The phosphorus cycle is a biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Phosphorus is an essential element for all life forms and is stored primarily in soil and sediment. The global phosphorus cycle includes four major processes:
- Tectonic uplift and exposure of phosphorus-bearing rocks such as apatite to surface weathering.
- Physical erosion, and chemical and biological weathering of phosphorus-bearing rocks to provide dissolved and particulate phosphorus to soils, lakes, and rivers.
- Riverine and subsurface transportation of phosphorus to various lakes and run-off to the ocean.
- Sedimentation of phosphorus in marine and lacustrine environments.
Phosphorus in soils generally occurs as the anions H2PO4- or HPO42-, and it reacts with calcium (Ca2+), magnesium (Mg2+), iron (Fe3+), and aluminum (Al3+). Phosphorus reactions in soil are pH dependent. In acid soils, soluble phosphorus in the soil solution reacts with Fe and Al to form low solubility Fe and Al phosphates. In calcareous soils, soluble phosphorus in the soil solution reacts with Ca to form low solubility Ca phosphates.
Phosphorus additions to soil occur due to additions of inorganic and organic (manure) fertilizer and the degradation and decomposition of organic (plant and animal) material. Export of P from soil occurs mainly through plant uptake, but it may also be exported from soil via surface runoff and erosion or subsurface loss through leaching.
Human activities have caused major changes to the global phosphorus cycle through shipping of phosphorus minerals, use of phosphorus fertilizer, and also the shipping of food from farms to cities, where it is lost as effluent.
