When crops are harvested and animals are slaughtered the nutrients they took from the soil are lost.

In order to replace these lost nutrients, farmers use natural (e.g. slurry, manure and compost) or artificial fertiliser.

Tractor spreading slurry

Fertilisers mainly contain nitrates that the crops can use for growth. They can also contain calcium, needed for the production of plant cell walls and magnesium, which is needed to make chlorophyll.

If nutrients are removed but not replaced the soil will eventually lose its ability to grow crops.


Eutrophication is a type of water pollution caused by the addition of sewage or fertiliser.

The sewage or fertiliser run-off increases the nitrate concentration of the water and has a negative effect on the aquatic ecosystem.

Dead fish in polluted water


  1. Sewage or fertiliser run-off increases the nutrient concentration of the water.
  2. Extra nutrients cause increased growth of the aquatic plants/algae – this is known as an algal bloom.
  3. The algal bloom covers the water’s surface killing any plants below the surface as light and oxygen cannot reach them.
  4. Algae also die as the nutrients run out.
  5. Aerobic bacteria decompose the dead plants.
  6. Bacteria use up oxygen for respiration.
  7. Fish and other organisms die from a lack of oxygen.

Controlling the use of fertiliser and storing manure and slurry more securely can reduce eutrophication.

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