorizon that is less than 100 cm deep.

Calcic

In these cases, calcium carbonate is abundant, and may be concentrated or diffuse between 0.20 and 0.50 m deep.

Arenic

Its texture is sandy-loam in the first half meter of depth.

Other types

Sodium, with a sodium saturation generally greater than 6%. Depending on the percentage of sodium saturation or the sum of sodium plus magnesium, they can be distinguished between endosodium and hypo-sodium.

Skeletal (endoskeletal and episkeletal), with a high content of gravels or other coarse fragments.

Dystric (epidritic, hyperdristic or orthidristic) and eutric (endoeutric, hypereutric or orthotic), depending on the base saturation and the depth at which it is found.

References

1. Andosol. On Wikipedia. Recovered from en.wikipedia.org
2. FAO (1999). World reference base for soil resources. World Soil Resources Reports. Rome
3. JJ Ibáñez, FJ Manríquez (2011). The andosols (WRB 1998): volcanic soils. Recovered from madrimasd.org
4. PV Krasilʹnikov (2009). A handbook of soil terminology, correlation and classification. Earthscan.
5. T. Takahashi, S. Shoji (2002). Distribution and classification of volcanic ash soils. Global Environmental Research
6. B. Prado, C. Duwig, C. Hidalgo, D. Gómez, H. Yee, C. Prat, M. Esteves, JD Etchevers (2007). Characterization, functioning and classification of two volcanic soil profiles under different land uses in Central Mexico. Geoderma