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Properties of humic substances
Humic acids - the fraction of humic
substances that is not soluble in water under acidic conditions (pH < 2) but is
soluble at higher pH values. They can be extracted from soil by various
reagents and which is insoluble in dilute acid. Humic acids are the major
extractable component of soil humic substances. They are dark brown to black in
Fulvic acids - the fraction of humic
substances that is soluble in water under all pH conditions. They remains in
solution after removal of humic acid by acidification. Fulvic acids are light
yellow to yellow-brown in color.
Humin - the fraction of humic substances
that is not soluble in water at any pH value and in alkali. Humins are black in
Many investigators now belive that all dark colored humic substances are
part of a system
of closely related, but not completely identical, high - molecular - weight
polymers. According to this concept, diferences between humic acids and fulvic
acids, can be explained by variations in molecular weight, numbers of functional
groups (carboxyl, phenolic OH) and extent of polymerization.
The postulated relationships are depicted in figure
, in which it can be seen that carbon
and oxygen contents, acidity and degree of polymerization all change
systematically with increasing molecular weight.
The low - molecular - weight fulvic acids have higher oxygen but lower carbon
contents than the high - molecular - weight humic acids. Fulvic acids contain
more functional groups of an acidic nature, particulary COOH. The total
acidities of fulvic acids (900 - 1400 meq/100g) are considerably higher than for
humic acids (400 - 870 meq/100g).
Another important difference is that while the oxygen in fulvic acids can be
accounted for largely in known functional groups (COOH, OH, C=O), a high portion
of the oxygen in humic acids seems to occur as a structural component of the
Electron microscope observations revealed the humic acids of different soils
to have polymeric structure, appearing in form of rings, chains, and clusters.
The sizes of their macromolecules can range from 60 - 500 A, what is mainly
decided of by the occuring humification process, which also exerts an influence
on their spatial sructure. Compared to other taxonomic units, the polymers of
podsol- earth soils showed to most loose structure.
Electron microscope observations of humic acids (Drozd 1978)
|Humic acids of peaty sandy hydro earth soil
||Humic acids of haplic phaeozems
||Humic acids of podzolic soil
Is apparent that humic substances consist of a heterogeneous mixture of
compounds for which no single structural formula will suffice.
Humic acids are thought to be complex aromatic macromolecules with amino
acids, amino sugars, peptides, aliphatic compounds involved in linkages between
the aromatic groups. The hypothetical structure for humic acid, shown in figure,
contains free and bound phenolic OH groups, quinone structures, nitrogen and
oxygen as bridge units and COOH groups variously placed on aromatic rings.
The hypothetical model structure of fulvic acid (Buffle's model) contains
both aromatic and aliphatic structures, both extensively substituted with oxygen
- containing functional groups.
Elemental composition of humic substances and several plant material
||% dry ash-free basis
||44 - 49
||3,5 - 5,0
||44 - 49
2,0 - 4,0
||52 - 62
||3,0 - 5,5
||30 - 33
3,5 - 5,0
||50 - 55
||6,5 - 7,3
||19 - 24
15,0 - 19,0
||62 - 69
||5,0 - 6,5
||26 - 33
The humic acid / fulvic acid ratio
The percentage of the humus which occurs in the various humic fractions
varies considerably from one soil type to another.
The humus of forest soils is characterized by a high content of fulvic acids
while the humus of peat and grassland soils is high in humic acids.(see figure)
The humic acid / fulvic acid ratio usually, but not always, decreases with
Humic acid / fulvic acid ratios of some surface soils (by Kononova)
Fulvic acid ratio
Fulvic acid ratio
2.0 - 2.5
1.5 - 1.7
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