Phosphorus is an essential nutrient for life. Too much in the wrong place is an environmental contaminant.

Phosphorus (chemical symbol P) is predominantly found as phosphate-based compounds (solid form) and is cycled through the lithosphere (the rigid outer surface of the earth), hydrosphere (all the water on the earth’s surface), and biosphere (the regions of the surface and atmosphere of the earth occupied by living organisms).

How is phosphorus cycled on my farm?

In the environment, the weathering of rocks and minerals releases phosphorus in a soluble form where it is taken up by plants, and subsequently transformed into organic compounds. On a farm, P this is typically added through the use of phosphate fertilisers. Unlike with nitrogen, the atmosphere does not play a significant role in the cycling of phosphorus.

In soil, phosphate is absorbed by iron oxides, aluminium hydroxides, clay surfaces, and organic matter particles, and becomes incorporated in the soil particle. When soil is lost by runoff, it takes the phosphorus with it. Organic phosphate is phosphorus that has been incorporated into plant or animal tissue (e.g., seeds, leaves).

In natural waters, phosphorus typically occurs in both inorganic and organic forms. 

A diagram showing the movement of Phosphorus though the soil, water, and atmosphere.
The phosphorus cycle is a slow process which involves the key steps of weathering and erosion, absorption by plants and animals, and return to the environment via decomposition and sedimentation. This figure shows the reservoirs of where phosphorus is stored in red, the actors that use it (i.e., us, animals, microorganisms), and the processes that phosphorus undergoes in the environment (yellow).

Why is too much phosphorus an issue?

Humans have had a significant impact on the phosphorus cycle due to a variety of human activities, specifically the use of fertiliser, the distribution of food products, and excessive nutrient concentrations in waterways (artificial eutrophication). P fertilisers increase the phosphorus levels in the soil and are particularly detrimental when lost into local aquatic ecosystems. When phosphorus is added to water at a rate typically achieved by natural processes, it is referred to as natural eutrophication. A natural supply of phosphorus over time provides nutrients to the water and serves to increase the productivity of the ecosystem. However, when levels of phosphorus are too high, the overabundance of plant nutrients enables the excessive growth of algae. A rapid increase in the population of algae in an aquatic ecosystem forms an algal bloom. Blooms can reduce the amount of light and oxygen available to other aquatic life. Some types of algae may be toxic if ingested or can be an irritant to skin and eyes.

Dark brown seaweed-looking sludge on a river stone.
In rivers, potentially toxic benthic algae generally form brown or black mats that grow on rocks in the riverbed. Benthic algae differ from harmless bright green algae, which often form long filaments.
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How is phosphorus measured in water?

When a water sample is analysed for phosphorus, different techniques are applied to isolate the various forms.

Phosphorus is typically analysed and reported as:

  • Total Phosphorus (TP)
  • Dissolved Reactive Phosphorus (DRP)
  • Particulate Phosphorus (PP) (PP = TP – DRP)
A flow diagram of how Phosphorus tests are combined to get 'Total Phosphorus (TP)'
The forms of phosphorus in water (organic and inorganic). Phosphorus is measured as Dissolved Reactive Phosphorus (DRP) and Total Phosphorus (TP). Particulate phosphorus (PP) is calculated.

For more information on water quality contaminants see Nitrogen, Sediment, and Microbes.