Fertilisers (AQA A Level Biology): Revision Note
Exam code: 7402
Natural & artificial fertilisers
In natural ecosystems, decomposition by saprobionts recycles nutrients from waste and dead organisms back into the soil
In agricultural ecosystems, crops and livestock are harvested and removed, so the nutrients contained in their biomass are not returned to the soil
This disrupts nutrient cycles and can lead to reduced soil fertility
To maintain productivity, fertilisers are used to replace lost mineral ions
Natural fertilisers
Natural fertilisers are made from organic matter, such as:
manure
compost
crop residues
sewage
Advantages of natural fertilisers | Disadvantages of natural fertilisers |
|---|---|
Release nutrients slowly over time, so nutrients are less likely to be washed away into rivers and lakes after rain | Nutrients are less concentrated, so large amounts needed |
Contain organic matter, so can improve soil structure and water retention | Nutrient content is variable and harder to control |
Artificial fertilisers
Artificial fertilisers are made up of inorganic matter in the form of powders or pellets that contain chemical compounds, e.g. ammonium nitrate
Advantages of artificial fertilisers | Disadvantages of artificial fertilisers |
|---|---|
Nutrients are concentrated and easy to apply | Are highly soluble in water so can be leached out of soil into rivers and lakes when it rains |
Precise nutrient content allows controlled dosing | Do not improve soil structure |
Environmental issues caused by fertilisers
Fertilisers are often applied in larger quantities than crops require; as a result, excess mineral ions may remain dissolved in the soil water
These mineral ions can be carried by rainwater into nearby rivers, lakes or streams; this is known as leaching
Leaching is more likely to occur:
after heavy rainfall
when using artificial fertilisers, as the inorganic ions are highly soluble
Leaching is less likely to occur with natural fertilisers as organic matter must be decomposed by microorganisms before minerals become water-soluble
Eutrophication
Leaching can lead to a process called eutrophication, which occurs as follows:
mineral ions enter water bodies, causing rapid growth of algae at the surface
This is known as an algal bloom
algae block sunlight, which prevents aquatic plants below the surface from photosynthesising
these plants, and eventually the algae, begin to die and dead organic matter accumulates
bacteria decompose the dead matter, respiring aerobically and using up the oxygen dissolved in the water
oxygen levels fall and aquatic animals such as fish and insects can no longer survive
Examiner Tips and Tricks
Avoid vague statements like “fertilisers wash away.”, but instead use the term leaching to describe the loss of minerals from the soil.
Make sure that you can describe the process of eutrophication in sequence, and that you can link cause and effect at each stage of the process.
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