Agriculture Reference
In-Depth Information
7.
Humification:
The process whereby the carbon of organic residues is transformed and
converted into humic substances through biochemical and abiotic processes.
8.
Humin:
The fraction of SOM that cannot be extracted from the soil with dilute alkali and
is most resistant to microbial oxidation.
9.
Humus:
The total of the organic compounds in soil exclusive of undecayed plant and ani-
mal tissues, their partial decomposition products, and the soil biomass. The term is often
synonymously used with SOM.
10.
Humus form:
A group of soil horizons, located at or near the surface of a pedon, which
have formed from organic residues either separated from or intermixed with mineral
material.
11.
Humic acid (HA):
The dark-colored organic material that can be extracted from the soil
with dilute alkali and other reagents and that is precipitated by acidification to pH 1.0-2.0.
HA is intermediate in resistance to microbial attack.
12.
Fulvic acid (FA):
The pigmented organic material that remains in solution after the removal
of HA by acidification. It is separated from the FA fraction by adsorption on a hydrophobic
resin at low pH values. FA is most susceptible to microbial degradation but is relatively
more stable to microbial attack than most freshly applied plant residues.
13.
Fluvic acid fraction:
A fraction of SOM that is soluble both in alkali and dilute acid.
14.
Organan:
A cutan composed of a concentration of OM.
15.
Parent material:
The unconsolidated and more or less chemically weathered mineral or
OM from which the solum of soils is developed by pedogenic processes.
16.
Soil organic residues:
Animal and vegetative materials added to the soil of recognizable
origin.
4.3 SOM FRACTIONS
SOM is a heterogeneous, dynamic substance that varies in C and N content, molecular structure,
decomposition rate, and turnover time (Oades, 1988; McLauchlan and Hobbie, 2004). However,
SOM can be conceptually defined as a series of fractions that comprise a continuum based on the
decomposition rate (Stanford and Smith, 1972; Paul and Clark, 1996). The various fractions of SOM
varied in the degree of decomposition, recalcitrance, and turnover rate and management practices
may affect these fractions differently (Schimel et al., 1985; Echeverria et al., 2004). Tirol-Padre
and Ladha (2004) and McLauchlan (2004), however, reported that SOM is mainly divided into
two main groups or fractions. These fractions are
labile
, that is, smaller in size and most rapidly
decomposable. The larger pool with a slow turnover is termed
recalcitrant
. The labile fraction of
OM may decompose in a few weeks or months. The labile fraction of OM constitutes plant litter,
macroorganic matter or light fraction, the living component or biomass, and nonhumic substances
that are not bound to soil minerals (Theng et al. 1989; Tirol-Padre and Ladha, 2004). The most com-
mon components of rapidly decomposable OM fractions are carbohydrates, amino acids, peptides,
amino sugars and lipids, cellulose, hemicellulose, waxes, fats, resins, and lignin. Labile SOM frac-
tions are highly responsive to changes in C inputs to the soil and will provide a measurable change
before any such change in the total OM (Gregorich and Janzen, 1996; Tirol-Padre and Ladha, 2004).
HAs, formed as a result of the humification process, are recalcitrant fractions of SOM (Dodla
et al., 2012). They play an important role in controlling C cycles and the biogeochemistry of soils.
HAs enhance soil C sequestration through hydrophobic protection (Spaccini et al., 2002) and sta-
bilizing soil aggregates (Hayes and Edward, 2001). The stabilized aggregates can, in turn, protect
easily degradable C such as polysaccharides (Bronick and Lai, 2005). Humic substances affect the
transport and retention of contaminants. Their amphipathic nature enables them to interact with a
wide variety of inorganic and organic pollutants, including heavy metals and charged organic pol-
lutants, via chemical bonding and with nonpolar organic pollutants through nonspecific physical
interactions (Li et al., 2003).
