Humin stability

There is a need to resume studies of soil saccharides and peptides. These can compose as much as 30-40% (when account is taken of the compositions of humin materials). Much is known about how polysaccharides of known structures interact with soil colloids, but it has not been possible as yet to know in sufficient detail the structures of the polysaccharides that persist in the soil. Hence we do not know the mechanisms of their binding to soil mineral colloids. The same applies for the peptide materials, though it is clear that polysaccharides and peptides have important roles in soil structure formation and stabilization. 

Campbell et al. (1967) applied sequential extraction to characterize FA, HA, and humin from gray podzolic and chemozemic soils. The fractions of FA and HA extracted by 0.5 M NaOH without acid pretreatment, which they called mobile humates (since the researchers assumed that they are not bound to minerals), had a lower mean residence time (ranging from 85 to 785 for HA, respectively, in the chernozemic and gray podzolic soils) as compared to Ca-humates extracted from humin (1410 years in the chemozemic soil) and to the total FA and HA extracted after acid pretreatment (195-1235 years for HA). This study showed that in the chernozemic soil, Ca-humates and clays play an equally important role in the stabilization of HS, whereas in the podzolic soil the oldest fraction was associated with clays. 

The MRTs of individual humus fractions also vary, as can be seen from Table 8. The MRT of the original soil (Melfort silt loam) was 870 years whereas the MRTs of individual fractions ranged from 25 to 1410 years. For both humic and fulvic acids, the acid hydrolyzable fractions (6A HCl for 18 hours) had lower MRTs than the nonhydrolyzable fractions. This result is in accord with expectations, because the hydrolyzable material would include the carbon of readily decomposable substrates (e.g., carbohydrates and amino acids). Stability of the major humus fractions followed the order humin = humic acid > fulvic acid (Campbell et al., 1967a). 

Humins (humus coal) are soluble neither in alkalies nor in acids. This group in humic substances have been very little investigated. They are characterized as an insoluble form of humic acids. Humins are considered as the oldest, and ultimate products of the humification process. They are assumed to be strongly carbonized substances, where the high stability is increased by a strong bond to the mineral fraction. Due to their high chemical stability or even inert character, humins do not participate in the soil formation process and they do not fulfil the function of the true humus. 

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