Geoscience Reference
In-Depth Information
are in agreement that the black horizon is linked to the establishment
of grassland vegetation known to favour the accumulation of humus
(Chap. 2, § 2.5.3). The capture of aluminium by organic substances at
the top of Ferralsols could thus be considered equivalent to a 'secondary
andosolization'.
But these grasses that would have been established at the beginning
of the Holocene should by now have given place to various forest types,
whereby there would be slow mineralization in the surface layer. At
the same time, andic properties disappear. Sometimes humus remains
in abundance at middle depth in the profiles, thus conferring on them
great novelty.
10.4
PROPERTIES OF ANDOSOLS
10.4.1 Physical Properties
The bulk density of an ordinary soil is between 1.3 and 1.5 kg dm
-3
.
That of an Andosol could be as low as 0.3 and must be, by definition,
less than 0.9 kg dm
-3
.
Bulk density
Porosity can be estimated by
(dp - db)/dp
, where
dp
is the particle
density and
db
the bulk density. If the bulk density is 0.3 kg dm
-3
and
the particle density 2.0 kg dm
-3
, the porosity is 0.7/2 = 85 per cent as
against about 50 per cent in most other soils.
Porosity
Very wet Andosols are structureless (no aggregates seen). But on
drying, the soil is transformed. It is then composed very small spherical
aggregates that contain all the constituents reviewed above. A theoretical
scheme (Fig. 10.8) proposed by Hétier (1975) takes this organization into
account.
Structure
These grains can agglomerate to form other, larger aggregates. Even
these are brought together and so on. Overall, the structure is more or
less 'fractal'. The elementary grains are faecal pellets and are related
to activity of the fauna such as Enchytraeid worms and Acari (Barois
et al
. 1998).
