Geoscience Reference
In-Depth Information
porcelain ware (Mandal and Banerjee 2004). In other words, red soils
develop mostly in environments poor in organic matter and poor in
living organisms. This explains their distribution on the Earth.
9.3 CLASSIFICATION AND TYPOLOGY
The red soils qualified by 'Rhodic' have iron dynamics in common,
as we have said, but in the WRB they are split up among all sorts of
'Reference Soil Groups': Regosols, Cambisols, Luvisols, Acrisols, Lixisols,
Alisols, Ferralsols and Nitisols. Apart from the common colour, some
features serve to separate them: (i) degree of profile differentiation (soils
without distinct horizons, soils having clay-enriched middle horizons,
soils with or without signs of degradation, and also (ii) mineralogical
evolution (Table 9.3).
Table 9.3
Mineralogical evolution of red soils.
Mediterranean climate Tropical climate with Equatorial
dry season
climate
Red soils
Cambisols Rhodic and
Nitisols
Ferralsols
Luvisols Rhodic
Primary
Present, including in B
More or less large
Absent except at
minerals
horizons
residues
base of profile
Mineralogical 2/1 clay minerals, a little Kaolinite/interstratified
Gibbsite and
suite
kaolinite especially near
smectites-kaolinite, at
kaolinite,
the surface in old soils
times boehmite
clay absent
at times
SiO
2
/Al
2
O
3
High
3.5 at the bottom
Æ
2.5 Low
ratio
or 2 at the top of the
profile
pH
Neutral to medium acid
Acid
Acid to strongly
acid
100 ×
50-80
30-60
(Fe
d
- Fe
o
)/Fe
t
Mottled zone
No
No
Yes
at base of
profile
Thus, with 1/1 clay minerals dominant, the Nitisols (earlier Nitosols)
appear as soils intermediate between typical Mediterranean soils (2/1
clay minerals considered obligatory) and soils of the hot humid zones
of the world (1/1 clay minerals + gibbsite). The equivalence 'Nitisols =
Mediterranean red soils' (of the French classification) is mentioned in
the WRB. But this is debatable.
