Agriculture Reference
In-Depth Information
The stable fraction of OM may persist in the soil for years or even decades. Stable organic
constituents in the soil include humic substances and other organic macromolecules that are highly
resistant to microbial decomposition, or that are physically protected by adsorption on mineral
surfaces or entrapment within clay and mineral aggregates (Theng et al., 1989; Tirol-Padre and
Ladha, 2004). A stable fraction of OM is probably a more appropriate and representative fraction
for C sequestration characterization (Cheng and Kimble, 2001; Tirol-Padre and Ladha, 2004). SOM
can also be divided into functional pools based on turnover rates (Tiro-Padre and Ladha, 2004). A
small pool (1-5%) with a rapid turnover that may take weeks to years and two large pools that are
designated as a slow turnover that may take decades and very slow turnover that may take centuries
(Scholes and Scholes, 1995).
4.4 OM CONTENT OF THE SOIL
The SOM content is generally measured as organic carbon (C) and/or total N content (Haynes,
2005). Although the organic fraction of soils typically accounts for a small, but variable, propor-
tion (typically 5-10%) of soil mass, it exerts far-reaching effects on soil properties (Haynes, 2005).
The input of C to the soil occurs mainly as aboveground plant litter, turnover of root material, and
exudation of carbonaceous material from the roots (Cadisch and Giller, 1997; Paustian et al., 1997;
Haynes, 2005). This C originates from atmospheric CO
2
that has been photosynthetically fixed and
incorporated into organic compounds in plants. Once the organic residues are added to the soil, they
are decomposed by the combined actions of soil fauna and microorganisms (Haynes, 2005). During
this process, the bulk of the residue C (about 70%) is returned to the atmosphere as CO
2
through
faunal and microbial respiration (Jenkinson et al., 1991). The remainder of added C, including that
incorporated into the microbial biomass, undergoes further transformations with the eventual for-
mation of relative recalcitrant humic substances (Haynes, 2005).
Soils having widely different OM content are often found even within the same climatic zone.
Such differences in the OM content of soils are normally attributed to the effects of vegetation,
microbial population, temperature and moisture content, and management practices adopted in crop
production. Natural processes leading to the development of soils having variable OM contents are
related to the so-called factor of soil formation (Stevenson, 1982).
OM =
f
f(time, climate, vegetation, parent material, topography,...)
where
f
stands for “depends” or “function of” and the dots indicate that other factors may be
involved.
Carbon is the chief element of SOM that is readily measured quantitatively. Hence, estimates of
OM are frequently based on organic-C that is mainly determined by two methods: (i) those based
on quantitative combustion procedures wherein C is determined as CO
2
and (ii) those based on the
reduction of the
Cr
2
2−
ion by OM, wherein the unreduced
Cr
2
2−
is measured by titration (Allison,
1965). Values for the organic-C content of soils may be expressed as such or may be reported as
total OM by multiplying the figure for organic-C by the conventional “Van Bemmelen factor” of
1.72. The use of this factor is based on the assumption that SOM contains 58% C. Organic nitrogen
may also be estimated from organic carbon values by dividing by 12 for most soils (Brady and Weil,
2002). Many studies have been reported on the other factor, with highly variable results indicating
that it is at best only an approximation (Allison, 1965). However, Broadbent (1953) reported that the
factor for converting organic-C into OM in surface soils is approximately 1.9, and that the factors
for subsoils are about 2.5. Generally, Histosols and Cambisols have a higher C content as compared
to other soil groups.
A typical prairie grassland soil (Mollisol) may contain 5-6% OM in the top 15 cm and a sandy
soil, <1%. Poorly drained soils (Aquepts) often have OM content approaching 10%. Tropical
Oxisols are known for their low content of OM (Stevenson, 1982). Fageria and Breseghello (2004)
