Fertilisers, pesticides and biological controlSpec 5.3, 5.4
In short
Fertilisers replace the mineral ions, such as nitrate, phosphate and potassium, that crops take from the soil, so plants grow bigger and the yield rises. Pests reduce yield, so farmers control them with pesticides, which are chemicals that kill pests, or biological control, which uses a living organism such as ladybirds eating aphids.
Fertilisers
Crops take mineral ions, such as nitrates, from the soil. When a crop is harvested, these ions are removed from the field. Fertilisers replace them. They contain mineral ions such as nitrate (for making proteins), phosphate and potassium.
With more mineral ions available, the plants grow faster and bigger, so the crop yield is higher. Without enough of them, growth is poor and the leaves may turn yellow.
Fertilisers are not plant food. Plants make their own food by photosynthesis. Fertilisers supply the mineral ions plants need to make proteins and other substances.
Why pests are controlled
A pest is an organism that damages a crop, such as an insect that eats the leaves. Pests reduce the yield, because they eat the plants or spread disease, and they lower the quality of the crop. Farmers control them to protect the yield.
Pesticides
Pesticides are chemicals that kill pests, for example insecticides that kill insects.
| Advantages | Disadvantages |
|---|---|
| Kill pests quickly and effectively, so the yield is protected | Can kill harmless or useful organisms, such as bees, that are not the pest |
| Easy to apply over a large area | Can build up in food chains, so top consumers get a high dose |
| Cheap compared with the value of the crop saved | Pests can evolve resistance, so the pesticide stops working |
| Can be used as soon as the pest appears | Can wash into rivers and harm wildlife; may leave residues on food |
Biological control
Biological control uses a living organism to reduce the pest population. For example, ladybirds are released to eat aphids, or a parasite is introduced that lays its eggs in the pest.
| Advantages | Disadvantages |
|---|---|
| No chemicals, so no harmful residues on food or in the environment | Slower than pesticides and rarely removes the pest completely |
| Usually targets only the pest, so other organisms are not harmed | The control organism may become a pest itself, or may feed on other species |
| Pests do not become resistant in the same way as they do to chemicals | The control organism may not survive or may leave the area, so it needs to be managed |
An evaluation needs both sides. Give at least one advantage and one disadvantage, each explained, not just named.
Quick check
Name two ways a glasshouse can increase the yield of a crop.
Show answer
Any two from: higher temperature, added carbon dioxide, extra light, protection from pests and weather.
Why does adding carbon dioxide increase yield?
Show answer
If carbon dioxide is limiting, photosynthesis is faster, so more glucose is made for growth.
What do fertilisers supply to crop plants?
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Mineral ions, such as nitrates, phosphates and potassium.
Give one disadvantage of using pesticides.
Show answer
Any one from: kill non-target organisms, build up in food chains, pests become resistant, pollute water.
What is biological control?
Show answer
Using a living organism to reduce the population of a pest.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026