AP® Environmental Science · Unit 1 · Topic 1.6
The Phosphorus Cycle
Phosphorus cycles only through rock, soil, water, and organisms, with no gas phase to move it through the air. That slow, rock-bound path keeps phosphorus scarce, which is why it usually limits plant and algae growth.
Carbon and nitrogen both have a gas phase: CO2 and N2 move through the atmosphere fast, reaching every corner of the planet in days. Phosphorus has no stable gas form, so it never enters the air. Its only source is phosphate rock, locked into mineral structures, and the only way out is weathering: rain and acids slowly dissolve the rock over thousands of years, releasing phosphate ions (PO43-) into the soil.
Once freed, phosphate follows a simple path. Plant roots absorb it from soil water and build it into DNA, ATP, and cell membranes. Animals get their phosphorus by eating those plants, or each other. When organisms die and decompose, phosphorus returns to the soil, where it can be taken up again, or it washes into rivers and eventually settles as sediment on the ocean floor. That sediment can take millions of years to uplift back into exposed rock, so once phosphorus reaches the deep ocean floor it is essentially removed from circulation for geologic time.
Because the only input is slow rock weathering, phosphorus income is fixed and tiny compared to what living things demand. Farming breaks the local cycle: harvesting crops hauls phosphorus off the field permanently, far faster than weathering can replace it, which is why farmers add phosphate fertilizer. That same scarcity is what makes phosphorus the limiting nutrient in most lakes: algae growth stays capped by whatever phosphorus trickles in, until an outside source arrives.
Key terms
- Weathering
- The slow physical and chemical breakdown of rock that releases phosphate into soil and water.
- Limiting nutrient
- The nutrient in shortest supply relative to need, which caps how much growth an ecosystem can support.
- Eutrophication
- Excess nutrient input to water causing algae overgrowth, followed by oxygen depletion as the algae decompose.
- Phosphate (PO43-)
- The dissolved ionic form of phosphorus that plants absorb from soil or water.
- Sediment
- Settled material on a lake or ocean floor where phosphorus can be stored for very long geologic timescales.
Watch it explained
- How it worksWalk me through phosphorus moving from rock to soil to plant to ocean sediment.
- Common mistakeWhy isn't phosphorus recycled fast enough to keep up with what farming removes?
- Going furtherWhat happens to a stream when phosphate detergent and fertilizer both wash into it?
- Ties togetherHow does the missing gas step make phosphorus the limiting nutrient in most lakes?
Check yourself
End of topic.