Phosphorus is an essential nutrient for plant growth but is often limited in soils. Phosphate solubilizing bacteria can aid in making phosphorus more available to plants. They release organic acids and phosphatases that solubilize insoluble phosphate compounds in soil into plant-available forms. This process allows plants to better acquire phosphorus for growth and reproduction. These bacteria are found in soil and can be selected for use as biofertilizers to enhance phosphorus availability and promote plant growth.

1. Economic importance of Phosphate
solubilising bacteria

2. Phosphorous is a major growth limiting nutrient. As like nitrogen, there is no large atmospheric source that
can be made biologically available .
• Phosphorous (P) plays an important role in plant growth and is the major plant growth limiting nutrient
despite its abundance in soils in both inorganic and organic forms .
• Phosphorus being a structural component of many coenzymes, phospho-proteins, phospholipids and also
forms a part of the genetic memory “DNA” of all living organisms. It is involved in the transfer and storage of
energy which is used for growth and reproduction.
• Also, it plays the vital role in root elongation, proliferation, and its deficiency affects root architecture seed
development and normal crop maturity.
Role of Phosphrous in plant growth

3.  A number of bacterial species provide beneficial effects to a plant and these are mostly present in rhizosphere and
hence called rhizobacteria.
 This group of bacteria has been termed plant growth promoting rhizobacteria. and among them, are
strains from genera such as Alcaligenes, Acinetobacter, Arthrobacter, Azospirillum, Bacillus, Burkholderia,
Enterobacter, Erwinia, Flavobacterium, Paenibacillus, Pseudomonas, Rhizobium, and Serratia. T. They are used as
biofertilizers or control agents for agriculture improvement, and there are numerous researchers for the area
with the agricultural environment conservation
 Plants acquire P from soil solution as phosphate anions.
 However, phosphate anions are extremely reactive and may be immobilized through precipitation with cations such
as Ca 2+ , Mg 2+ , Fe 3+ and Al 3+.
 In these forms, P is highly insoluble and unavailable to plants.
 Different bacterial species has ability to solubilize insoluble inorganic phosphate compounds, such as tricalcium
phosphate, dicalcium phosphate, hydroxyapatite, and rock phosphate
Phosphate solubilising bacteria

4. Mechanism of action of PSM
• Plants absorbs Phosphorus in only two soluble forms, the monobasic(HPO4ˉ )and the dibasic(H2PO4ˉ ).
• Solubilizing Bacteria (PSB) enhance the solubilization of insoluble phosphorous compounds through the release of
organic acids and phosphatase and phytase enzymes, which is present in a wide variety of soil microorganisms.
• During the conversion process, a part of phosphorous is assimilated by microorganisms, but the amount made
soluble and released is in excess to the requirement of the microorganisms.
• The excess amount thus released is made available to plants. During this conversion process, organic acids play an
important role.
• Equally important are nitric acid and sulphuric acid. As a result, these organic and inorganic acids convert calcium
phosphate to di or monobasic phosphates and are then easily made available to plants phosphates .

5.  The principal mechanism for mineral phosphate solubilization is the production of organic acids,
and acid phosphatases
 phosphate solubilization is the action of organic acids synthesized by soil microorganisms

6.  Bacillus were found to produce mixtures of lactic, isovaleric, iso-butyric and acetic acids.
 Other organic acids, such as glycolic, oxalic, malonic, and succinic acid, have also been
identified among phosphate solubilizers.
 Strains from the genera Pseudomonas, Bacillus and Rhizobium are among the most
powerful phosphate solubilizers

7. Possible using plate screening
methods.
Phosphate solubilizers produce
clearing zones around the microbial
colonies in media.
 Insoluble mineral phosphates such
as tricalcium phosphate or
hydroxyapatite are contained in the
media.
Bromophenol blue method produce
yellow halos following pH drop
through the release of organic acids`

8.  Potential materials that are able to support good growth and survival of bacteria are needed in
inoculant production such as - coals, bentonite, corn oil, mineral soils, peat, peat moss,
vermiculite, and perlite
 STEP -1 (CULTIVATION)
Bacteria are cultivated on suitable medium in suitable conditions.
ERIAL)
 STEP-2 (COLLECTION)
After incubation period collect microbial cells by centrifuge.
Wash cells with distilled water or diluted saline solution.
STEP-3(FORMULATION OF INOCULANTS USING CARRIE MAT
Mix cells using carrier materials such as peat, vermiculite, perlite etc.
Centrifuged
Culture

11. Immobilization in soil science is the conversion of inorganic compounds to organic compounds by
microorganisms or plants by which the compounds become inaccessible to plants. Immobilization is the
opposite of mineralization
Mineralisation refers to the conversion of organic matter to mineral nutrients, which are readily absorbed by plants.