Phosphate-Solubilizing Bacteria (PSB)

Phosphorus (P) is one of the key nutrients limiting plant growth. The amount of plant-available phosphorus in soil is relatively low because, in calcareous soils, most phosphorus tends to bind with calcium (Ca) and magnesium (Mg), while in acidic soils, it binds with iron (Fe) and aluminum (Al). This makes phosphorus less available to plants. Naturally occurring phosphate-solubilizing bacteria (PSB) can dissolve insoluble phosphorus compounds and release soluble phosphorus into the soil solution, where it can be absorbed by plant roots. By applying PSB, it is possible to reduce the use of chemical phosphorus fertilizers, improve the soil microbial community, and protect the soil environment—contributing to more sustainable agricultural practices.

Main Mechanism of Action of PSB

Currently, most legally registered PSB-based fertilizers primarily contain strains of Bacillus spp. When applied to soil, these phosphate-solubilizing microbes secrete organic acids, which help dissolve phosphorus compounds such as calcium phosphate, iron phosphate, and aluminum phosphate, making the phosphorus available for plant uptake. However, the solubilizing ability and the range of phosphorus compounds each PSB strain can act upon vary widely. Some strains can dissolve calcium-bound phosphorus compounds but not iron-bound phosphorus compounds, while others are capable of solubilizing a broader variety of otherwise insoluble phosphorus compounds.

Functions of PSB

01 Enhancing Phosphorus Availability in Soil

PSB are often applied in combination with phosphate rock powder to improve phosphorus uptake and boost crop yields. Once inoculated into the soil, PSB can enhance the solubilization of phosphate rock, superphosphate, and aluminum-bound phosphate compounds, making phosphorus more available to plants.

02 Secretion of Plant Growth Hormones

In addition to increasing phosphorus availability for root absorption, PSB can secrete plant growth hormones such as indole-3-acetic acid (IAA) and naphthaleneacetic acid (NAA). These auxin-type hormones promote plant growth. Experiments with Gerbera and lilies have shown significant positive effects from PSB inoculation.

03 Reduction in Chemical Fertilizer Use

Proper use of PSB can raise the level of available phosphorus in the soil, allowing for a 30% to 50% reduction in the use of chemical phosphate fertilizers. This reduces input costs and supports more sustainable farming practices.

04 Additional Benefits

PSB also offer multiple other benefits, such as:

• Rhizosphere protection

• Promoting root growth and improving water and nutrient uptake

• Extending root lifespan

• Neutralizing or breaking down toxic substances

• Enhancing plant autoimmune system when facing common risks

• Improving transplant survival rates under stress conditions

These multifunctional properties make PSB valuable allies in sustainable and resilient agriculture.