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 color.

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 color.

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 nucleus.

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 soilHumic acids of haplic phaeozemsHumic 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 (by Kononova)

Substances% dry ash-free basis
Fulvic acids44 - 493,5 - 5,044 - 49
2,0 - 4,0
Humic acids52 - 623,0 - 5,530 - 33
3,5 - 5,0
Proteins50 - 556,5 - 7,319 - 24
15,0 - 19,0
Lignin62 - 695,0 - 6,526 - 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 increasing depth.

Humic acid / fulvic acid ratios of some surface soils (by Kononova)
Humic acid/
Fulvic acid ratio
SoilHumic acid/
Fulvic acid ratio
Chernozem ordinary
2.0 - 2.5
Gray forest
Chernozem deep
Sod podzolic
Chestnut dark
1.5 - 1.7

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