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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. source: http://www.ar.wroc.pl/~weber/kwasy2.htm
Humic substances are ubiquitous in the environment.
Their importance in agriculture and soil sciences has been acknowledged for over
150 years. Aquatic scientists have been slower in appreciating their importance,
but now realize that they may constitute as much as 95% of the total dissolved
organic matter in aquatic systems and often are equal to or greater than the
concentrations of inorganic ions present. In many cases they act as the major
buffering system, which has serious implications for acidification of lakes and
rivers. source: http://www.hagroup.neu.edu/abouthafrm.htm
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