The Benefits of Using Phosphate-Solubilizing Biofertilizers: Comparison
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Chemical phosphatic fertilizers are mainly produced from phosphate rocks, a natural reserve that is depleting rapidly. These chemical phosphatic fertilizers are polluting the environment at an alarming rate as a result of injudicious application to farmlands. On the other hand, phosphate-solubilizing biofertilizers (PSBs) are often considered better alternatives to industrial phosphatic fertilizers in many ways. PSBs are microorganisms capable of solubilizing insoluble forms of phosphate into soluble plant-usable forms. This paper is written with the objective of discussing the impacts of phosphatic fertilizers and making the case for why we should shift to PSBs instead. Phosphatic fertilizers have numerous impacts on the environment (water bodies, land resources, and air), and micro- and macro-organisms, including humans. Chemical fertilizers also tend to be more expensive, especially for farmers in developing countries. On the contrary, PSBs tend to be safer and way more beneficial than their chemical counterparts in that they are environmentally friendly and cheaper options of availing plant-usable phosphorus. PSBs are also involved in other beneficial roles such as the production of phytohormones and secretion of anti-phytopathogenic metabolites. The phytohormones enhance plant growth and the metabolites render crops immunity against phytopathogens. Hence, it is vital to replace chemical phosphatic fertilizers with PSB inoculants both to prevent the irreversible impacts of chemical fertilizers and to take advantage of the numerous benefits of PSBs. 

  • phosphate-solubilizing biofertilizers
  • fertilizer
  • phosphate

1. The Role of Phosphate-SBolubilizing Biofertilizer in Cutting Crop P-Fertilizer Requirements

The phosphate-solubilizing biofertilizers (PSBs) have been proved to be economically sound alternative to the more expensive and environmentally pollutant chemical fertilizers and possess a greater agronomic utility [65,99][1][2]. Inoculation with PSB has been shown to increase the efficiency of different forms of both inorganic and organic fertilizers. PSBa increased the available phosphorus from the organic fertilizer olive residue and manure by 97.8% (28.3 mg kg−1) and 3.5% (35.14 mg kg−1), respectively [64][3]. This is compared to sole application of olive residue (14.31 mg kg−1) and organic manure (33.96 mg kg−1). Well, this reveals that PSB inoculation can reduce the amount of manure going out to farmlands, thereby preventing its impact on the environment. Though they most often ignored and the focuses are mainly on chemical fertilizers, manures are one of the prominent contributors to the greenhouse gas methane [100,101][4][5].
Bacillus FS-3 and Aspergillus FS9 inoculation resulted in phosphatic fertilizer savings of up to 149 kg P ha–1 and 102 kg P ha−1, respectively, for the same amount of strawberry fruit yield gained when 200kg P ha–1 was applied [102][6]. This shows how PSBa inoculation increases the efficiency of inorganic fertilizer, leading to a reduced fertilizer requirement and its subsequent impacts on the environment [103,104][7][8]. The same study revealed that inoculation of Bacillus FS-3 increased phosphorus uptake and promoted the uptake and concentration of other nutrients including N, K, Ca, and Fe in strawberry fruits and leaves. Thus, PSBa could actually play a role in reducing the requirement of chemical N, k, Ca, and Fe fertilizers as well [102][6].
Wang et al. [105][9] studied the role of different plant growth-promoting rhizobacteria (PGPR) (P-solubilizers, K-solubilizers, and N-fixers) in reducing chemical fertilizer doses commonly used by farmers on wheat. No significant difference was found in soil available N, P, K, as well as N, P, K uptake by the plant between PGPR combination +75% fertilizer and 100% fertilizer without PGPR. This shows that although the efficacy of it may depend on numerous other environmental factors, inoculation of PGPR projecting phosphate solubilizing ability plays a role in cutting crop fertilizer requirement.
Co-inoculation of PSB has also been shown to reduce chemical phosphorus application by 50% without any significant reduction in the grain yield of maize [106][10]. Wheat grain yields failed to significantly differ when supplied with different strains of PSBa with and without calcium phosphate fertilizer [107][11]. This implicates the possibility of avoiding the chemical fertilizer 100%. Here, it all comes down to a choice of whether to use the phosphatic fertilizer or PSB as both fertilizer options had a statistically equal effect on most of the growth parameters of the wheat crop. Well, all this implies that PSB could partly or fully replace chemical phosphatic fertilizers in promoting growth and yield of crops. Unlike chemical phosphatic fertilizers, PSB also plays a role in boosting the uptake of other important plant nutrients.

