Phosphorus Cycle

Phosphorus Cycle Definition

The phosphorus cycle is the process by which phosphorus moves through the lithosphere, hydrosphere, and biosphere. Phosphorus is essential for plant and animal growth, as well as the health of microbes inhabiting the soil, but is gradually depleted from the soil over time. The main biological function of phosphorus is that it is required for the formation of nucleotides, which comprise DNA and RNA molecules. Specifically, the DNA double helix is linked by a phosphate ester bond. Calcium phosphate is also the primary component of mammalian bones and teeth, insect exoskeletons, phospholipid membranes of cells, and is used in a variety of other biological functions. The phosphorus cycle is an extremely slow process, as various weather conditions (e.g., rain and erosion) help to wash the phosphorus found in rocks into the soil. In the soil, the organic matter (e.g., plants and fungi) absorb the phosphorus to be used for various biological processes.

Phosphorus Cycle Steps

The phosphorus cycle is a slow process, which involves five key steps, as shown in the diagram below and described as follows:
Simple phosphorus cycle


Since the main source of phosphorus is found in rocks, the first step of the phosphorus cycle involves the extraction of phosphorus from the rocks by weathering. Weather events, such as rain and other sources of erosion, result in phosphorus being washed into the soil.

Absorption by Plants and Animals

Once in the soil, plants, fungi, and microorganisms are able to absorb phosphorus and grow. In addition, phosphorus can also be washed into the local water systems. Plants can also directly absorb phosphorus from the water and grow. In addition to plants, animals also obtain phosphorus from drinking water and eating plants.

Return to the Environment via Decomposition

When plants and animals die, decomposition results in the return of phosphorus back to the environment via the water or soil. Plants and animals in these environments can then use this phosphorus, and step 2 of the cycle is repeated.

Human Impact on the Phosphorus Cycle

Humans have had a significant impact on the phosphorus cycle due to a variety of human activities, such as the use of fertilizer, the distribution of food products, and artificial eutrophication. Fertilizers containing phosphorus add to the phosphorus levels in the soil and are particularly detrimental when such products are washed into local aquatic ecosystems. When phosphorus is added to waters 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 that particular ecosystem. However, when foods are shipped from farms to cities, the substantial levels of Phosphorus that is drained into the water systems is called artificial or anthropogenic eutrophication. When levels of phosphorus are too high, the overabundance of plant nutrients serves to drive the excessive growth of algae. However, these algae die or form algae blooms, which are toxic to the plants and animals in the ecosystem. Thus, human activities serve to harm aquatic ecosystems, whenever excess amounts of phosphorus are leached into the water.


1. The most abundant source of phosphorus on the planet is:
A. Soil
B. Water
C. Rocks formed from the Earth’s crust
D. Algae blooms

Answer to Question #1
C is correct. All Phosphorus on the Earth is derived from rock. Weathering and the decay of organic material washes the phosphorus into the soil and water.

2. Which of the following is NOT a critical function of Phosphorus:
A. Nucleotide formation
B. Cell membranes
C. Plant and animal growth
D. All of the above
E. None of the above
F. A and C only

Answer to Question #2
D is correct. Phosphorus is a critical component of nucleotides, which are used to make DNA and RNA. In addition, phosphorus is also needed in the formation of the phospholipid bilayer observed in cell membranes. Thus, phosphorus is an essential nutrient required for both plant and animal growth.

3. Excess phosphorus caused by fertilizers leaching into water systems is dangerous because:
A. It causes algae blooms which are toxic to plants in water ecosystems
B. It causes algae blooms which are toxic to animals in water ecosystems
C. It causes algae blooms which are toxic to the animals which drink the affected water
D. All of the above

Answer to Question #3
D is correct. Excess phosphorus results in the overproduction of algae or algae blooms within affected water ecosystems. These algae blooms are toxic to both the plants and animals which live in the ecosystem, as well as the terrestrial animals which consume the water.


  • Emsley, J. (1980). The Phosphorus Cycle. The Natural Environment and the Biogeochemical Cycles: Chapter in The Handbook of Environmental Chemistry.: pp.147-167.
  • Mu D, Ruan R, Addy M, Mack S, Chen P, Zhou Y. (2017). Life cycle assessment and nutrient analysis of various processing pathways in algal biofuel production. Bioresour Technol. 230:33-42.
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