Maarg Manthan · Topic 5.8

Phosphorus Cycle and Human Influence on It

Environment & Ecology › Nutrient Cycling (Biogeochemical Cycles) · Topic 5.8

The phosphorus cycle is the movement of phosphorus between rocks, soil, water and living things. It is a sedimentary cycle with no significant gaseous phase, so the atmosphere plays almost no part. The main reservoir is phosphate rock, such as apatite. Weathering and erosion release phosphate into soil and water, plants absorb it, and it passes through food chains. Phosphorus is a component of DNA, RNA, ATP and cell membranes, and of bones and teeth. Decomposers return it to the soil, but much is carried by rivers to the sea, where it settles in sediments and becomes rock again over millions of years, so the cycle is slow and open. Mining phosphate for fertiliser and detergents and sewage have greatly increased flows, causing eutrophication of lakes and coasts.

5 min read · UPSC Prelims · Environment & Ecology · Nutrient Cycling (Biogeochemical Cycles)

At a Glance

Type Sedimentary
Reservoir Phosphate rock
Gas phase Almost none
Used in DNA, ATP, bones
Pollution effect Eutrophication
Exam link UPSC Prelims, Environment & Ecology: Nutrient Cycling (Biogeochemical Cycles)

Where Does It Fit?

This is the last topic of the macro on nutrient cycling. It contrasts with carbon and nitrogen because it lacks a gaseous phase, and leads into biodiversity in the next macro. Previous topic: Sulphur Cycle and Human Impact on It. Next topic: Definition and Types of Biodiversity (Genetic, Species, Ecosystem).

What Are the Steps of the Phosphorus Cycle?

Step	Process
Weathering	Rain and erosion break phosphate rocks, releasing phosphate ions into soil and water
Uptake	Plant roots, helped by mycorrhizal fungi, absorb phosphate; algae absorb it from water
Food chain transfer	Herbivores and carnivores obtain phosphorus from plants and prey
Decomposition	Bacteria and fungi convert organic phosphorus in dead matter and waste back to phosphate
Run-off to water bodies	Phosphate is carried by rivers into lakes and the sea
Sedimentation	Phosphate settles in bottom sediments and forms new rock over millions of years
Uplift	Geological uplift brings phosphate rocks to the surface again to complete the cycle

How Do Humans Influence the Phosphorus Cycle?

Mining for fertiliser: phosphate rock is mined and processed into fertilisers like single super phosphate and diammonium phosphate
Detergents and sewage: phosphate in detergents and human waste reaches water bodies
Livestock manure: adds large phosphorus loads to run-off
Eutrophication: phosphorus is often the limiting nutrient in fresh water, so small additions cause algal blooms
Consequences: oxygen loss, fish kills, toxic cyanobacteria, loss of biodiversity and water treatment costs
Scarcity: phosphate rock is finite and concentrated in a few countries, causing supply and price risks
Import dependence: India imports much of its phosphate fertiliser or raw material; check the latest data
Recycling: recovering phosphorus from sewage, manure and food waste can reduce mining

What Else Should You Remember?

Apatite: calcium phosphate mineral and the main phosphate ore
Guano: bird droppings, once a major phosphate source
Morocco: holds the largest phosphate rock reserves
Peak phosphorus: a concept about limits on phosphate supply
ATP: adenosine triphosphate, the energy currency of cells
DNA and RNA: phosphate forms the backbone
Bones and teeth: calcium phosphate gives hardness
Phospholipids: form cell membranes
Mycorrhiza: fungal partnership that helps plants gain phosphorus from soil
Soil phosphorus: binds to iron, aluminium and calcium, so little is available at any time
Phosphate solubilising bacteria: used as a biofertiliser to release locked phosphate
Cultural eutrophication: human-driven nutrient enrichment, as against natural ageing of lakes
Lakes affected: many Indian lakes suffer from sewage nutrients and weed growth, such as water hyacinth blooms
Dal Lake: affected by nutrient load, with restoration efforts under way
Detergent rules: many countries limit phosphate in detergents
Wastewater treatment: can remove phosphorus by chemical precipitation or biological methods
Struvite: a mineral recovered from sewage as a slow-release fertiliser
Fertiliser subsidy: India’s nutrient based subsidy scheme sets rates for phosphorus and other nutrients; check the latest rates
Hypoxic zones: coastal dead zones linked to river phosphorus and nitrogen
Link to previous: see Sulphur Cycle and Human Impact on It
Link to nitrogen: see Nitrogen Cycle and Human Impact on It
Link to water pollution: see Water Resources and Water Pollution macro
Caution: phosphorus has no major gas phase, though small amounts move as dust and aerosols
Slow cycle: a phosphorus atom can take millions of years to return from ocean sediments to land
Exam hint: repeat that phosphorus is a sedimentary cycle and the key limiting nutrient in freshwater eutrophication
Phosphorus in oceans: coastal upwelling returns phosphate from deep water to the surface, supporting rich fisheries such as those off Peru
Fertiliser phosphorus use: most mined phosphate goes into fertilisers, and crops take up only a part of what is applied, the rest accumulating in soil or running off
Legacy phosphorus: phosphate stored in agricultural soils can keep leaking into water for decades after fertiliser use stops
Algal toxins: blooms of cyanobacteria can produce toxins harmful to livestock, fish and people
Water treatment: blooms add taste and odour problems and raise treatment costs for city water supplies
Precision agriculture: soil testing and soil health cards help farmers apply the right amount of phosphate
Buffer strips: vegetated strips along streams trap phosphorus before it reaches water
Constructed wetlands: reed beds can remove nutrients from sewage and farm run-off
Organic sources: compost, farmyard manure and bone meal return phosphorus to soils

