Andosols - Major Soil Groups of the World - page 292

Chemically and mineralogically, Andosols exhibit the following
characteristics (Quantin et al . 1985), precisely defined later in this
chapter: presence of crystalline or paracrystalline minerals, the reaction
with NaF, great variability of exchange capacity with pH, PZNC close
to 4.4 or 5, base saturation less than 50 per cent, high selectivity for
phosphate, often high content of aluminium complexed by humic and
fulvic acids. Figure 10.1 presents an example of Andosol.
Fig. 10.1 Andosol of the Coirons (Massif Central, France). Deep soil, organic, morphologi-
cally poorly differentiated. Photo : J. Moinereau.
Successive volcanic eruptions very often form horizons of geological
origin, which are not significant in the context of soil differentiation.
10.1.2 Conditions of Formation
Andosols are found in the following conditions:
￿ Pyroclastic volcanic rocks , easily weatherable, that is, finely divided
and rich in glasses such as lapilli, pumice, breccia and ash.
Volcanic ejecta make it possible for Andosols to form beyond the
volcanic mounds proper.
￿ Uniformly humid, cold climate , favourable for mineral weathering
and accumulation of organic matter; this means absence of a dry