Agriculture Reference
In-Depth Information
dynamic than from other sources (Vazquez et al.
2000
). The PSB are ubiquitous,
varying in forms and composition. The population of PSB is affected by physical
and chemical properties of soils, soil organic matter content, P content, and cultural
activities (Kim et al.
1998
). Higher populations of PSB are found in agricultural and
range land soils (Yahya and Azawi
1998
). The PSB also plays a vital role in
combination with chemical fertilizers, for example, single super phosphate (SSP)
and PR, and application of microbial phosphatic fertilizers has been found to reduce
the synthetic P levels by 25-50 % in agricultural practices (Sundara et al.
2002
).
Direct application of PR is mostly not effective for annual crops (Goenadi
et al.
2000
), the availability of which however can be enhanced by applying some
acid-producing microorganisms: able to solubilize PR (Gyaneshwar et al.
2002
).
Rodr´guez and Fraga (
1999
) in a study suggested that certain PSB strains were able
to solubilize P; examples included were those of
Pseudomonas putida
(51 %),
P. fluorescens
(29 %), and
P. fluorescens
(62 %) (Ghaderi et al.
2008
).
Pseudomo-
nas striata
and
Bacillus polymyxa
solubilized 156 and 116 mg P l
1
, respectively;
Pseudomonas fluorescens
solubilized 100 mg P l
1
containing Ca
3
(PO
4
)
2
,
92 mg P l
1
containing AlPO
4
, and 51 mg P l
1
containing FePO
4
(Henri
et al.
2008
).
9.3.1 Production of Organic Acids by Phosphate-Solubilizing
Bacteria
There are different mechanisms by which PSB can transform insoluble P into
soluble P forms. These include acidification, enzymatic dissolution of phosphates,
and ammonium assimilation. Generally, P solubilization is correlated with the
production of organic acids through oxidation process that happens on the outer
face of the cytoplasmic membrane and is related with the drop in pH of the medium
(Maliha et al.
2004
; Pradhan and Sukla
2005
). Bacteria and fungi can produce
organic acids and acidify their surroundings with the release of P ions from mineral
P by the substitution of H
+
for Ca
2+
(Goldstein
1994
). The findings by Asea
et al. (
1988
) revealed that fixed P in acidic soils accumulated Fe or Al ions, and
there was no correlation found between pH and P solubilization. Hence, there might
be other alternative possibilities than organic acids for the insoluble inorganic P
solubilization, such as the release of H
+
and production of chelating substances and
inorganic acids (Khan et al.
2007
). Illmer and Schinner (
1995
) reported that the
production of organic acids by the bacterial cells is not the only reason for P
solubilization; therefore, acidification is not the only mechanism of P solubiliza-
tion, as the capability to decrease the pH in some cases did not correlate with the
solubilized mineral P. The chelating ability of the organic acids is also significant
(Kucey
1988
). Altomare et al. (
1999
) has examined the ability of plant growth-
promoting and biocontrol fungus
Trichoderma harzianum
T-22 to solubilize P in
in vitro conditions including PR, whereas organic acids were not detected in culture
