Agriculture Reference
In-Depth Information
Table 2.1
Common phosphorous (P) minerals found in acid, neutral and calcareous soils
S.No.
Minerals
Chemical formula
Acid soils
Strengite
FePO
4
.2H
2
O
Variscite
AlPO
4
.2H
2
O
Neutral and calcareous soils
B-tricalcium phosphate
Ca
3
(PO4)
2
Dicalcium phosphate
CaHPO
4
Dicalcium phosphate dihydrate
CaHPO
4
2H
2
O
∙
Fluorapatite
Ca
5
(PO4)
3
F
Hydroxyapatite
Ca
5
(PO4)
3
OH
Octacalcium phosphate
Ca
4
H(PO4)
3
∙
2-5 H
2
O
Adapted from Yadav and Verma (
2012
)
which are then incorporated into their living cells. Mineralization and immo-
bilization of P occur simultaneously and are influenced by structure and composi-
tions of microbes and physico-chemical characteristics of soils besides the exudates
of various plant genotypes.
2.2 Phosphate Solubilization by Microbes: Current
Perspective
The insoluble forms of P such as tricalcium phosphate (Ca
3
PO
4
)
2
, aluminium
phosphate (Al
3
PO
4
), iron phosphate (Fe
3
PO
4
), etc. may be converted to soluble P
by P-solubilizing organisms inhabiting different soil ecosystems (Gupta et al.
2007
;
Song et al.
2008
; Khan et al.
2013
; Sharma et al.
2013
). Soil microorganisms in this
regard have generally been found more effective in making P available to plants
from both inorganic and organic sources by solubilizing (Toro
2007
; Wani
et al.
2007a
) and mineralizing complex P compounds (Bishop et al.
1994
;
Ponmurugan and Gopi
2006
), respectively. Several workers have documented
their findings in order to better understand as to how the microbial populations
cause the solubilization of insoluble P (Illmer and Schinner
1995
; Khan et al.
2007
,
2009
; Buch et al.
2008
). Of the various strategies adopted by microbes, the
involvement of low molecular mass organic acids (OA) secreted by microorgan-
isms has been a well-recognized and widely accepted theory as a principal means of
P-solubilization, and various studies have identified and quantified organic acids
and defined their role in the solubilization process (Maliha et al.
2004
; Khan
et al.
2010
; Marra et al.
2012
). The OA produced by many P-solubilizers, for
example, bacterial cultures (Table
2.2
) or fungi (Table
2.3
), in the natural environ-
ment or under in vitro conditions chelate mineral ions or decrease the pH to bring P
into solution (Pradhan and Shukla
2005
). Consequently, the acidification of micro-
bial cells and their surrounding leads to the release of P-ions from the P-mineral by
H
+
substitution for Ca
2+
(Goldstein
1994
; Mullen
2005
; Trivedi and Sa
2008
). The
