# Mechanism of phosphate solubilization and phosphate mobilization

Ajeet Kumar

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To cite this version:

Ajeet Kumar. Mechanism of phosphate solubilization and phosphate mobilization. Master. Biopesticide and Biofertilizer (Elective course), Dr. Rajendra Prasad Central Agricultural University, Samastipur, India. 2025. hal-05267064

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Submitted on 18 Sep 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Dr. Rajendra Prasad Central Agricultural
University

Pusa (Samastipur)-Bihar- 848125
A study Material

On
Mechanism of phosphate solubilization and
phosphate mobilization

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COURSE TITLE: Bio-pesticides and Bio-fertilizers

*(Elective course)

COURSE NO.: ASS-307

CREDIT HOUR: 3 (2+1)

YEAR/SEMESTER: 3rd YEAR / 6th SEMESTER

Dr. AJEET KUMAR

Assistant Professor-cum-Scientist (Soil Science-Soil Microbiology)

Department of Microbiology

Collage of Basic Sciences and Humanities (CBS&H), Dr. RPCAU, Pusa

2025
# Mechanism of phosphate solubilization and phosphate mobilization

## INTRODUCTION

Much of the soluble phosphorus applied as fertilizer to soil is fixed into one of the many sparingly soluble forms (tricalcium phosphate, aluminium phosphate, Iron phosphate and manganese phosphate) and only 10-18 % is utilized by the standing crop. A variety of mechanism have been accounted for the increased mobilization of soil phosphorus in plant rhizosphere such as excretion of organic acids (Moghimi et al. 1978), exudation of reducing substances (Gardner, 1982), the production of mucilage and sloughed off root cells which provide an energy substrate for microbial activity in the rhizosphere which subsequently dissolve mineral and organic phosphorus (Banik and Dey, 1982), Hydrolysis of organic phosphorus by phosphatase enzymes (Helal and Sauerbeck, 1984). Many phosphates solubilizing microorganism isolated from the rhizosphere of different crops predominantly of the genera *Bacillus polymixa*, *B. megaterium*, *B. circulans* *Pseudomonas striata*, species of *Aspergillus* and *Penicillium* have shown the ability to solubilise sparingly soluble phosphate *in vitro* as well as *in vivo* condition by secreting the organic acid and inorganic acids. Kumar and Maurya (2006) observed a more rapid initial release of phosphorus from tricalcium phosphate in the presence of the fungal species of *Aspergillus sulfurious*, *Aspergillus niger*, *Aspergillus flavus*, *Aspergillus terreus*, and *Rhizopus stolonifer*. Fungi in soils are able to traverse distances more easily than bacteria thus it is more important to phosphorus solubilisation (Kucey, 1983). The solubilizing microorganisms convert insoluble inorganic phosphatic compounds into soluble form. Such microbes not only assimilate phosphorus but they cause a large portion of soluble phosphate to be released in quantities in the excess of their own requirement. With this view an attempts has been made to summarize the informations about the mechanisms of microbial phosphorus solubilisation.

## PHOSPHATE FIXATION AND RETENTION

Phosphate ions attracted to soil constituents with such a bond that they become insoluble and difficultly available to plants. This process is called phosphate fixation or phosphate retention.

### Phosphate Fixation in Acidic Soil
Acid soils usually contain significant amount of soluble and exchangeable Al<sup>+3</sup>, Fe<sup>+3</sup>, and Mn<sup>2+</sup> ions. These ions serve as bridge in between clay and phosphate ions. **Precipitation and Adsorption:** The phosphate ion enters into a chemical reaction with these free metal ions. Al<sup>3+</sup> + 3 H<sub>2</sub>PO<sub>4</sub> → Al (H<sub>2</sub>PO<sub>4</sub>)<sub>3</sub>↓

The product formed is difficultly soluble in water and precipitates in solution. With the passage of time the Al phosphate ppt become less soluble consequently less available to plants. Phosphate is fixed through adsorption on the surface of hydrous oxide of iron and aluminium.

## Phosphate Fixation in Alkali Soils

An alkaline soil contains high amounts of soluble and exchangeable Ca<sup>2+</sup> and CaCO<sub>3</sub>. Phosphate is reported to react with the ionic and carbonate form of calcium.

$$3\text{Ca}^{2+} + 2\text{PO}_{4}^{3-} \rightarrow \text{Ca}_{3}(\text{PO}_{4})_{2}\downarrow \quad \text{Insoluble}$$

$$3\text{CaCO}_3 + 2\text{H}_2\text{PO}_4^- \rightarrow \text{Ca}_3(\text{PO}_4)_2 + 2\text{H}_2\text{O} + 2\text{CO}_2\uparrow \quad \text{Insoluble}$$

### Bacteria (PSB)

*Bacillus polymixa*

*Bacillus subtilis*

*Bacillus megaterium*

*Bacillus circulans*

*Pseudomonas striata*

*Pseudomonas rathonia*

### Fungi (PSF)

*Aspergillus niger*

*Aspergillus flavus*

*Aspergillus sulfurious*

*Aspergillus awamori*

*Aspergillus wentii*

*Aspergillus fomitopsis*

*Penicillium fuscum*

*Penicillium digitatum*

*Penicillium bilaii*

*Penicillium lilacinum*

## PHOSPHATE SOLUBILISING MICROORGANISM (PSM)

Fungal species are more efficient solubilisers than bacteria.

