Agriculture Reference
In-Depth Information
(Mahidietal.
2011
). The organic acids produced by PSM acidify the microbial cells
and their surroundings (Richardson et al.
2009
) and the release of P ions from the
phosphate mineral by H
+
substitution for Ca
2+
through induction of metabolic pro-
cesses that are effective in directly solubilizing and mineralizing P from sparingly
available forms of P (Illmer and Schinner
1995
). In soil, organic acids further reduce
the pH of their surroundings and can either dissolve the P directly by lowering the pH of
soil or they can chelate heavy metal ions such as Ca, Al and Fe and release associated P
with them (Awasthi et al.
2011
). The mechanism of P solubilization also involves
lowering of pH by release of proton/bicarbonate, gaseous exchange, chelation of
cations and by competing with P for the adsorption sites in soil (Nahas
1996
). Some
of the inorganic acids (e.g. HCl) are also helpful in solubilizing P, but they are less
effective as compared to organic acids (Kim et al.
1997
). There are other mechanisms
by which microorganisms solubilize inorganic P other than the secretion of organic
acids, for example, by producing siderophores (Vassilev et al.
2006
) and secretion of
phenolic compounds and humic substances (Patel et al.
2008
).
4.4 Types of Phosphate Solubilization
Mainly two forms of P, namely, organic and inorganic forms, occur in soils and are
important for plants as a specific source of P. The relative amounts of P in both
forms, however, vary from soil to soil. Most inorganic P compounds in soil belong
to one of the two groups: (i) those in which calcium is the most dominant
controlling cation (calcium phosphate) and (ii) those in which iron and aluminium
are the controlling cations (iron and aluminium phosphate). Calcium phosphates,
including rock phosphate ores (fluorapatite, 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
). Phosphate-solubilizing microorganisms
increase the P nutrition of plants through enhanced solubility of Ca phosphates
(Vassilev et al.
2006
) and their solubility increases with a consequent decrease in
soil pH. Phosphate solubilization is mainly due to the combined effect of pH
decrease and organic acids production (Khan et al. 2010). Microorganisms through
secretion of different types of organic acids and pH lowering mechanisms dissoci-
ate the bound forms of P like Ca
3
(PO
4
)
2
. Nevertheless, buffering capacity of the
medium reduce the effectiveness of PSMs in releasing P from tricalcium phos-
phates (Stephen and Jisha
2009
). Carboxylic anions produced by PSMs 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 solubili-
zation if the organic acid structure favours 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 or production of microbial metabolites (Abd-Alla
1994
). Calcium
phosphate (Ca-P) release results from the combined effects of pH decrease and
