A andosol It is a soil that is formed 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 to retain water, as well as 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 of humid and perhumid climates..
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- It has an AC or ABC horizon profile; the first of these is usually the darkest.
- It has a high natural productivity: the content of organic matter is in the range of 8 to 30% in the surface horizon.
- Its texture is sandy loam, very porous, so it presents good drainage.
- Sometimes it has high water saturation and is quite resistant to erosion caused by this..
- It presents minerals such as allophane, imogolite, ferrihydrite and complexes formed by organic material and aluminum or iron.
- It weathers easily, creating amorphous mixtures of silicate and aluminum.
- Its apparent density is low.
- Generally has low base saturation values.
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 arrangement of clays in this type of soil will also change as it ages..
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):
It is characterized by having in its first meter a horizon in which glass and other minerals of volcanic origin predominate..
Additionally, it should not have a horizon from the moderate decomposition of rocks of volcanic origin (andic horizon) above it..
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..
It has a horizon with a silica content of 0.6%, or with a ratio of less than 0.5 of aluminum extracted with pyrophosphate with respect to that extracted using oxalate..
It has gray, green, blue colors 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.
It has a very thick dark horizon. Its organic matter content is high, mainly due to the decomposition of grass roots. Presents abundant allophane and complexes formed by aluminum and organic compounds.
It has a horizon similar to the melanic one, but the organic matter does not come from the decomposition of the roots of grasses.
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..
It has a well structured and dark horizon. Its organic matter content is medium to high, saturated (molic) or unsaturated with bases (umbric). Has a thickness greater than 0.50 m.
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:
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.
It differs from the previous one because less than 17% of the organic material comes from still recognizable plant remains.
It is located between 0.40 and 1 m deep.
It has a well-defined, dark and basic horizon, with a medium or high content of organic matter.
The first 100 cm of soil presents nodules compacted with silica and microcrystalline forms of this same material.
It is similar to molicum, but is not saturated with bases.
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..
It presents a horizon with a large amount of iron oxides and organic matter at a depth greater than 0.50 m deep, followed by a very thin sub-horizon joined by a complex of organic matter and aluminum. Iron can be present or absent.
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).
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 sub-horizons in the first meter of depth..
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.
In these cases, calcium carbonate is abundant, and can be concentrated or diffuse between 0.20 and 0.50 m deep..
Its texture is sandy-loam in the first half meter of depth..
Sodium, with a sodium saturation generally greater than 6%. Depending on the percentage of sodium saturation or the sum of sodium plus magnesium, a distinction can be made between endosodic and hypo-sodium.
Skeletal (endoskeletal and episkeletal), with a high content of gravels or other coarse fragments.
Dystric (epidritic, hyperdristic or orthidritic) and eutric (endoeutric, hypereutric or orthotic), depending on base saturation and depth.
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