Agriculture Reference
In-Depth Information
changes in the microbial biomass and/or SOM characteristics (Doran et al., 1987; Liebhardt et al.,
1989; Wander et al., 1994).
The importance of OM in maintaining the sustainability of cropping systems is
Indiscutivel
.
Allison (1973), Campbell (1978), and Haynes (2005) reported that SOM has long been suggested
as the single most important indicator of soil productivity. This is because OM greatly affects the
chemical, physical, and biological properties and processes in soils. Several workers have tabu-
lated and discussed these effects in detail (Stevenson, 1994; Baldock and Nelson, 2000; Haynes,
2005).
The ability to increase pools of soil organic carbon (SOC) in agricultural ecosystems is of interest
both for sequestering atmospheric CO
2
and for restoring OM pools important to soil health (Hooker
et al., 2005). The decline in SOC following cultivation and the detrimental effects of decreased SOC
have been well documented (Follett, 2001; Mann et al. 2002; Hooker et al., 2005). The continuous
crop production potential of soils has a direct relationship to its OM content (Lal, 1998; Mann et al.,
2002; Wilhelm et al., 2004). Within limits, productivity is positively related to the SOM content
(Reicosky and Forcella, 1988). Many of the characteristics of productive soils are associated with
the organic fraction of the soil (Doran et al. 1998; Doran, 2002; Wilhelm et al., 2004).
SOM is a reactive, dynamic component of soils. It is a major component of biogeochemical
cycles of major nutrients, including N and the quantity and quality of SOM, both of which reflect
and control crop productivity (Burke et al., 1989). SOM, a major source of system stability in agro-
systems, is controlled by many factors that have complex interactions. The SOM content depends
on the soil type (Schimel et al., 1994; Webb et al., 2003), frequency and type of cultivation (Heenan
et al., 1995), cropping and residue management (Grace et al., 1995), fertilizer N input (Bhogal et al.,
1997), and climatic conditions (Webb et al., 2003). Understanding the process that controls SOM
dynamics and its response to management is essential for the appropriate use of agricultural land
(Burke et al., 1989). OM is the main component that supplies N to crop plants. The objective of this
chapter is to provide updated information on the role of OM in improving the crop productivity and
sustainability of cropping systems and suggesting management practices to improve/maintain the
OM content of the soils.
4.2 SOM-RELATED TERMS
Providing definitions of SOM-related terms in the beginning of the chapter may contribute to an
understanding of the concepts and discussion on OM dynamics in soil-plant systems. The Soil
Science Society of America (2008) and Fageria (2012) provide the following definitions of the most
common terms generally used in the OM contest.
1.
Organic farming:
Crop production system that reduces, avoids, or largely excludes the use
of synthetically compound fertilizers, pesticides, growth regulators, and livestock feed
additives.
2.
Organic fertilizer:
A by-product from the processing of animals or vegetable substances
that contains sufficient plant nutrients to be of value as fertilizers.
3.
Organic soils:
A soil in which the sum of the thickness of layers containing organic soil
materials is generally greater than the sum of the thickness of mineral layers.
4.
Organic soil materials:
Soil materials that are saturated with water and have 174 g kg
−1
or more organic carbon if the mineral fraction has 500 g kg
−1
or more clay, or 116 g kg
−1
organic carbon if the mineral fraction has no clay, or has proportional intermediate con-
tents, or, if never saturated with water, has 203 g kg
−1
or more organic carbon.
5.
Organotroph:
An organism able to derive carbon and energy for growth and cell synthesis
by utilizing organic compounds.
6.
Humic substances:
A series of relatively high-molecular-weight, yellow-to-black-colored
organic substances formed by secondary synthesis reactions in soils.
