ABSTRACT
Characterization of humic substances ƒ For a long time, scientists had distinguished humin (insoluble in alkali), fulvic acids (soluble in the mixture NaOH + Na4P2O7, 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, H2O2, 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, Fouriertransform infrared spectroscopy (FTIR), nuclear magnetic resonance for 13C (13C 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.
