4.13B - Agriculture and food demand
Explore how fertilisers and biological control can help agriculture meet rising food demand. Connect each method to useful crop yield and weigh its benefits against environmental and practical limitations.
4.13B — Food demand and useful yield
A growing human population increases the total demand for food. Agriculture therefore needs to produce enough useful food reliably, but simply increasing the total mass of living material is not always useful. A wheat crop is grown for its grain, for example, so larger roots without more grain would not necessarily meet the demand.
Crop yield
Crop yield is the amount of useful product obtained from a crop.
Different interventions act on different limits to yield:
| Limit on yield | Possible intervention | Immediate biological effect |
|---|---|---|
| too few available mineral ions in the soil | apply fertiliser | roots can absorb more of the limiting mineral ion |
| pests eating or damaging the crop | use biological control | a natural enemy suppresses the pest population |
| the crop lacks a useful inherited characteristic | selective breeding or genetic engineering | future plants inherit a characteristic such as disease resistance |
The best choice is not automatically the one with the largest possible yield. A useful evaluation asks how large and reliable the benefit is, what biological harm could occur, whether the method is practical, and whether its effects on animals, people and access to food are ethically acceptable.
4.13B — Fertilisers and their trade-offs
Fertilisers add mineral ions to soil. If nitrate ions are limiting growth, roots absorb the added nitrate and the plant uses its nitrogen to make amino acids and then proteins. This supports the production of new cells and tissues, so the plant may produce more grain, fruit or other useful material and the crop yield increases.
This benefit has a limit. Once another factor, such as light, water or temperature, becomes limiting, adding still more mineral ions may not produce a matching increase in yield. Farmers also have to pay for the fertiliser and apply a suitable dose at a time when the crop can take it up.
Excess soluble nitrate or phosphate ions can be leached from soil by rain or carried by surface run-off into a lake or river. The added nutrients can cause rapid growth of algae. When this extra biomass dies, decomposer microorganisms respire as they break it down, lowering the dissolved oxygen concentration; aquatic animals may then be harmed or killed. This is a disadvantage of poorly managed excess, not evidence that every correct fertiliser application causes water pollution.
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