Phosphorus solubilization

What is phosphorus solubilization?

Phosphorus is essential for energy transfer, root growth and seed formation in plants, and it is taken up only as dissolved phosphate ions in the soil water. Most of the phosphorus in soil is not in that form. Phosphorus solubilization is the process of converting bound phosphorus into soluble phosphate that roots can absorb. It happens naturally through soil chemistry and microbial activity, and it is the mechanism behind phosphate-solubilizing biological products.

Why soil phosphorus becomes unavailable

Soluble phosphate reacts quickly with other elements in soil:

• In acidic soils, it binds with iron and aluminum to form compounds of very low solubility.
• In alkaline and calcareous soils, it binds with calcium to form calcium phosphates.
• In organic matter, a large share of soil phosphorus is held in organic forms that must be broken down before plants can use them.

This is why phosphate applied as fertilizer is quickly converted to less soluble forms. The FAO notes that the crop recovers only a small share of applied phosphorus in the season it is applied, commonly cited as 10 to 25 percent, though much of the rest remains in the soil and becomes available slowly over later years.

How microorganisms solubilize phosphorus

Phosphate-solubilizing bacteria and fungi use several mechanisms, often together:

• Organic acids. Microbes release acids such as gluconic and citric acid, which dissolve calcium phosphates and bind the iron, aluminum and calcium that hold phosphate.
• Acidification. Microbial activity lowers pH in the immediate root zone, making mineral phosphates more soluble.
• Enzymes. Phosphatase and phytase enzymes release phosphate from organic compounds in soil organic matter.

Mycorrhizae contribute differently: their fine fungal threads extend far beyond the root and collect phosphate from a larger volume of soil.

Phosphate-solubilizing products

Biological products that contain phosphate-solubilizing microorganisms are sold as biofertilizers or biostimulants, depending on the market and claims. They are intended to improve the use of both native soil phosphorus and applied fertilizer, part of improving nutrient use efficiency. Their effect depends on the soil’s phosphorus reserves, pH, organic matter and moisture, and on whether the organisms establish in the root zone, so results vary by field.

Why it matters

Phosphorus fertilizer is made from mined phosphate rock, a finite resource, and phosphorus that runs off fields contributes to algal blooms in lakes and rivers. Making better use of phosphorus already in the soil can reduce the need for new applications and the risk of loss.

Phosphorus solubilization at Locus

Locus’s Pantego is a biological for specialty crops that Locus describes as improving phosphorus availability in the root zone, converting bound phosphate into plant-available forms.

Frequently asked questions