phosphate-solubilizing microorganism can be a promising source for plant
growth-promoting agent in agriculture (Rodriguez et al. 2006 ).
Using phosphate-solubilizing microorganism as inoculants will increase the P
uptake through plants (Chen et al. 2006 ). The production of bioinoculants on a
commercial scale and their acceptance by farming communities are closely linked
as it is not easy. Furthermore, environmental variables including salinity, pH,
moisture, temperature and climatic conditions of the soil largely affect the estab-
lishment and performance infield cum demonstrations trials of these PSM inocu-
lants developed under laboratory conditions. Hence, there is a great need for proper
development of suitable technology for the isolation of effective inoculants of PSM
based biofertilizers for their adoption under farmer’sfields. Current approach and
developments in our understanding of the functional diversity, rhizosphere colo-
nizing ability, mode of actions and judicious application are likely to facilitate their
use as reliable components in the management of sustainable agricultural systems
(Zaidi et al.2009a).4.3 Mechanism of P-Solubilization
Organic acid production by soil microorganisms is predominant mechanism of
phosphate solubilization. Organic acids result in a decrease in pH of microbial cell
and its surroundings (Halder et al. 1990 ; Khan et al.2014a) (Fig.4.1). In soil,
phosphorus is present in the organic and inorganic form. Soil microorganisms
release phosphorus by organic and inorganic P solubilization. Organic P solubi-
lization is mineralization process (Richardson and Simpson 2011 ). Numerous
mechanisms are opted by soil microorganisms in order to perform P solubilization
such as lowering of pH, organic acid production, chelation and exchange reactions
(Gerke 1992 ). Microorganisms secrete different types of organic acids during sol-
ubilization and lower the pH of rhizosphere and consequently dissociate the bond
form of phosphates like Ca 3 (PO 4 ) (Tri Calcium Phosphate) in calcareous soil.
Furthermore, these microorganisms also serve as a sink for P in the vicinity of labile
C. Soil microorganisms immediately immobilize it even in low P soils. Ecological
changes, for example, freezing–thawing or drying–rewetting, can bring about
flush-events, a sudden increment in accessible P because of high extent of microbial
cell lysis (Butterly et al. 2009 ).
The major processes employed by microorganisms for soil P solubilization
summarized here:(1) Secretion of mineral dissolving compounds e.g. organic acid anions, protons,
hydroxyl ions, CO, siderophores
(2) Biochemical P mineralization by release of extracellular enzymes and
(3) Biological P mineralization by liberation of P during substrate degradation
4 Endophytic Bacteria: Role in Phosphate Solubilization 67