2.2. Generality on Andosols
2.2.1. Definition
Andosols are soils that are dominated by amorphous (or
short-range order) Aluminium Silicates and/or Al-organic matter complexes. They
usually have an Ah - BW - C horizon Sequence. The Ah horizon is dark-coloured
and normally very high in organic matter (Often more than 10%) stabilized by
Aluminium. The B-horizon is usually dominated by amorphous aluminium silicates
(allophane, imogolite). Andosols form mainly on volcanic ashes, but can also
be found on other highly weatherable rock such as Amphibolites, arkoses, etc.
Andosols have many peculiar properties, such as a high phosphate fixation,
aluminium toxicity, irreversible drying, high water retention with low water
availability, and high erosion resistance. They usually have sedimentary
stratification, with the most unweathered materials occurring on top.
One of the most important characteristics of Andisols is their
high capacity to immobilize (Fix) phosphorus (P) on the surface of the
amorphous minerals. This is perhaps the principal chemical constraint of
Andisols. It seems that the P fixing capacity varies with the type of clay
mineral, affecting the residual value of phosphate applications. (Dahlgren
et al., 2004).
2.2.2. Phosphorus Fixation Mechanisms in Andosols
Initially, P fixation in Andosols was considered to occur only
on the active surface of allophane and imogolite. Fixation mechanisms include
chemiadsorption and displacement of structural silicon (Si). The importance of
Al complexes in the P fixation processes has attracted attention. Soil humus in
Andosols readily forms metal complexes with transition metals like Al.
Furthermore, hydroxyl groups attached to the complexed Al
enter into ligand exchange reactions with phosphates
(HPO4-- and H2PO4-).
Formation of allophane and imogolite is restricted by the
accumulation of humus and the subsequent formation of humus-Al complexes.
The strong complexation of Al with humus limits the
possibility of co precipitation of Al and Si released from the weathering of
volcanic ash. This process is common in Andosols of high altitude. Accumulation
of organic matter is higher in volcanic soils located at higher altitude (more
than 2,000 m above sea level).
The clay fraction of Andosols is dominated by allophane and
imogolite (amorphous, short range ordered minerals) which come from the
weathering of pyroclastic material produced from recent volcanic depositions.
Research conducted in the last 20 years has demonstrated that humus-aluminum
(Al) complexes also play a significant role in Andosols chemical behavior.
Phosphate fixation potential of Andosols appears to be related to the presence
of different materials in the clay fraction as a result of different weathering
conditions of volcanic ash. Soil dominated by humus-Al complexes seems to have
higher P fixing potential, which is apparently difficult to satisfy. (Juo and
Valverde, (1996).
2.2.3. Volcanic soils and agriculture
The volcanic soils are considered among the most productive
soils of the world for agriculture. According to Shoji et al. (1993b),
the agricultural areas most productive of the world are localized close to
volcano, and support a strong population density in general.
Between the principal chemical limitations of agricultural
productivity of Andosols is the strong capacity of fixing of phosphorus a
strong acidity and thus, the possibility of aluminum. However, the volcanic
soils are generally easy to plow and the well developed, and is relatively
resistant to erosion and compaction (Dahlgren et al., 2004).
The unique Chemical and physical characteristics of Andosols,
in particular organic matter contents, porosity, apparent density and capacity
of retention of water, influence largely their productivity (Dahlgren et
al., 2004).
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