The phosphorous cycle is how the element phosphorous, a nutrient like nitrogen, is passed from the geosphere to the biosphere and back again. Phosphorous is typically found on Earth in the form of phosphate (PO₄^3-). Phosphorous is used by animals and plants to make cell membranes and nucleic acids (important in DNA). Additionally, phosphorous is found in the teeth and bones of animals.

Phosphorous generally moves between the geosphere and biosphere. It is found in the atmosphere in small amounts, usually attached to dust and soil particles that have been blown into the atmosphere. Most phosphorous occurs in the rocks that make up Earth's crust or ocean sediments. When rocks that contain phosphate are weathered, the phosphate that is dissolved is eroded by wind and/or water and deposited in the sediments of lakes and rivers, or in soils. Plants can then take up the phosphate from the soil and incorporated into their tissues. Animals obtain phosphorous by consuming plants or - if they are carnivores or omnivores - other phosphate-eating animals. The phosphorous is then used by the animals to create cell and body structures. Phosphate is returned to the soil through animal waste and when organisms die. Bacteria decompose this matter and release the phosphorous back to the soil, making it available once again for uptake by plants.

Sometimes the phosphorous can be buried and becomes part of the sediments and rocks. Phosphorous can be washed into the oceans when rocks weather, where it filters down through the water column and is deposited on the seafloor. Phosphate that is buried in sediments can become sedimentary rock over millions of years. When the phosphate-bearing rocks are uplifted via tectonic processes and then weathered, the phosphate is again released for use by plants and animals.