Fertilisers raise yield but can pollute water
Definition
Fertiliser
A substance added to soil to supply the mineral ions, such as nitrate, phosphate and potassium, that plants need for healthy growth.
- Fertilisers supply mineral ions that may limit plant growth, including nitrate ions for amino acids and magnesium ions for chlorophyll.
- Correct application can increase growth and crop yield, helping more food to be produced from the same area of land.
- Excess soluble fertiliser can be washed from soil into rivers and lakes by rain.
- The extra nitrate stimulates rapid algal growth, forming an algal bloom that blocks light.
- Aquatic plants die, decomposers multiply and their respiration removes dissolved oxygen.
- Low oxygen can kill fish and other aerobic organisms, a process called eutrophication.
Exam technique
In a eutrophication explanation, link fertiliser run-off to algal growth, blocked light, plant death, decomposition, oxygen loss and death of aquatic organisms.
Biological control uses natural enemies
Definition
Biological control
The use of a natural predator, parasite or pathogen to reduce the population of a pest, instead of chemical pesticides.
- Biological control reduces a pest population by introducing or encouraging a predator, parasite or pathogen of that pest.
- It can reduce chemical pesticide use, leave fewer chemical residues and provide longer-term control if the control organism reproduces.
- The control organism may not remove the pest quickly enough and usually does not eradicate it.
- An introduced organism may attack non-target species, spread beyond the intended area or disrupt food webs.
- The control species should be host-specific, tested in containment and monitored after release.
Example
Ladybirds can be used to reduce aphids because they feed on the pest, but the population and effects on other insects still need monitoring.
Agriculture needs a balanced judgement
- A growing population increases demand for reliable food production, but higher yield can create environmental costs.
- Fertilisers give rapid, predictable mineral supply, while biological control can reduce repeated pesticide use.
- The best method depends on crop, climate, pest, cost, speed, environmental risk and the evidence available.
- Integrated pest management combines monitoring, targeted chemicals, resistant varieties and biological controls to reduce harm while maintaining yield.
Exam technique
Compare methods using the same criteria, then make a conclusion that follows from the context rather than stating that one method is always best.
Self review
- How can nitrate fertiliser increase plant growth?
- Explain eutrophication as a sequence.
- What is biological control?
- Give one benefit and one risk of biological control.
