Phosphorus is an essential element in DNA, RNA, ATP, cell membranes, bones, and teeth. The phosphorus cycle describes how phosphorus moves through rocks, soil, water, sediments, and living organisms. It matters because plant growth often depends on how much usable phosphate is available in soil or water.
Unlike carbon and nitrogen, phosphorus does not have a major atmospheric step, so its movement is usually slow and closely tied to geology and water flow.
Most phosphorus begins locked in phosphate minerals in rocks. Weathering releases phosphate ions into soil and water, where plants and algae take them up and animals get phosphorus by eating plants or other animals. Decomposers return phosphate to soil and water when organisms produce waste or die.
Some phosphate washes into lakes and oceans, settles into sediments, and may eventually form rock again over long geologic time.
Key Facts
- Phosphorus is commonly used by organisms as phosphate, PO4^3-.
- Weathering releases phosphate from rocks into soil and water.
- Plants absorb phosphate through roots, and animals obtain phosphorus through food.
- Decomposition returns organic phosphorus to inorganic phosphate in soil and water.
- Runoff can carry phosphate into aquatic ecosystems, where excess phosphate may cause algal blooms.
- There is no major gaseous phosphorus reservoir, so the phosphorus cycle is slower than many other biogeochemical cycles.
Vocabulary
- Phosphate
- Phosphate is an ion containing phosphorus and oxygen, written as PO4^3-, that plants and algae can absorb.
- Weathering
- Weathering is the breakdown of rocks and minerals by water, wind, acids, temperature changes, or living organisms.
- Sedimentation
- Sedimentation is the process in which particles, including phosphate-containing material, settle and build up in layers at the bottom of water bodies.
- Limiting nutrient
- A limiting nutrient is a substance in short supply that restricts the growth of organisms in an ecosystem.
- Eutrophication
- Eutrophication is the over-enrichment of a body of water with nutrients, often causing rapid algal growth and low oxygen levels.
Common Mistakes to Avoid
- Adding a large atmospheric phosphorus stage is wrong because phosphorus has no major gaseous reservoir under normal Earth surface conditions.
- Treating phosphorus movement as fast is wrong because much of it is stored in rocks and sediments, which change over long time scales.
- Assuming all fertilizer phosphorus is absorbed by crops is wrong because some phosphate binds to soil particles or is carried away by runoff.
- Confusing phosphorus with nitrogen is wrong because nitrogen cycles through the atmosphere as N2, while phosphorus mainly cycles through land, water, organisms, rocks, and sediments.
Practice Questions
- 1 A farmer applies 60 kg of phosphate fertilizer to a field. If 25% is lost in runoff during a storm, how many kilograms of phosphate leave the field?
- 2 A lake receives 8 kg of phosphate per week from runoff. If algae remove 3 kg per week and 2 kg per week settle into sediments, what is the net weekly increase of phosphate in the water?
- 3 Explain why phosphorus is often a limiting nutrient in freshwater ecosystems and why adding too much phosphate can disrupt the ecosystem.