### MECHANISM OF PHOSPHATE SOLUBILIZATION

1. Production of organic acids

Lowering of pH

Chelating effect

2. Production of inorganic acid

3. Production of CO<sub>2</sub>

4. Production of H<sub>2</sub>S

5. The production of mucilage and sloughed off root cells.

### Production of organic acids

Phosphate solubilising microorganism produces various kinds of organic acids which are the main cause of solubilisation of insoluble phosphate by lowering the pH. Amount of phosphate dissolution depends up on many factors like types of organic acids (Monocarboxylic acid: Formic acid, Acetic acid,
Lactic acid, Gluconic acid; **Dicarboxylic acid**: Oxalic acid, Tartaric acid, Malic acid, Succinic Acid; **Tricarboxylic acid**: Citric acid) produced such as aliphatic, aromatic; Structures of organic acids and amount of organic acids.

## CHELATION

The nonvolatile organic acids mainly responsible for phosphate liberation in solution possibly by their chelating action. These acids were produced as a result of the decomposition of glucose in Pikovskaya's medium (1948) by *Bacillus megaterium*, *Bacillus circulans*, *Escherichia freundii*, *Penicillium digitatum* and *Penicillium fuscum*.

### Production of inorganic acid

Nitrifying bacteria like *Nitrosomonas* and *Nitrobacter* produces the nitric acids which reacts with the insoluble phosphate and convert into soluble form. The possible chemical reactions are as follows:

$$Ca_3(PO_4)_2 + 4 HNO_3 \rightarrow Ca(H_2PO_4)_2 + 2Ca(NO_3)_2$$

Similarly sulphur oxidizing bacteria like *Thiobacillus* spp. Oxidizes the sulphur containing compounds and produces SO₂ which react with water and form sulphuric acid which reacts with the insoluble phosphate and convert into soluble form.

$$Ca_3(PO_4)_2 + 2H_2SO_4 \rightarrow Ca(H_2PO_4)_2 + 2CaSO_4$$

### Production of CO₂

The CO₂ produced by microorganism and plant roots is responsible for solubilization of inorganic phosphate in soil.

$$CO_2 + H_2O \rightarrow H_2CO_3$$

$$Ca_3(HPO_4)_2 + 2H_2CO_3 \rightarrow Ca(H_2PO_4)_2 + 2CaCO_3$$

### Production of H₂S

The H₂S produced by sulphate reducing bacteria (*Desulfovibrio*) particularly in the anaerobic condition.

$$FePO_4 + H_2S \rightarrow FeS + H_2PO_4^-$$

### Exudation of organic compounds from plant root

A variety of organic substances available at the root region, which directly or indirectly influences the quality and quantity of microorganism.

1. Sugars : Glucose, fructose, sucrose, maltose.

2. Amino acids : glutamine, glycine, methionine.

3. **Organic acid**: Tartaric, oxalic, citric, butyric, Succinic, Fumaric, glycolic.

4. Fatty acids and sterols: Palamitic, stearic, oleic, linolenic, cholesterol, sitosterol.
5. Nucleotides, flavonones and enzymes: flavonones; Adenine, guanine, uridine, cytidine; phosphatase, invertase, proteinase.

Mucilage and sloughed off root cells provide an energy substrate for microbial activity in the rhizosphere which subsequently dissolve insoluble phosphorus.

Conclusions

When soluble phosphates are added to soil, it is quickly fixed into insoluble forms, which greatly reduces the efficiency of phosphatic fertilizers.

* In acidic soils, the phosphorus is fixed primarily by Fe, Al, and Mn and in alkaline soils by Ca and Mg.

* Production of organic acid in microenvironment of root zone of plant is the main cause of phosphorus solubilization.

PSF are more effective in solubilization than PSB.

* Besides concentration of organic acid, types of organic acid produced by PSM is more important.

References:

Gardner, W.K (1982) The acquisition of phosphorus by Lupinus albus L. I. Some characteristics of the soil root interface. *Plant and Soil*. **68**: 19-32.

Moghimi, A; Lewis, D.G and Oades, J.M (1978) Release of phosphate from calcium phosphates by rhizosphere products. *Soil biology and biochemistry*. **10**: 277-281.

Banik, S and Dey, B.K (1982) Available phosphate content on an alluvial soil as influenced by inoculation of some isolate phosphate solubilising microorganism. *Plant Soil*. **69**: 353-364.

Helal, H.M; and Sauerbeck, D. (1984) Influences of plant roots on C and P metabolism in soil. *Plant and Soil*. **76**: 175-182.

Kucey, R.M.N (1983) Phosphate solubilizing bacteria and fungi in various cultivated and virgin Alberta soils. *Canadian Journal of Biological Science*. **63**: 671-678.

Pikovskaya, R.I (1948) Mobilization of phosphorus in soil in connection with vital activity of some microbial species. *Mikrobiologiya*. **17**: 362-370.

Kumar, A and Maurya, B.R (2006) Efficacy of phosphate solubilising fungi on acidity and solubilisation of tricalcium phosphate in broth. *Plant Archive*. **6(1)**: 309-311.