Factors of Pedogenesis - Major Soil Groups of the World - page 59

For a long time, scientists had distinguished humin (insoluble in
alkali), fulvic acids (soluble in the mixture NaOH + Na 4 P 2 O 7 , sodium
pyrophosphate) and the humic acids , soluble in alkali and insoluble
in acid; the three fractions together were presumed to represent the
relatively stable organic matter. But recent investigations have shown
that these reagents separate fractions that are only partially different
in their composition, age and molecular weight. The question has
been raised by a large number of scientists who had tested all sorts of
extraction methods (NaOCl, HF, HCl, H 2 O 2 , NaOH, citric acid, etc.) to
attempt to identify meaningful organic compartments. A very complete
review covers the subject (von Lützow et al . 2007). The question has not
been resolved yet.
This realization of the imperfect nature of the separation humic
acids/fulvic acids/humin is related to the revival of methods of
characterization of SOM that supplemented the traditional elemental
analysis, that is, chiefly: observations with SEM and AFM, Fourier-
transform infrared spectroscopy (FTIR), nuclear magnetic resonance
for 13 C ( 13 C NMR—Table 2.6), gas chromatography and viscometry. If
necessary, the principles of these methods can easily be obtained from
the Web. They have greatly improved the detection of functional groups
in the soil (Table 2.6). The special issue of Geoderma [ 80 (3-4), 1997],
already somewhat dated, may be referred to.

Characterization of humic substances

Table 2.6 Examples of the functional groups detectable by Nuclear Magnetic
Resonance.
Peaks, ppm Functional groups Formula Principal sources

Carboxyl COOH Organic acids

220-160 Carbonyl >C=O Aldehydes, ketones

(tetravalent C)

Amide R-CO-NH 2

R-CO-NHR´,

R-CO-NR´ 2

160-140 Aromatic groups COR or CNR Lignins (polyphenols),

tannins, coals

160-110 Aromatic carbons C-H Lignins, coal

110-90 Anomeric carbon OOH Cyclic form of oses and

(the only one represented osides

in the scheme) H

90-60 Carbohydrates C 2 , C 5 Sugar (lexose)

Table 2.6 Contd...