Geoscience Reference
In-Depth Information
fairly numerous features that should not be found. For exam-
ple, salts that are characteristic of a
natric
horizon are not
permitted.
In the US Soil Taxonomy, the brown soils are Inceptisols and soils
enriched with clay in the B are Alfisols, characterized by an
argillic
horizon. But even if a soil that has an argillic horizon is, by priority,
classified in the Alfisols, it may also be a Mollisol, Spodosol (surface
podzolization) or Ultisol (highly degraded soil). For example, Argi-ud-
olls do exist.
The WRB distinguishes Cambisols and Luvisols, the latter having
an argic horizon (Table 8.2). Other than in Luvisols and Albeluvisols,
the argic horizon may also occur in rarer soils that are not discussed
here: Gypsisols, Alisols, Acrisols or Lixisols. Planosols are not affected
by the argic horizon because they do not have a B with signs of clay
translocation. Nevertheless, in the field there is a great family likeness
between Albeluvisols and Planosols, so much so that investigators
hesitate to choose between the two terms.
Classification
Table 8.2
Comparative classification of soils (approximate equivalents).
Soil Taxonomy
WRB
Inceptisols
Cambisols
Alfisols
Luvisols
Gloss-ud-alfs
and
Albeluvisols (
earlier
Glossic Fragi-ud-alfs
Podzoluvisols—FAO)
The term
fragipan
is of American origin (Grossman and Carlisle 1969).
It was introduced in 1946 by G. Smith, who later (in 1960) authored the
U.S. classification (Chap. 4). The literal meaning is: hardened but still
fragile (from L.
fragilis
) horizon (pan). It pertains chiefly to B horizons of
Luvisols differentiated in loess typically containing 50-60 per cent silt,
10-35 per cent clay and little sand (Assallay
et al
. 1998). The fragipan
is characterized by a high bulk density (1.7 to 1.9) and low porosity of
about 30 per cent, instead of the 50 per cent average for soils (Lozet
and Herbillon 1971). It appears below 40, 50 or 80 cm depth and has
a very coarse prismatic structure. It is penetrated in its upper part by
Fragipans
