Geoscience Reference
In-Depth Information
Table 1.1
Definition and characterization of soil organic matter (Stevenson
1994
)
Term
Definition
Litter
Macroorganic matter (e.g., plant residues) that lies on the soil surface
Light fraction
Undecayed plant and animal tissues and their partial decomposition products
that occur within the soil proper and can be recovered by flotation with a
liquid of high density
Soil biomass
Organic matter present as live microbial tissue
Humus
Total of the organic compounds in soil, exclusive of undecayed plant and
animal tissues, their ''partial decomposition'' products, and the soil
biomass
Soil organic
matter
Same as humus
Humic
substances
A series of relatively high-molecular-weight, yellow- to black-colored
substances formed by secondary synthesis reactions. The term is used as a
generic name to describe the colored material or its fractions obtained on
the basis of solubility characteristics. These materials are distinctive to the
soil (or sediment) environment in that they are dissimilar to the
biopolymers of microorganisms and higher plants (including lignin)
Nonhumic
substances
Compounds belonging to known classes of biochemistry, such as amino acids,
carbohydrates, fats, waxes, resins, and organic acids. Humus probably
contains most, if not all, of the biochemical compounds synthesized by
living organisms
Humin
The alkali-insoluble fraction of soil organic matter or humus
Humic acid
The dark-colored organic material that can be extracted from soil by dilute
alkali and other reagents and is insoluble in dilute acid
Hymatomelanic
acid
Alcohol-soluble portion of humic acid
Fulvic acid
fraction
Fraction of soil organic matter that is soluble in both alkali and acid
Generic fulvic
acid
Pigmented material in the fulvic acid fraction
Soil organic matter is found wherever organic matter is decomposed, mainly in
the near surface. However, soil organic matter may also be transported as sus-
pended particles into deeper layers of the vadose zone or via surface- and
groundwater-forming sediments. Although these components form a minor part of
the total solid phase, they are of major importance in defining the surface prop-
erties of the solid phase and have a great impact on the chemical behavior.
Organic matter extracted from earth materials usually is fractionated on the
basis of solubility characteristics. The fractions commonly obtained include humic
acid (soluble in alkaline solution, insoluble in acidic solution), fulvic acid (soluble
in aqueous media at any pH), hymatomelamic acid (alcohol-soluble part of humic
acid), and humin (insoluble in alkaline solutions). This operational fractionation is
based in part on the classical definition by Aiken et al. (
1985
). It should be noted,
however, that this fractionation of soil organic matter does not lead to a pure
compound; each named fraction consists of a very complicated, heterogeneous
mixture
of
organic
substances.
Hayes
and
Malcom
(
2001
)
emphasize
that
