The Phosphorus Cycle: Why It Depends on Rock Weathering

1.1 What is the phosphorus cycle?

The phosphorus cycle is the path phosphorus takes through rock, soil, water and living things. Phosphorus is an essential nutrient for life. Plants need it to grow. Animals get it by eating plants or other animals.

1.2 In what form does phosphorus move?

In nature, phosphorus occurs as the phosphate ion. Think of phosphate as the usable form. Rock holds it. Water carries it. Roots take it up.

2.1 How does phosphate leave rock?

Weathering leaches phosphate out of phosphate-containing rock. Runoff then sends it into rivers, lakes and the ocean. Groundwater flow and river flow also carry it to the sea.

2.2 How does it get back to rock?

In the sea, organisms take up phosphate. Their bodies and waste fall to the sea floor as sediment. This sediment forms the rock of the cycle.

Over long ages, uplift lifts that sediment onto land. Then weathering starts again. The average phosphate ion spends 20,000 to 100,000 years in the ocean.

2.3 Does rock supply everything?

Not always. In remote regions, volcanic ash, aerosols and mineral dust may also be significant phosphate sources.

3.1 Where do carbon and nitrogen enter life?

Carbon and nitrogen both have a large air store. Carbon dioxide sits in the air. Nitrogen gets into living things from the air through bacteria that fix it.

Source: OpenStax Biology 2e, biogeochemical cycles.

3.2 Where does phosphorus come from?

The phosphorus route runs through rock, soil and water. The OpenStax description of the cycle lists weathering, runoff, groundwater, rivers, ocean food webs and sediment. It names no step through air.

That is the idea behind the UPSC question on weathering. Rock weathering is the main natural source of phosphate. It is not the main source for carbon or nitrogen.

4.1 Why does life need phosphorus?

Phosphorus is a major part of nucleic acids and phospholipids. As calcium phosphate, it also makes up the supporting parts of bones. Two more roles show how basic it is.

4.2 What does ATP have to do with phosphorus?

Adenosine triphosphate (ATP) is the cell’s main energy currency. Its name means adenosine triphosphate. It has an adenosine backbone with three phosphate groups attached.

OpenStax says ATP powers the majority of energy-requiring cellular reactions. Without phosphate, a cell cannot make it.

4.3 What about membranes and bone?

• Membranes. A phospholipid has a modified phosphate group on the third carbon of its glycerol backbone.
• Bone and enamel. They share the same mineral, hydroxyapatite.

5.1 Is phosphate rock spread evenly?

Apatite is the most abundant phosphate mineral. It holds more than 95% of the phosphorus in Earth’s crust.

But the rock is not spread evenly. The US Geological Survey (USGS) is the US government’s earth-science agency. It lists Morocco with 50,000 million tonnes of reserves out of a world total of 73,000 million. That is about 68%. The next largest, China, has 3,400 million tonnes.

5.2 Are we about to run out?

No. USGS says world resources of phosphate rock are more than 300 billion tons. It sees no imminent shortage of phosphate rock.

The real worry is a different one. USGS says there are no substitutes for phosphorus in agriculture. When most reserves sit in one country, supply can be disrupted even if the total is large.

6.1 How much rock does India have?

USGS lists India with reserves of 31 million tonnes. That is tiny next to Morocco’s 50,000 million. India’s best-known mine is at Jhamarkotra near Udaipur, Rajasthan.

Mining began there in 1968. Mongabay-India says the area has the largest phosphate reserve in the country. RSMML, a Rajasthan government enterprise, has leased 1,370.37 hectares.

6.2 How does India fill the gap?

India imports. In 2024, it took 1.88 million tonnes of diammonium phosphate (DAP). DAP is a common phosphate fertiliser. It also took 250,000 tonnes of nitrogen-phosphorus-sulphur (NPS) fertiliser from Saudi Arabia alone. World Fertilizer reports this from Argus data.

In July 2025, IPL, Kribhco and Coromandel signed a five-year deal with Saudi Arabia for 3.1 million tonnes a year, mostly DAP.

6.3 How does the government keep prices low?

The Cabinet approved Nutrient Based Subsidy rates for Rabi 2025-26. They cover phosphatic and potassic fertilisers. The expected cost is about Rs 37,952.29 crore. Farmers get 28 grades, including DAP, at subsidised prices.

7.1 Can wasted phosphorus be reused?

Yes. Sewage carries phosphorus that can be caught and sold again. One method makes struvite, a crystal that contains phosphorus, nitrogen and magnesium.

7.2 Is this only a lab idea?

No. On 17 June, a 900 tonne a year struvite unit was opened at the Amersfoort sewage works in the Netherlands. It treats sewage from 300,000 people-equivalent. The aim is to recover 40% of the works’ input phosphorus as struvite.

Ostara’s Pearl process combines phosphorus and nitrogen with magnesium. The product is sold as fertiliser.

8.1 What is nutrient pollution?

The US Environmental Protection Agency (EPA), the US government’s environment regulator, says nutrient pollution means too much nitrogen and phosphorus in air and water. Both nutrients help algae and water plants grow. Too much makes algae grow faster than ecosystems can handle.

8.2 Why does it end in low oxygen?

When the algae die, they use up oxygen as they decompose. Dead zones are areas where aquatic life cannot survive because there is little or no dissolved oxygen. OpenStax adds that fertiliser runoff and sewage drive this.

8.3 Is there a real example?

Yes. USGS says the main driver of Lake Erie’s harmful algal blooms is high phosphorus loading. Much of it comes from the Maumee River watershed. The target for March-July Maumee loads is 186 tonnes of dissolved reactive phosphorus. That is a 40% cut from 2008 loads.

9.1 What did UPSC ask?

The other options were the nitrogen, carbon and sulphur cycles. Weathering of rock is the main natural source only for phosphorus.

9.2 Which mix-ups come up?

• Air or rock. Phosphorus comes from rock. Carbon and nitrogen come from the air.
• Shortage or concentration. USGS sees no imminent shortage. Supply sits mostly in Morocco.
• One cause. Nitrogen matters too in blooms.
• Rock or fertiliser. Fertiliser is made from the mined rock.
• Sewage waste or resource. Struvite recovers it.