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Administrative data

Hazard for aquatic organisms

Hazard for air

Hazard for terrestrial organisms

Hazard for predators

Additional information

Phosphorus is usually the limiting nutrient in ecosystems: it is an essential nutrient of microorganisms, plants and animals. Phosphoric acid can be readily utilisable by aquatic/terrestrial plants and microorganisms. Phosphorus compounds are an important component of living matter and all organisms contain a certain quantity; however, excessive bioaccumulation is unlikely due to natural metabolic processes for essential nutrients (Frausto da Silva & Williams, 1991). Thanks to its high water solubility and low vapour pressure, phosphoric acid is mainly found in soil (migrating towards the groundwater table) and water compartments: there, phosphoric acid progressively dissociates affecting the pH of the receiving compartment.

Phosphoric acid progressively dissociates as pH is raised from about 3.0 to above 7.0.

H3PO4 <=> H+ + H2PO4- <=> 2H+ + HPO42 - <=> 3H+ + PO43 -; respectively pKa 2.2; pKa 7.2; pKa 12.3.

The third H+ is generally only lost in alkaline conditions. Under relevant environmental pH range (pH5 to pH8), the monovalent and divament phosphates are dominant (H2PO4 - and HPO42 -). As reported in "Acid soils of the tropics" from Robert D.Harder, because of this dissolution, when phosphorus fertilizer is placed in a band, very low pH values can be attained. The acidity will then gradually diffuse into the soil surrounding the band. According to Lindsay and Stephenson (1959), pH values as low as 1.5 can be found in a zone immediately around a fertilizer band. Natural pH can vary significantly as well as organisms sensitivity in adapting to such change. A pH range of 6 -9 is generally considered safe for aquatic organisms. Moreover, pH change due to anthropogenic release of phosphoric acid is influenced by the buffer capacity of the receiving water.


Frausto da Silva JJR and Williams RJP, The Biological Chemistry of the Elements: The Inorganic Chemistry of Life; Oxford University Press: Oxford, 1991.

Harder RD (2002) Acid soils of the tropics, An Echo Technical Note

Lindsay WL and Stephenson HF (1959) Nature of the reactions of monocalcium phosphate monohydrate in soils, Soil Sci. Soc. Am. J. 23(1): 12-

Conclusion on classification