Geoscience Reference
In-Depth Information
True Andosols (with amorphous materials) are replaced by halloysitic
soils when the climate is less humid, the soils older, the environment
less drained or the weathering of the rock more difficult (lavas).
On some plutonic rocks such as syenites or diorites, under cold
humid climate, are found soils that satisfy the criteria necessary for
classifying them in the 'Nonallophanic Andosols' of the WRB (Delvaux
et al
. 2004). This is to be expected because a diorite (plutonic rock) has
the same chemical composition as an andesite (volcanic rock). Also, this
has the merit of underscoring the convergence, at the limit, of aluandic
soils with humiferous soils on crystalline rocks (Umbrisols).
10.5.2 Classification
The WRB has given up describing an 'Andic' diagnostic horizon and,
in its latest version, defines '
Vitric properties
' and '
Andic properties
'. T h e
latter obey the following major criteria:
bulk density less than 0.9 kg dm
-3
,
Al
o
+ ½Fe
o
of 2 per cent or more in the fine earth,
phosphate retention greater than 85 per cent,
less than 25 per cent carbon by weight,
thickness of at least 30 cm.
The method for phosphate retention is simple in principle. A sample
is mixed with a known volume of a standard phosphate solution and
the quantity of phosphate left in the solution and could not be retained
by the sample is determined.
But the WRB does not limit the categories of Andosols to the three
presented earlier. It adds to the term 'Andosol' qualifiers that take into
account a major characteristic. This sometimes considers the constituents
(e.g. Vitric, Fulvic, Silandic and Aluandic Andosols). But this major
feature could also pertain to base saturation (e.g. Eutric), morphology
(e.g. Fragic), hydric regime (Gleyic, Hydric), or chemistry (e.g. Calcic -
not so strange: see end of §10.6.2), etc.
WRB
Americans at first grouped Andosols in the Inceptisols (suborder
Andepts). This is why we find many Vitr-andepts, Cry-andepts, etc.
in old publications. Then, in 1978, they introduced a new
Soil Order
,
Andisols. There is no diagnostic andic horizon corresponding to this
Soil Taxonomy
