Geoscience Reference
In-Depth Information
Sodium pyrophosphate, which extracts the fractions named Fe
p
,
Al
p
and Si
p
.
Oxalate-oxalic acid, which extracts Fe
o
, Al
o
and Si
o
.
Citrate-bicarbonate-dithionite, a method developed by Mehra
and Jackson (1960); it extracts Fe
d
, Al
d
and Si
d
.
Table 10.1 identifies which compounds are extracted by which
reagents (Lassausse 1991). It has been simplified and slightly modified
to include recent data.
Table 10.1
Extractability in reagents of various compounds containing Fe, Al
and Si (Lassausse 1991).
Organo-mineral
Oxyhydroxides
Silicates
complexes
Mobile Immobile Amorphous Crystalline Allophane, Crystalline
imogolite
Fe
Pyrophosphate Al
p
Si
Fe
Oxalate
Al
o
Si
Fe
CBD
Al
d
Si
Fe
Total
Al
t
Si
From the results of the extractions, many parameters can be
calculated, particularly:
Al
p
/Al
o
is used in classification systems as we shall discuss
later;
(Al
p
+
Fe
p
)/C
p
defines the degree of complexing of metals by
humic substances; it is between 0.1 and 0.2 in Andosols;
Al
o
/Al
t
is considered as an index of weathering (Vacca
et al
.
2009);
various expressions are used to calculate the content of allo-
phane, for example,
Si
o
×
6 according to R.L. Parfitt in 1990,