2. Phosphate-SBolubilizing Biofertilizer as Phytohormone Producers

Phytohormones are organic compounds that are produced at one part of the plant and move to other parts causing physiological responses, such as growth [108][12]. Most PSBs have been shown to also possess the ability to produce growth-promoting hormones such as indole acetic acid [109][13], cytokinin, and gibberellins [110][14]. The PSBa strain Bacillus tequilensis has been reported to secrete plant growth hormones such as abscisic acid, auxin, and gibberellins (GA1, GA3, GA5, GA18, and GA19), and its inoculation on soybean improved shoot biomass, leaf structure, and photosynthetic pigment under heat stress [111][15]. Many strains of PSBa under the bacterial genus Pseudomonas were shown produce phytohormones such as indole acetic acid [109][13] and gibberellins [112][16]. Reduction in the level of abscisic acid, as well as an increment in the jasmonic acid and salicylic acid content in the rhizosphere were reported as a result of PSB inoculation [113][17].
The prominent phytohormone produced most frequently by PGPRs including PSB is IAA. Numerous bacterial genera such as Pseudomonas, Mycobacterium, rhizobium, and Bacillus uphold the ability to produce IAA. IAA influences numerous processes of the host plant ranging from phytostimulation to pathogenesis [114][18]. Enhancement in seed germination and physiomorphological changes have been reported in the orchids that were treated with IAA-producing PGPRs such as Azospirillum brasilense and Bradyrhizobium japonicum [115][19]. PSBa isolated from aerobic rice grown in Penang Malaysia was able to produce IAA [116][20]. The same study revealed that PSB with IAA-producing abilities applied as biofertilizers has elevated root expansion through lateral and adventitious root formation, thereby increasing surface area for increased uptake of nutrients and water. Apart from regulating cellular processes, IAA also stimulates vascular bundle formation and nodule formation [117][21]. All this shows the contribution of PSBs not just as soluble phosphate providers but also as growth promoters through the probable production of phytohormones, something chemical phosphatic fertilizers cannot do, of course.

3. Phosphate-SB asolubilizing Biofertilizersa Biocontrol against Plant Pathogens

In addition to their role in availing soluble phosphorus to crops and phytohormone synthesis, most PSBs have been shown possess lethal features against several soilborne pathogens. The prominent mechanism through which PSBs do so is by secretion of metabolites such as siderophores, lytic enzymes, and the production of acids (HCN and other organic acids). These acids possess the ability to suppress the growth and survival of various phytopathogens [118,119][22][23].
Siderophores are secondary metabolites that have low molecular weight (<10 KD), produced and utilized by bacteria and fungi in iron (Fe) acquisition and as biocontrol agents [119,120][23][24]. They are produced in response to iron deficiency that normally occurs in neutral-to-alkaline pH soils [121][25]. The mechanism behind the suppression of fungal pathogens by PSBa is through the production and release of siderophores that compete for and deprive the fungus of the essential iron needed for its survival and reproduction.
Lytic enzymes produced by PSBa serves as biocontrol by attacking pathogen cell walls at different levels of its growth and development by excreting cell wall hydrolases [122,123][26][27]. Bacillus is one of the prominent PSBa capable of producing numerous lytic enzymes including amylase, esterase, lipase, protease, cellulase, pectinase, chitinase, gluconase, protease, and chitosanase, which were shown to be highly effective against different phytopathogenic fungi [124][28]. Two strains of Bacillus, B. mojavensis and B. thuringiensis were shown to be effective in inhibition of biomass (30.4%) and spore germination (33.1%) of the fungus Aspergillus niger through the production of chitinase and protease [125][29].
The simultaneous role of several other bacteria and fungi, as both phosphate solubilizers and biological controls against various plant pathogens, is further reviewed and discussed in Vassilev et al. [126][30]. All this shows that PSBs can play a role in averting the adverse impacts of chemical pesticides applied as a control against various pathogens, on soil fertility, human health, and the environment. This implies that PSBs do not only protect the environment from pollution by chemical fertilizers, but pesticides too, a role which is very crucial to sustainable agriculture and the environment.

4. Phosphate-SBolubilizing Biofertilizer as Crop Abiotic Stress Reliefers

Inoculating plants with PSBs has become a subject of interest, especially when it comes to dealing with varying abiotic stresses including, drought, salinity, metal toxicity, etc., in this era of climate change [127,128,129][31][32][33]. The mechanisms by which PSBs help plants overcome such stress include, but are not limited to, the production of phytohormones, improvement of nutrient uptake, initiation of osmolytes and antioxidant build up, downregulation or upregulation of stress-responsive genes, and enhancement of root morphology [130][34]. Addition of PSBa to the soil significantly enhanced the immobilization rate of Pb and Cd from 69.95 to 80.76% and from 28.38 to 30.81%, respectively, thereby removing heavy metal toxicity [131][35]. Jiang et al. [132][36] isolated six PSBa isolates that can grow under saline conditions of up to 1.5 M NaCl with a potential to be used as bioinoculants to protect plants against salt stress.
Inoculation of the plant Quercus brantii with two strains of PSBa (Microbacterium sp. (M.) and Streptomyces sp.), individually and in combination, significantly enhanced growth and physiological traits of seedlings under a water-stressed condition [133][37]. Inoculation of PSBa were shown to improve physiological parameters of tomato crop including the proline, protein, chlorophyll, and relative water content under water stressed condition [134][38]. Inoculation of foxtail millet with the strains Acinetobacter calcoaceticus and Penicillium sp. efficiently ameliorated the adverse effects of drought on the crop by enhancing accumulation of glycine betaine sugars and decreasing lipid peroxidation [135][39]. Yield and several yield related parameters of green mung crop were also improved as a result of inoculation with the PSBa strain Bacillus polymixa under drought stress [136][40].

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