When crops are harvested and animals are slaughtered the nutrients they took from the soil are lost.
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.
Controlling the use of fertiliser and storing manure and slurry more securely can reduce eutrophication.