Phosphorus is required for nucleic acids, ATP, phospholipids and many cellular reactions. In ecosystems, it cycles mainly through rocks, soils, water and organisms rather than through a large atmospheric gas reservoir.

Most environmental phosphorus occurs as phosphate in minerals or dissolved inorganic forms.

Weathering—the physical and chemical breakdown of exposed rock—releases phosphate into soils and water. Primary producers assimilate inorganic phosphate and incorporate it into organic molecules. Consumers obtain phosphorus by eating other organisms.

When organisms excrete waste or die, decomposition returns organic phosphorus to inorganic phosphate that can be reused.

Some phosphate is transported by runoff into lakes and oceans. It can settle into sediments and eventually become incorporated into new rock. Over long timescales, tectonic uplift can raise buried rock toward Earth's surface, where weathering exposes its phosphorus again.

A simplified pathway is

phosphate rock
   ↓ weathering
inorganic phosphate in soil/water
   ↓ biological uptake
organic phosphorus in biomass
   ↓ consumption
consumers
   ↓ waste/decomposition
inorganic phosphate
   ↓ sedimentation
sedimentary rock
   ↓ uplift and weathering
inorganic phosphate

Because the major geological return pathways are slow, biologically available phosphorus can be scarce even when total phosphorus in rocks is large. This helps make phosphorus an important limiting nutrient in many ecosystems.