Agriculture Reference
In-Depth Information
The return of plant residue to the soil is a very important source of organic
matter affecting soil dynamic and properties. Plant roots can also exudate
different organic compounds as well as mineral nutrients influencing plant
rhizosphere. Plant rhizosphere is very different from the surrounding bulk soil.
Accordingly, microbial activities and diversity differ very much in the plant
rhizosphere, relative to the bulk soil (Bais et al., 2006). Root exudates interact
with both beneficial soil microbes including AM fungi,
rhizobium
and PGPR
and non-beneficial soil organisms including pathogens, parasitic weeds and
invertebrate herbivores (Badri and Vivanco, 2009).
There are different chemicals used to increase the performance of crop
plants. For example, use of nutrient fertilization, or chemicals, which are used
to inhibit the activity of weeds and plant pathogens in soil, is very common.
Chemical fertilization can supply the necessary nutrients for plant growth and
crop production; however it can also adversely affect soil structure, especially
when applied at excess amounts (Miransari and Mackenzie, 2010, 2011a,b). In
addition to the controlling effects of different chemicals on the activity of
different organisms in soil affecting the ecosystem performance, they can also
adversely
affect
soil
biological
activity
and
hence
soil
productivity
(Samarbakhsh et al., 2009).
T
OPSOIL AND
O
RGANIC
M
ATTER
The importance of soil organic matter for maintaining soil productivity
has been illustrated since the time that human kind started farming (Allison,
1973). SOM is originated from biotic tissues with different forms and types
(Coleman et al., 1989), mainly found in the upper soil layer (Kramer and
Gleixner, 2008), and is a very important parameter affecting different soil
properties. Compared with soil mineral phase, SOM is of lower bulk density,
higher porosity, dark color, higher available nutrients, higher cation exchange
and hence bufferic capacity and higher biological activity. These properties
make soil organic matter a very important component of soil, significantly
affecting soil properties and hence its efficiency (Riley et al., 2008).
A lower bulk density of SOM can be favorable for modifying soil bulk
density and providing a more suitable medium for plant growth and crop
production (Miransari et al., 2007; 2008). SOM higher porosity including
micropores can affect soil water behavior as well as flux of gases such as
oxygen (O
2
), carbon dioxide (CO
2
), methane (CH
4
) and nitrogen oxides (NO
x
)
(Robertson et al., 2000; Franzluebbers, 2005). In addition, the activity of soil
