Humic acid is a principal component of humic substances, which are the major organic constituents
of soil, peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced
by biodegradation of dead organic matter. It is not a single acid; rather, it is a complex
mixture of many different acids containing carboxyl and phenolate groups so that the
mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid.
Humic acids can form complexes with ions that are commonly found in the environment creating
humic colloids. Humic and fulvic acids are commonly used as a soil supplement in agriculture,
and less commonly as a human nutritional supplement. As a nutrition supplement, fulvic acid can
be found in a liquid form as a component of mineral colloids. Fulvic acids are poly-electrolytes
and are unique colloids that diffuse easily through membranes whereas all other colloids
do not. "Synthesis of fulvic acid was accomplished by a route involving selective ozonization
of 9-propenylpyranobenzopyran, obtained by a regioselective cyclization of the 2-methylsulphinylmethyl
1,3-dione(3c)."
Formation and description Humic substances are formed by the microbial
degradation of dead plant matter, such as lignin. They are very resistant to further
biodegradation. The precise properties and structure of a given sample depend on the
water or soil source and the specific conditions of extraction. Nevertheless, the average properties
of humic substances from different sources are remarkably similar.
Humic substances in soils and sediments can be divided into three main fractions: humic
acids, fulvic acids, and humin. The humic and fulvic acids are extracted as a colloidal
sol from soil and other solid phase sources into a strongly basic aqueous solution of
sodium hydroxide or potassium hydroxide. Humic acids are precipitated from this solution
by adjusting the pH to 1 with hydrochloric acid, leaving the fulvic acids in solution.
This is the operational distinction between humic and fulvic acids. Humin is insoluble
in dilute alkali. The alcohol-soluble portion of the humic fraction is, in general, named
ulmic acid. So-called "gray humic acids" are soluble in low-ionic-strength alkaline media;
"brown humic acids" are soluble in alkaline conditions independent of ionic strength;
and fulvic acids are soluble independent of pH and ionic strength.
Liquid chromatography and liquid-liquid extraction can be used to separate the components that
make up a humic substance. Substances identified include mono-, di-, and tri-hydroxy acids,
fatty acids, dicarboxylic acids, linear alcohols, phenolic acids, and terpenoids.
Chemical characteristics of humic substances
A typical humic substance is a mixture of many molecules, some of which are based on
a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together;
the illustration shows a typical structure. The functional groups that contribute most
to surface charge and reactivity of humic substances are phenolic and carboxylic groups.
Humic acids behave as mixtures of dibasic acids, with a pK1 value around 4 for protonation
of carboxyl groups and around 8 for protonation of phenolate groups. There is considerable
overall similarity among individual humic acids. For this reason, measured pK values
for a given sample are average values relating to the constituent species. The other important
characteristic is charge density. The molecules may form a supramolecular structure held together
by non-covalent forces, such as Van der Waals force, π-π, and CH-π bonds.
The presence of carboxylate and phenolate groups gives the humic acids the ability to
form complexes with ions such as Mg2+, Ca2+, Fe2+ and Fe3+. Many humic acids have two or
more of these groups arranged so as to enable the formation of chelate complexes. The formation
of complexes is an important aspect of the biological role of humic acids in regulating
bioavailability of metal ions. Determination of humic acids in water samples
The presence of humic acid in water intended for potable or industrial use can have a significant
impact on the treatability of that water and the success of chemical disinfection processes.
Accurate methods of establishing humic acid concentrations are therefore essential in
maintaining water supplies, especially from upland peaty catchments in temperate climates.
As a lot of different bio-organic molecules in very diverse physical associations are
mixed together in natural environments, it is cumbersome to measure their exact concentrations
in the humic superstructure. For this reason, concentrations of humic acid are traditionally
estimated out of concentrations of organic matter or dissolved organic carbon.
Extraction procedures are bound to alter some of the chemical linkages present in the soil
humic substances. The humic extracts are composed of large numbers of different bio-organic
molecules that have not yet been totally separated and identified. However, single classes of
residual biomolecules have been identified by selective extractions and chemical fractionation,
and are represented by alkanoic and hydroxy alkanoic acids, resins, waxes, lignin residues,
sugars, and peptides. Ecological effects
Organic matter soil amendments have been known by farmers to be beneficial to plant growth
for longer than recorded history. However, the chemistry and function of the organic
matter have been a subject of controversy since humans began their postulating about
it in the 18th century. Until the time of Liebig, it was supposed that humus was used
directly by plants, but, after Liebig had shown that plant growth depends upon inorganic
compounds, many soil scientists held the view that organic matter was useful for fertility
only as it was broken down with the release of its constituent nutrient elements into
inorganic forms. At the present time, soil scientists hold a more holistic view and at
least recognize that humus influences soil fertility through its effect on the water-holding
capacity of the soil. Also, since plants have been shown to absorb and translocate the complex
organic molecules of systemic insecticides, they can no longer discredit the idea that
plants may be able to absorb the soluble forms of humus; this may in fact be an essential
process for the uptake of otherwise insoluble iron oxides.
A study on the effects of Humic acid on plant growth was conducted at Ohio State University
which said in part “humic acids increased plant growth” and that there were “relatively
large responses at low application rates” Ancient masonry
In Ancient Egypt, according to archeology, straw was mixed with mud in order to produce
building bricks. Straw produces stronger bricks that are less likely to break or lose their
shape. Modern investigations have found that humic acid is released from straw when mixed
with mud, basically a mixture of sand and clay. Humic acid increases clay's plasticity.
See also glomalin
Potassium humate Further reading
Hessen, D.O.; Tranvik, L.J.. Aquatic humic substances: ecology and biogeochemistry. Berlin:
Springer. ISBN 3-540-63910-1. References