Geoscience Reference
In-Depth Information
10
Andosols
The word
Andosol
comes from the Japanese
an
= black and
do
= soil.
Andosols are largely associated with volcanic rocks over the entire
Earth. But they cover only a small area (about 110 million ha, or
less than 1 per cent of the lands of the world). Very fertile, they are
cultivated wherever possible. In the regions with these soils, population
density can reach 1000 persons per sq. km. At the physical, chemical
and mineralogical levels they have exceptional properties, the study of
which is particularly instructive and indirectly helps to understand the
properties of all the other soil types.
10.1
PRINCIPAL FEATURES AND ECOLOGY
10.1.1 Typical Profile
The soil is humiferous and dark brown or black. The organic carbon
content often exceeds 25 kg m
-2
. The profile is poorly differentiated (AR),
sometimes with a more ochreous intermediate layer [A(B)C profile].
The soil is rather deep, up to a metre or more. The structure depends
on moisture content and on cultural practices. Under cultivation and in
dry state, it is powdery. When wet, the soil mass becomes continuous
but not resistant. It is porous, light and easily penetrated by a knife.
The consistence is then creamy, but without any plasticity. In all cases,
abundance of organic compounds is observed and also a low bulk
density (0.3-0.9 kg dm
-3
). In very moist environments, the water retained
at 15-bar tension is considerable and higher than one gram water per
gram of the <2
ยต
m fraction. In this case, kneading of the soil between
thumb and index finger almost always re-releases the water. This is
thixotropy
. To put it plainly, application of pressure is liable to change
the state of the soil abruptly from a paste to a liquid.
