Fjölrit RALA - 15.06.2004, Síða 95

Soils with ‘andic’ properties developed from non-volcanic materials.
Genesis and implications in soil classification
E. García-Rodeja, T. Taboada, A. Martínez-Cortizas, B. Silva and C. García
Departamento de Edafología y Química Agrícola, Facultad de Biología, USC
The central concept of Andosols refers to soils developed from volcanic materials whose
colloidal fraction is dominated by short-range-order minerals (allophane, imogolite,
ferrihydrite) and/or Al(Fe)-humus complexes. Nevertheless, the weathering of primary
minerals in parent materials of non-volcanic origin may lead to the formation of soils with
these type of components, some of which can be also classified as Andosols. Since the late
70’s the presence of soils with properties close to those of Andosols was identified in NW
Spain (Macías et al., 1978; García-Rodeja et al., 1987). These soils, typically their organic
matter rich Ah horizons, had properties like high content in Al(Fe)-humus complexes, low
bulk density, strong reaction with NaF, dominance of variable charge, high phosphate and
sulphate retention capacity, etc., which make them to fulfil the requirements defined for
ECDAM, ‘andic properties’, ‘andic materials’ (García-Rodeja, 1985; García-Rodeja et al.,
1987) or the ‘andic’ horizon. These properties are mainly related to the abundance of Al-
humus complexes accompanied by halloysites with very poor crystallinity and allophane. The
formation of non-volcanic Andosols with similar properties have also been reported in Nepal
(Baumler and Zech, 1994), India (Caner et al., 2002) and Austria (Delvaux et al, 2004).
The climate of Galicia, with mean temperatures between 8° and 15°C and annual
precipitation ranging from 900 to 2500 mm has an important influence in pedogenesis which
makes soils and saprolites differ from those of surrounding areas. A great lithological
variabihty, together with variations in temporal and biotic factors leads to a great diversity in
pedogenetic processes, within a general trend to acidification in well drained systems, that in
some cases leading to the formation of Andosols or soils with properties close to them:
- In rocks where weatherable minerals are dominant (gabbros, amphibohtes, granulites and
some biotite and plagioclase-rich schists) on old surfaces, saprolites are thick and composed
almost exclusively of kaolinite and goethite (fermonosiallitization). In areas that undergo
rejuvenation by erosion, two situations can be differentiated, depending on the importance of
the biotic factor: i) in abiotic conditions the weathering systems are characterised by a mixture
of primary materials, products of mica degradation and secondary minerals with gibbsite,
halloysite and disordered materials (imogolite, allophane, A1 hydroxides, iron oxihydroxides,
ii) under the influence of organic matter, soils are more acidic, leading to highly reactive
secondary products (mainly Al(Fe)-humus complexes, halloysite and small quantities of
allophane). In many cases, these soils meet the requirements to be classified as Andosols. The
organic matter accelerates weathering and together with humidity, contributes to the
stabihsation of the disordered phases. Therefore, the andosolization process occurs frequently
in organic matter rich horizons of soils developed from intermediate to basic rocks.
- The weathering of granitic and quartz rich metamorphic rocks is slower than in the above
mentioned situations. In abiotic systems the formation of kaolinite (monosiallitization) and/or
gibbsite (allitisation) is accompanied by degradation of micas. In the presence of organic
matter the dominant process is aluminosiallitisation, and the soils are characterised by the
abundance of Al-humus complexes, Al-intelayered vermiculites, halloysite and less
frequently, allophane. In some cases the Ah horizons also meet the requirements of the andic
horizon.
In summary, the process of andosolization that affects parent materials rich in weatherable
minerals (but also other as granites or schists if weathering conditions are more aggressive),
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