**3. Phosphate solubilization sources and mechanism of action**

Phosphate-solubilizing microorganisms technology improves saline-alkaline soil fertility and agricultural use without causing any environmental or deleterious health implication that accompanies the continuous use of synthetic fertilizers [24]. The phenomenon of P desorption by PSM usually occurs along with the drop of pH [25]. The ability of PSM to convert insoluble organic and inorganic phosphorus depends on the nutritional richness of the soil and the physiological and growth status of the organism. PSM isolated from saline-alkaline soils–soils with a high level of nutrient deficiency, or soils from extreme temperature environments, have the tendency to solubilize more phosphate than PSM of soils from moderate conditions [14].

P solubilization has been reported to be secretion of organic acids (e.g., oxalic acid, citric acid, acetic acid, succinic acid, etc.) produced by phosphate-solubilizing microbes to solubilize insoluble P by lowering of pH. The hydroxyl and carboxyl groups present in the organic acids chelating the cations (mainly calcium) bound to phosphate, leading to increased solubility and availability of mineral phosphates, to ensure the microbial growth, organic acid production, and RP solubilization of metabolizable carbon compounds [26–30].

Mechanism of action of P solubilization could be observed when a phosphatesolubilizing microbe is grown in a specialized highly alkaline medium known as Pikovskaya medium. These phosphate-solubilizing microbes are then incubated at 30°C for six days. There has been a conflicting report on the influence of temperature on phosphorus solubilization by microbes. White et al. [31] found 20–25°C as the optimum temperature for maximum microbial phosphorus solubilization while 28°C was reported by [32, 33]. In addition, others including [34–37] have recorded 30°C as the best temperature for P solubilization. [36, 38] reported P solubilization at extreme temperature of 45°C in desert soil while [39] reported solubilization at a low temperature of 10°C. The ability of these phosphate-solubilizing microbes to form halo zones when cultured on the PVK medium is a clear indication that the organism can solubilize phosphate. Later, the PSB/AMF could then be centrifuged at 5000rpm for 10min [40]. Supernatant of blended cultures was filtered through 0.20-μm syringe filters (cellulose acetate), and the organic acid could then be detected using HPLC (shimadzu SPD-M20A) with PDA detector by using methanol 5% and 0.1% TFA 95% as mobile phase with flow rate of 1ml per minute with

column particle size 5 μm, length 25 cm, and column diameter 4.6mm (Merck C18 column). The extent of P solubilization by the fungal strain could then be determined by measuring the clear zone (in mm) around the colonies by taking the halo zone formation into consideration [25, 41–44].
