Geoscience Reference
In-Depth Information
Table 2.6 Contd...
60-45
Methoxyl group
CH
3
O-
Pectins
45-25
Methylene group
-CH
2
-
Derived from methane
25-0
Terminal methyls, alkyl
CH
3
-
Polysaccharides,
-CH
2
-CO-R
proteins, lignins
The organic fraction of the soil has a very variable composition according
to the environment, and there is no definite chemical formula for humic
or fulvic acid. We only look for the formulae of the
building blocks
, that
is, molecular fractions whose nature and position of functional groups
confer on them a structure and chemical properties closest to those
observed in the soil (Jansen
et al
. 1996; Alvarez-Puebla
et al.
2006), as
shown in Figure 2.21. The modelling will always remain approximate.
See
Soil Science
,
vol.
164
, no.
11, November 1999.
Models of substances/discussion
O
COOH
O
O
OH
OH
COOH
H
2
O
OH
OH
COOH
O
O
OH
OH
OH
O
COOH
HOOC
O
SH
OH
OH
OH
OH
OH
OH
COOH
Fig. 2.21
Model of fulvic acid (Alvarez-Puebla
et al
. 2006).
After all is said and done, these organic substances are perhaps
not macromolecules but smaller molecules liable to bond between
themselves strongly or weakly according to the environment. This idea
was floated by the American scientist Robert L. Wershaw in 1986. It has
been revived several times since then (Piccolo
et al
. 2001; Alvarez-Puebla
et al
. 2006). Actually it is difficult to accept that very large and very
well organized molecules appear in degrading and mineralizing organic
matter, contrary to the direction of general development. In acid medium
with high concentration of salts, the molecular aggregates assume a
