Andosols - Major Soil Groups of the World - page 306

Gutiérrez-Castorena et al . 2007) and is particularly distinct in oceanic
islands known for their variations in rainfall with altitude: Reunion,
the Canary islands, Madeira, Guadeloupe, the West Indies, Hawaii,
New Zealand, the new Hebrides, etc. Many scientific publications refer
to this aspect.

Andosolization requires only a few centuries on recent volcanic flows
(Dahlgren et al . 1997). On the other hand, allophanes can persist unaltered
for more than 40,000 years (Kirkman 1980). In a study of a chronological
series of buried Palaeosols in Mexico, at 3000-m altitude with an annual
rainfall of about 800 mm, a very slow evolution to Luvisols was observed.
The process could take tens of thousands of years (Torn et al . 1997; Sedov
et al . 2003). Other authors feel that de-andosolization could be sudden.
Bringing under cultivation and its consequence of mineralization of
organic matter will be sufficient to start formation of clay minerals
(Moinereau 1977). The powdery single-grain structure gives place to a
coarse polyhedral structure. More recent work also shows that andic
properties disappear after 30 years of cropping (Verde et al . 2005). But
this has been disputed. Cultivated soils risk being truncated by erosion
exposing their deeper horizons devoid of andic character (Quantin 2004).
Also the simple dehydration of allophane, in which the Si/Al ratio is
less than unity, is not enough to give a kaolinite with Si/Al = 1. Silica
has to be found (Fig. 10.6).

Role of time

Plateaus that are not very well drained are more favourable for
andosolization than dry slopes.
It is strange to see soils with A horizons of non-allophanic
Andosols resting, at 70 or 80 cm, on a very thick altérite of ferrallitic
type including sesquioxides, kaolinite and gibbsite but without
poorly crystallized minerals. These soils are spectacular because they
are black over red. They were earlier classified as Sombric Oxisols,
Humox, Sombric Ferralsols… They are common in the mountains of
the intertropical zone: South America, Asia, Africa 1 (Zaïre, Burundi...)
(Caner 2000; Caner and Bourgeon 2001; Dümig et al . 2008a). The authors

Other factors

1 Exploratory surveys by Innocent Musoni of Burundi who, sadly, died before completing
his thesis.