The Phosphorus Cycle

25 min

The element with no gas

Carbon and nitrogen can travel round the world as gases in the air. Phosphorus cannot: it has no common gas. It moves only through rocks, soil, water and living things, and much of it moves very slowly. A phosphate ion released from a rock today may spend a few years cycling between soil, plants and animals, then be washed into a lake, sink into the mud, and not return to dry land for millions of years. That is one reason why phosphorus is often the nutrient that runs short in farm soils and, when too much is added, the one that turns lakes green.

Definition

Phosphorus cycle: the movement of phosphorus, mostly as phosphate ions $\mathrm{(PO_4^{3-})}$ , between rocks, soil, water, sediments and living things. It has no gas stage.

Weathering: the slow breaking down of rocks by rain, heat, cold and living things, which releases the mineral ions they contain.

Sediment: particles and dead remains that settle on the bed of a lake or sea and, over very long times, can be pressed into new rock.

Eutrophication: the enrichment of water with nutrients such as phosphate and nitrate, causing algae to grow so much that the water loses its oxygen (you studied its steps in Senior 2).

Why living things need phosphorus

DNA and RNA: every nucleotide contains a phosphate group.
ATP: the energy carrier of every cell is adenosine triphosphate.
Cell membranes: made mainly of phospholipids.
Bones and teeth: hardened by calcium phosphate.

Plants short of phosphate grow slowly, develop poor roots and often have dull, purplish leaves.

The phosphorus cycle drawn

Figure 1: The phosphorus cycle. There is no box for the air, because phosphorus has no common gas.

Arrow	Process	Speed
1	Weathering of rocks releases phosphate into soil and water	very slow
2	Plants absorb phosphate through their roots	fast
3	Animals take in phosphate by eating plants (or other animals)	fast
4, 5	Death, faeces, urine, shed leaves and bones	fast
6	Decomposers break down the remains and release phosphate	fast
7	Runoff and erosion carry phosphate into lakes and the sea	fast to slow
8	Phosphate settles into sediments	slow
9	Sediments become rock and are lifted up by movements of Earth's crust	millions of years
10, 11	People mine phosphate rock and spread it on fields as fertiliser	fast

Worked example 1: phosphorus in bones

The mineral in bone is mainly calcium phosphate, $\mathrm{Ca_3(PO_4)_2}$ . Calculate the percentage of phosphorus in it. (Ca = 40, P = 31, O = 16.)

Relative formula mass: $3 \times 40 + 2 \times (31 + 4 \times 16) = 120 + 2 \times 95 = 310$ .
Mass of phosphorus: $2 \times 31 = 62$ .
Percentage: $\frac{62}{310} \times 100\% = 20\%$

Link to the cycle: bones decay very slowly, so a large share of an animal's phosphorus returns to the soil only long after it dies. Ground bones (bone meal) have long been used as a phosphate fertiliser.

Worked example 2: phosphate below the farms

Students tested a stream at four points: in a forest (A), just above a farmed valley (B), just below it (C) and 2 km further down (D). (Illustrative data.)

Site	A	B	C	D
Phosphate / mg per litre	0.02	0.05	0.30	0.25
Algae / thousand cells per mL	0.2	0.5	5.2	4.1

(a) By how many times does the phosphate increase from B to C? (b) Describe the link between phosphate and algae. (c) Suggest the source of the phosphate and one way to reduce it.

(a) $\frac{0.30}{0.05} = 6$ times.
(b) Where phosphate is higher, there are more algae: from B to C, algae rise from 0.5 to 5.2 thousand cells per mL, about ten times. Phosphate is often the nutrient that limits algal growth in fresh water, so adding it lets algae multiply.
(c) Fertiliser and soil washed off the fields by rain (runoff and erosion), and possibly animal waste. Farmers can apply fertiliser only when crops need it, avoid spreading it before heavy rain, and keep strips of grass or trees along the stream banks to trap soil.

Worked example 3: comparing the cycles

Explain why phosphorus returns to dry land much more slowly than carbon or nitrogen.

Carbon and nitrogen have gas stages ( $\mathrm{CO_2}$ , $\mathrm{N_2}$ ): they can be released into the air anywhere and carried by winds over the land within days.
Phosphorus has no gas stage: phosphate washed into lakes and oceans can only return to land in food (for example fish eaten by birds and people) or when sediments become rock and are lifted up, which takes millions of years.
So phosphorus lost from farm soils by erosion is not quickly replaced, and it has to be put back with manure, compost or fertiliser.

Warning

Common mistakes. (1) Drawing an "air" box or a gas arrow in the phosphorus cycle: there is no gas stage. (2) Thinking mined phosphate is unlimited: phosphate rock forms over millions of years, so the rock that can be mined is a finite resource. (3) Forgetting that decomposers are needed to release phosphate from dead matter, just as in the carbon and nitrogen cycles.

Nitrogen and Phosphorus Cycles: Always Recycle! Part 2 (CrashCourse Ecology #9). Watch the phosphorus section: why it has no gas stage and how fertiliser runoff causes algal blooms.

Tip

Memory hook: "rock to rock". The phosphorus cycle starts and ends in rock: rock → soil → plants → animals → decomposers → soil → water → sediment → rock. The short loop (soil ↔ living things) is fast; the long loop (through sediments and rock) is very slow.

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Practice: the phosphorus cycle

Give three uses of phosphorus in living organisms.
In what form do plants absorb phosphorus?
State one important difference between the phosphorus cycle and the nitrogen cycle.
Name the process that releases phosphate from rocks.
Calculate the percentage of phosphorus in $\mathrm{KH_2PO_4}$ (K = 39, H = 1, P = 31, O = 16).
A stream has 0.04 mg of phosphate per litre above a town and 0.36 mg per litre below it. By how many times has the phosphate increased?
Explain why eroded soil washed into Lake Kivu takes phosphorus out of the farm's cycle for a very long time.
A student draws an arrow from "phosphate in soil" to "air". Evaluate this diagram.

Show answersHide answers

Any three: DNA and RNA, ATP, phospholipids in cell membranes, calcium phosphate in bones and teeth.
As phosphate ions ( $\mathrm{PO_4^{3-}}$ ) dissolved in soil water, absorbed by the roots.
The phosphorus cycle has no gas stage in the air (the nitrogen cycle has $\mathrm{N_2}$ ); phosphorus cycles much more slowly; bacteria have a much smaller special role (no fixation, nitrification or denitrification).
Weathering.
$M_{r} = 39 + 2 + 31 + 64 = 136$ ; $\frac{31}{136} \times 100\% = 22.8\%$ .
$\frac{0.36}{0.04} = 9$ times.
The phosphate settles into the lake's sediments. With no gas stage, it can only return to land through food chains (fish) or, after millions of years, when the sediments become rock and are lifted up.
It is wrong: phosphorus has no common gas, so no arrow should go to the air. The phosphate in soil can go to plants (absorption) or to water (runoff and leaching).

Key points

Phosphorus is needed for DNA, ATP, membranes, bones and teeth; plants absorb it as phosphate.
The cycle has no gas stage: rock → soil → living things → decomposers → soil → water → sediments → rock.
Mined phosphate fertiliser is finite; phosphate runoff causes eutrophication.

1The element with no gas

2Why living things need phosphorus

3The phosphorus cycle drawn

The element with no gas

Why living things need phosphorus

The phosphorus cycle drawn