Agriculture Reference
In-Depth Information
most of the P remain in insoluble forms (Omar
1998
; Zaidi et al.
2009
). Further-
more, a large portion of the phosphatic fertilizers applied to soil is rapidly fixed/
immobilized and becomes unavailable to plants (Xiao et al.
2011
). The insoluble
and fixed forms of P, therefore, alter the fertility of soil (Bhattacharyya and Jha
2011
) and limit plant growth (Prejambda et al.
2009
; Victoria et al.
2009
; Plassard
and Dell
2010
). Due to this P fertility problems, growers often apply severalfold
excess P than required by plants (Goldstein
1986
) which after accumulation/
deposition gradually results in soil pollution and pollution of other water resources
such as lakes, streams, and groundwater (Del Campillo et al.
1999
; Reddy
et al.
2002
). On the contrary, the rate of replenishment and availability of P in
soil is determined largely by so many environmental factors, for example, soil pH,
temperature, humic substances, soil P concentration, its fixation by soil, microbial
composition including PSM (Hameeda et al.
2008
; Henri et al.
2008
; Srividya
et al.
2009
) and their functions, and various plant exudates (Hoffland et al.
1989
;Ae
et al.
1990
; Gillespie and Pope
1990
; Hartwig et al.
1991
; Cook et al.
1995
). Apart
from these, the P solubilization process, a complex phenomenon, also depends on
many other factors such as the nutritional richness of soils and growth dynamics and
physiological functions of the organisms involved in solubilization of complex
organic P (mineralization) or inorganic P (solubilization) (Cunningham and Kuiack
1992
; Reyes et al.
1999
; Chen et al.
2006
). Moreover, the performance of PSM has
also been found to be severely affected by stressors and vegetation (Yoon
et al.
2001
; Gupta et al.
2007
;S
ยด
nchez-Porro et al.
2009
; Yadav et al.
2010
),
while for proper growth, establishment, and normal functioning of PSM in soils, the
organisms should be provided with a healthy and nutrient-rich environment
(Vassileva et al.
1999
). Despite conflicting reports on the P solubilization efficiency
of PSM in fluctuating environment (Kern et al.
2012
), such microbes, when coated
onto seeds or applied in soils, have shown a variable yet profound increase in P
uptake by plants and in turn enhance the crop yields (Zaidi et al.
2003
; Afzal and
Bano
2008
; Hamdali et al.
2012
; Ahmad et al.
2013
) under changing/stressed
environmental conditions. The impact of various environmental variables on struc-
tural and functional diversity of PS microbes is reviewed and discussed in the
following section.
3.2 Factors Affecting Inorganic P Solubilization
3.2.1 Hydrogen Ion Concentration (pH)
Among the various environmental factors affecting the growth and metabolic
activities of microbial populations including PSM (Table
3.1
; Fig.
3.1
) is the pH
of the medium in which organisms are growing (Narsian and Patel
2000
; Reyes
et al.
2002
; Khan et al.
2007
). Functionally, the optimum pH for maximum
solubilization of inorganic P by bacteria has been found to be neutral or slightly
