Fertilisers and eutrophicationAQA A-Level Biology: Revision notes
Section 1
Why fertilisers are needed
In a natural ecosystem, minerals such as nitrate and phosphate ions are recycled: dead plants, dead animals and faeces are decomposed by saprobionts, which return the ions to the soil for plants to take up again.
In farming this cycle is broken. When a crop is harvested, or when livestock are sold, the minerals in their biomass are removed from the field. If nothing is added, the concentration of nitrate and phosphate in the soil falls and crop yields fall with it.
Farmers therefore add fertilisers to replace the nitrate and phosphate that are lost, so that mineral ions do not limit the growth of the next crop.
Always link the need for fertiliser to removal: harvesting or selling livestock takes minerals out of the ecosystem and they are not returned by decomposition.
Section 2
Natural and artificial fertilisers
Natural fertilisers are organic materials such as farmyard manure, slurry, compost and crop residues. The nitrogen and phosphorus are mostly in organic compounds, so saprobionts must decompose them before the ions are available. Release is therefore slow and the composition varies. They also add organic matter, which can improve soil structure.
Artificial (inorganic) fertilisers are manufactured, for example ammonium nitrate and NPK compounds. They contain a known proportion of nitrate, phosphate and other ions in a soluble form, so plants can take them up quickly and the farmer can apply an exact amount.
The soluble ions are also the problem: they are easily washed out of the soil.
Do not say that artificial fertilisers 'give plants more nutrients'. Say that they supply soluble ions in a known amount that plants can use immediately.
Section 3
Leaching
Leaching is the washing of soluble mineral ions, especially nitrate, out of the soil by rainwater draining through it. The ions end up in groundwater, streams, rivers and lakes.
Leaching is more likely when:
- too much fertiliser is applied, so the crop cannot take it all up
- fertiliser is soluble, as artificial fertilisers are
- heavy rain falls soon after application
- the land is sloping or the soil is sandy and drains quickly
Leached nitrate is a waste of money for the farmer and can pollute drinking water.
Section 4
Eutrophication: the sequence
Eutrophication is the effect of excess mineral ions, mainly nitrate and phosphate, in water. The sequence is:
- Nitrate and phosphate leach from fertilised land into a lake or river.
- The mineral ions are no longer limiting, so algae grow rapidly, forming an algal bloom on the surface.
- The surface algae absorb the light, so submerged plants and algae cannot photosynthesise and die.
- Saprobionts, such as aerobic bacteria, decompose the dead plants and algae and use up oxygen in aerobic respiration. The oxygen concentration of the water falls (the biochemical oxygen demand rises).
- Fish and other aerobic organisms die from lack of oxygen, so biodiversity falls.
The fish do not die because of the nitrate itself, and the algae do not use up the oxygen. They die because saprobionts respiring aerobically use up the oxygen.
Section 5
Evaluating fertiliser use
Fertilisers raise yields and make farming productive, but the benefit must be balanced against the environmental harm.
- Rate of application: beyond a certain rate extra fertiliser gives little extra yield (diminishing returns) but a greater risk of leaching.
- Timing and place: applying fertiliser in dry weather, in the growing season and away from watercourses reduces leaching.
- Natural or artificial: natural fertilisers release minerals slowly and recycle waste, so they leach less, but their composition is variable and they are bulky. Artificial fertilisers are precise and fast acting but are more easily leached.
When evaluating, use data from the question (for example percentage increase in yield for each extra amount of fertiliser) and finish with a justified recommendation.
In a 'suggest' or 'evaluate' question, give one benefit and one environmental drawback, then state a clear conclusion.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Fertilisers and eutrophication
- A farmer grows wheat on the same field every year. At harvest the grain is sold off the farm and the straw is baled and removed. Soil tests show that the concentrations of nitrate ions and phosphate ions in the field fall each year.Explain why nitrate and phosphate concentrations fall in this field but would not fall in an undisturbed woodland.2 marks
- A freshwater lake lies next to arable fields. In spring the farmer spread fertiliser on the fields and there was heavy rain the following week. Over the next month, a green layer of algae spread across the lake surface. By midsummer, submerged plants had died and many fish were found dead.Explain why the fish died after the submerged plants and the surface algae died.2 marks
- A farmer grows potatoes in a field that slopes down towards a stream. She is deciding how much nitrogen fertiliser to apply. In a trial she applied ammonium nitrate at four rates and recorded the mean yield of potatoes: no fertiliser gave 28 tonnes per hectare, 100 kg per hectare gave 41 tonnes per hectare, 200 kg per hectare gave 47 tonnes per hectare and 300 kg per hectare gave 48 tonnes per hectare. She can also spread manure from a neighbouring farm.Suggest why a heavy downpour is more likely to cause nitrate to leach into the stream if she has just applied ammonium nitrate than if she has just applied manure.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).