Biofertilizer Potential of Indigenous Rhizosphere Bacteria: Phosphate Solubilization and Indole Acetic Acid Production by Isolates from Ocimum gratissimum in Benin City, Edo State, Nigeria

Phosphorus is vital for plant growth, yet much of it in soil becomes locked in insoluble forms, limiting availability to crops. Farmers often rely on chemical phosphate fertilizers to overcome this challenge, but their overuse contributes to environmental problems such as eutrophication, soil degradation, and disruption of beneficial microbes. A sustainable alternative lies in phosphate-solubilizing bacteria (PSB), which naturally release phosphorus and promote plant health. This study explored PSB from the rhizosphere of Ocimum gratissimum (scent leaf), a medicinal and culinary plant widely cultivated in Benin City, Nigeria. Soil samples from three farms were analyzed, yielding eighteen bacterial isolates, of which six (TA1–TA6) showed clear phosphate-solubilizing activity. These isolates were tested for their ability to lower pH, solubilize phosphate, and produce indole-3-acetic acid (IAA), a plant growth hormone. Among them, isolate TA4 stood out with the highest phosphate solubilization index (1.57 ± 0.02), efficiency (57.13 ± 0.57%), and IAA production (15.7 µg/mL). Statistical analysis confirmed that TA4’s performance was significantly higher than other isolates (p < 0.05), indicating robust phosphate-solubilizing potential. TA1 also performed strongly (PSI: 1.50 ± 0.02; PSE: 50.00 ± 0.75%; IAA: 15.0 µg/mL), with differences between TA1 and TA4 remaining statistically significant (p < 0.05). All isolates caused a measurable decline in medium pH during incubation, and ANOVA tests revealed significant variations among isolates in acidification capacity (p < 0.01). Biochemical tests identified the most effective strains as Bacillus licheniformis (TA4) and Bacillus cereus (TA1). Both demonstrated strong potential as plant growth-promoting rhizobacteria (PGPR). These findings highlight the promise of indigenous PSB from the O. gratissimum rhizosphere as biofertilizers. Their application could improve soil phosphorus availability, enhance crop productivity, and reduce dependence on synthetic fertilizers, offering a statistically validated pathway toward more sustainable agriculture.