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## AP®︎/College Environmental science

### Unit 1: Lesson 2

Biogeochemical cycles

# The phosphorus cycle

AP.ENVSCI:
ERT‑1 (EU)
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ERT‑1.F (LO)
,
ERT‑1.F.1 (EK)
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ERT‑1.F.2 (EK)
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ERT‑1.F.3 (EK)
Review your understanding of the phosphorus cycle in this free article aligned to AP standards.

## Key points

• Phosphorus is an essential nutrient for living organisms. DNA, RNA, and cell membranes all contain phosphorus atoms.
• Phosphorus is often the limiting nutrient in ecosystems. This means that phosphorus is the nutrient in shortest supply, so it puts a limit on plant growth. Phosphorus is scarce in ecosystems because plants use available phosphorus quickly, but natural processes replenish phosphorus slowly.
• Phosphorus moves between living organisms and the environment during the phosphorus cycle.
• The largest reservoirs of phosphorus are buried in rock and deep ocean sediments. When rocks and sediments weather, or wear down, phosphate ions ($\ce{PO4^3-}$) are released. These ions leach into surface water and soils where they are more accessible to plants and other organisms.
• In soil, phosphate ions can be taken in by plants and incorporated into biological molecules in the plants’ tissues. This allows phosphorus-containing compounds to move up the food chain as plants are eaten by animals, who are themselves eaten.
• Phosphorus-containing biological molecules may stay inside the bodies of organisms, or be broken down and released as waste. As the bodies and wastes of organisms break down, they become compacted into layers of soil and rock, returning phosphorus to the environment.
• Humans influence the phosphorus cycle by using animal waste and phosphate sediments to make fertilizer, detergent, and other products. When these products are released into the environment they can enter aquatic ecosystems as runoff. This can accelerate plant growth in these ecosystems, leading to harmful algal blooms.
• The phosphorus cycle is slow compared to other nutrient cycles because there is no atmospheric component to move phosphorus directly from the ocean to land.
A series of arrows indicate the movement of phosphorus from: aerosol, to food webs, to soil and water, to rocks, to ocean sediment and oceans, and from fertilizer runoff to oceans.
Phosphorus moves between living organisms and the environment during the phosphorus cycle. Image credit: “Figure 46 03 07.jpg" by CNX OpenStax, CC BY 4.0 .

## Want to join the conversation?

• Since plants use phosphorus quick, and the replenishment is slow, is there a possibility of running out of usable phosphorus, especially since it gets compacted back into the earth when plants die?