Agriculture Reference
In-Depth Information
Fig. 7.1
Schematic diagram of soil P mobilization and immobilization in bacteria (Richardson
and Simpson
2011
)
Soil phosphates mainly the apatites and metabolites of phosphatic fertilizers are
fixed in the form of calcium phosphates under alkaline conditions. Many of the
calcium phosphates, including rock phosphate ores (fluoroapatite, francolite), are
insoluble in soil with respect to the release of inorganic P (Pi) at rates necessary to
support agronomic levels of plant growth (Goldstein
2000
). Gerretsen (
1948
) first
showed that pure cultures of soil bacteria could increase the P nutrition of plants
through increased solubility of Ca-phosphates. Their solubility increases with
consequent decrease in soil pH. Microorganisms through secretion of different
types of organic acids, e.g., carboxylic acid (Deubel and Merbach
2005
), and
rhizospheric pH-lowering mechanisms (He and Zhu
1988
) dissociate the bound
forms of phosphate like Ca
3
(PO
4
)
2
. Nevertheless, buffering capacity of the medium
reduces the effectiveness of PSB in releasing P from tricalcium phosphates (Ste-
phen and Jisha
2009
).
Acidification of the microbial cell surroundings releases P from apatite by proton
substitution/excretion of H
+
(accompanying greater absorption of cations than
anions) or release of Ca
2+
(Goldstein
1994
; Illmer and Schinner
1995
; Villegas
and Fortin
2002
) (Fig.
7.2
), while the reverse occurs when uptake of anions exceeds
that of cations, with excretion of OH
/HCO
3
exceeding that of H
+
(Tang and
Rengel
2003
). Carboxylic anions produced by PSB have high affinity to calcium
and solubilize more P than acidification alone (Staunton and Leprince
1996
).
Complexing of cations is an important mechanism in P solubilization if the organic
acid structure favors complexation (Fox et al.
1990
). It is controlled by nutritional,
physiological, and growth conditions of the microbial culture (Reyes et al.
2007
),
but it is mostly due to the lowering of pH alone by organic acids (Moghimi and Tate
1978
) or production of microbial metabolites (Abd Alla
1994
). Organic anions and
associated protons are effective in solubilizing precipitated forms of soil P (e.g.,
Fe- and Al-P in acid soils, Ca-P in alkaline soils), chelating metal ions that may be
associated with complexed forms of P or may facilitate the release of adsorbed P
