**4. Diversity of phosphate-solubilising microorganisms (PSM)**

PSM are diverse and can be found in various environments such as water, and plant tissues while the soil is the key habitat [10, 12]. The population of PSMs in soil ranges between 104 and 106 g−1 of soil, and they are accumulated at the rhizosphere of plants, and those organisms are highly metabolically active [10, 24]. In the microbial profile of soil, phosphate-solubilising bacteria (PSB) were responsible for 1–50%, whereas phosphate-solubilising fungi (PSF) represent 0.1–0.5%. The population density of PSB ranged between 8 × 105 and 5.33 × 109 in the different rhizospheres of vegetable fields, with PSB accounting for 3.98% of the total population of bacteria [25]. Most of PSBs are coccus, bacillus or spirillum in shape while the bacillus is the most abundant and spirillum are rare. However, there is a high diversity of PSM in the soil. Among these microbial species, *Bacillus* and *Pseudomonas* are the foremost bacterial genera [4, 12, 24].

The number of PSM is not sufficiently high to compete with other microbial species in the rhizosphere. The profile and population of these PSM vary between ecosystems due to the influence of complex biological factors [25]. Physical-chemical factors of soil such as the soil nutrient status, moisture content, organic matter, soil pH, and soil enzyme activities affect these variations [4, 12, 24]. The diversity of PSM (**Table 1**)




#### **Table 1.**

*Major phosphate solubilizing microorganisms.*

is important for maintaining healthy ecosystems, as they play a critical role in the phosphorus cycle by making this important nutrient more available to plants [25].
