Agriculture Reference
In-Depth Information
(Whitelaw
2000
) and chelates Fe and Al ions with phosphate, and after transforma-
tion, phosphates become available for plant uptake. Different carboxylic anion lowers
the P desorption potential with decrease in the stability constants of Fe- or Al-organic
acid complexes (log KAl or log KFe) in the order: citrate
>
oxalate
>
malonate/
malate
>
tartrate
>
lactate
>
gluconate
>
acetate
>
formiate (Ryan et al.
2001
).
9.3.2.2 Solubilization of Ca-Bound Soil P by Phosphate-Solubilizing
Bacteria
At high pH, soil P forms a complex with Ca and remains unavailable to plants. In
the alkaline soil conditions, phosphatic fertilizers and its metabolites are fixed as
calcium phosphates. Rock phosphate in soil is insoluble and can become soluble
following the release of inorganic P to maintain plant growth (Goldstein
2000
).
Calcium phosphate solubilization occurs through the secretion of organic acids by
microbes (Deubel and Merbach
2005
), and lowering of rhizosphere pH (He and Zhu
1988
), that break down the bound forms of P like Ca
3
(PO
4
)
2
; however, the
buffering capacity of the medium decreases the effectiveness of PSB in releasing
P from tricalcium phosphates (Stephen and Jisha
2009
). Thus, any microorganism
that acidifies its external medium will result in some level of PS activity. In majority
of the soils, proton substitution reactions are determined by microbial production of
organic acids, which is shown by the following equation:
m
PO
4
3
n
þ
þ
A
ðÞ
Ca
2
þ
H
ðÞ
PO
4
3
Ca
2
þ
ðÞ¼
HA
There is no stoichiometry in the equation above because of the complexity of
Ca-P chemistry and the multiplicity of microbially produced organic acids with
differing numbers of dissociable protons (Goldstein
1986
).
9.3.2.3 Phosphate Solubilization from Phosphate Rock
Various bacterial species have been reported to solubilize insoluble inorganic P
compounds, like TCP, DCP, hydroxyapatite, and PR (Goldstein
1986
).
Pseudomo-
nas
,
Bacillus
,
Rhizobium
,
Burkholderia
,
Achromobacter
,
Agrobacterium
,
Micro-
coccus
,
Aerobacter
,
Flavobacterium
, and
Erwinia
are the most common genera
which have the ability to solubilize P. For inorganic P solubilization by PSB, there
is a relationship between bacterial growth, supernatant acidification, and P solubi-
lization from Ca
3
(PO
4
)
2
. In vitro study conducted to determine the P solubilization
by PSB isolates using different levels of organic acids demonstrated a significant
variation in the organic acid secretion by microbes. Of the organic acids, oxalic acid
at 20 mM was significantly better than others (Table
9.2
).