Why Does It Matter for the Exam?

Type: sedimentary
Reservoir: rocks
Effect: eutrophication
Current affairs link: nutrient based subsidy and phosphate supply
Common errors: giving phosphorus an atmospheric reservoir or confusing it with nitrogen

As an analytical point, phosphorus is both essential and non-renewable, yet it is wasted on a large scale through run-off. Efficient fertiliser use and recovery from sewage and manure can protect water bodies and secure future food production.

Why Is This Topic Important?

Type Sedimentary
Role DNA, ATP
Problem Eutrophication

What Came Next?

The next macro covers biodiversity basics and classification. Next topic: Definition and Types of Biodiversity (Genetic, Species, Ecosystem).

Key People and Terms

Apatite Phosphate ore
Phosphate Usable form
Eutrophication Nutrient overload
Guano Bird droppings
Mycorrhiza Root fungus partner
Struvite Recovered fertiliser mineral

Exam Corner

Points to Remember

Phosphorus has no gas phase.
Reservoir: phosphate rock.
Part of DNA, ATP and bones.
Often limits freshwater growth.
Excess causes algal blooms.

Do Not Confuse With

Phosphorus and nitrogen cycles Sedimentary versus gaseous
Phosphate and phosphorus Ion form versus element
Eutrophication and bloom Nutrient enrichment versus its visible result

Memory Hook

Rock to root to rock again, no air route; too much on the water makes green soup.

Mains Angle

Questions ask for type, role and impact. These points help.

Cycle Steps
Human Fertiliser
Remedy Recycling

A Question You May Face

An original practice question, not a past paper question.

“Explain the phosphorus cycle and discuss how human use of phosphate fertilisers and detergents affects aquatic ecosystems.”

How to Answer

Cycle Steps.
Human Inputs.
Effect Eutrophication.
Remedy Recycle.

GS Relevance

Prelims: sedimentary nature, uses of phosphorus, eutrophication, limiting nutrient. GS Paper 3: agriculture and pollution.

Frequently Asked Questions

What is the phosphorus cycle?

The phosphorus cycle is the movement of phosphorus through rocks, soil, water and living things. Weathering releases phosphate from rocks, plants absorb it, animals obtain it from food, decomposers return it to the soil, and run-off carries it to the sea, where it settles and forms new rock over millions of years.

Why is the phosphorus cycle called a sedimentary cycle?

It is called a sedimentary cycle because the main reservoir is phosphate rock in the Earth's crust, and the element returns to the rocks as sediments in the sea. There is no major gas form of phosphorus, so the atmosphere plays almost no role, unlike in the carbon and nitrogen cycles.

Why is phosphorus important to living things?

Phosphorus is a component of DNA and RNA, which carry genetic information, of ATP, which carries energy in cells, of phospholipids in cell membranes, and of calcium phosphate in bones and teeth. Plants need it for growth, root development and flowering, so it is an essential nutrient in fertilisers.

How does phosphorus cause eutrophication?

In many lakes and rivers phosphorus is the nutrient that limits plant growth, so extra phosphate from fertiliser run-off, detergents and sewage triggers rapid growth of algae. When the algae die and decay, bacteria use up the oxygen in the water, causing fish deaths and loss of biodiversity.

Is phosphorus a renewable resource?

No. Phosphate rock is a finite resource that formed over millions of years and is mined in a few countries. Because the natural cycle is so slow and much phosphorus is lost to the sea, saving it by efficient fertiliser use and recovering it from manure and sewage is important for long-term food security.

PYQ Practice — Statement Analysis

1 The phosphorus cycle is a sedimentary cycle.

True

The reservoir is rock.

2 The atmosphere is the main reservoir of phosphorus.

False

It is phosphate rock.

3 Phosphorus is a component of DNA and ATP.