Agriculture Reference
In-Depth Information
4
Management of Soil
Organic Matter
4.1 INTRODUCTION
Soil organic matter (SOM) has long been recognized as an important indicator of soil productivity
(Haynes, 2005). Organic matter (OM) refers to the solid, nonmineral portions of the soil, originat-
ing from plant and animal residues (Aust and Lea, 1991). According to the Soil Science Society of
America (2008), SOM can be defined as the organic fraction of the soil exclusive of undecayed plant
and animal residues. Hayes and Swift (1983) defined OM as the term used to refer more specifically
to the nonliving components, which are a heterogeneous mixture, largely composed of products
resulting from microbial and chemical transformations of organic debris. This transformation, col-
lectively known as the humification process, gives rise to humus, a mixture of substances, which has
a degree of resistance to further microbial attack. An adequate amount of OM in the soil plays an
important role in improving the soil physical, chemical, and biological properties, and consequently
improves or maintains the sustainability of cropping systems. In agricultural systems, maintenance
of SOM has long been recognized as a strategy to reduce soil degradation (Mikha and Rice, 2004;
Baldock and Broos, 2012).
The major role that OM plays in soil is to stabilize soil aggregates, make soil easier to cultivate,
increase the soil water-holding and buffering capacity, and release plant nutrients upon mineral-
ization (Carter and Stewart, 1996; Fageria, 2012). There is no critical level of OM established for
different cropping systems below which soil quality decreases markedly or irreversibly; decreasing
SOM is still of concern since it might adversely affect some or all of the above properties (Webb
et al., 2003). OM also adsorbs heavy metals, which may be toxic to plants or may contaminate soils
and reduce their quality. Wander et al. (1996) reported that SOM characteristics are potentially
the single best integrator of inherent soil productivity and should be developed as an index of soil
quality. Maintenance of soil quality, which is the capacity of soils to sustain productivity, maintain
environmental quality, and promote plant and animal health (Doran and Parkin, 1994), is the key to
agricultural sustainability (Wander et al., 1996).
SOM consists of a heterogeneous mixture of components with hydrophilic and hydrophobic
functional groups (Jenkinson, 1988; Ellerbrock et al., 2005). The SOM formation is a consequence
of a feedback relationship between organic carbon input and decomposition (Hsieh, 1996). Hence,
the amount of OM in a soil that has been under a given system of cropping and management for a
long time depends on how much OM enters the soil each year and how fast this OM decomposes
in the soil (Jenkinson and Ayanaba, 1977). The turnover of SOM represents energy (C) and nutri-
ent flows of a soil and, therefore, is closely related to intrinsic soil productivity (Hsieh, 1996).
Improving the SOM content is difficult in arable lands due to the rapid decomposition rate of added
organic materials. In cultivated soils, fertility management practices may not change the SOM con-
tents by more than 10% during time periods of 0-10 years (Paustian et al. 1992; Wander and Traina,
1996). The small magnitude of C change may easily be overshadowed by natural soil C heterogene-
ity. This may be the reason why, even though it is well recognized that SOM should be maintained
to sustain soil productivity, SOM contents are generally not effectively used within sites to assay
management practice impacts on soil productivity or fertility (Wander and Traina, 1996). Although
by addition of organic materials the total SOM content may not improve, there may be beneficial
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