By the end of this lesson
- Explain why fertilisers increase crop yield.
- Compare natural and artificial fertilisers.
- Evaluate the environmental risks of fertiliser use.
BIG QUESTION
Why do crops need fertiliser if they make their own food?
Photosynthesis makes glucose, but plants need minerals such as nitrogen to make proteins and chlorophyll.
Minerals are removed by harvesting
Plants absorb mineral ions from the soil. When crops are harvested, the minerals in them are removed from the field rather than returned by decomposition.
fertiliserA substance added to soil to provide mineral ions, such as nitrate, phosphate and potassium, to increase crop yield. replace these minerals. Common fertilisers supply nitrogenAn element plants absorb as nitrate (or ammonium) ions to make amino acids and proteins for growth; a lack causes stunted growth and often yellowing of older leaves. (as nitrate or ammonium), phosphateMineral ions that supply plants with phosphorus, used to make DNA (nucleic acids), ATP and cell membranes; a lack can cause poor root growth. (as phosphate) and potassiumAn element plants absorb as potassium ions, needed for enzyme activity and water balance (including stomatal function). (NPK).
Why minerals increase yield
Nitrate is needed to make amino acids and proteinA large molecule made of chains of amino acids, used for growth and repair and to make enzymes, antibodies and some hormones. for growth. Magnesium is needed for chlorophyllThe green pigment inside chloroplasts that absorbs light energy and transfers it to chemical energy in photosynthesis., and phosphate for DNA and cell membranes. Potassium helps enzyme activity and water balance.
With enough minerals, plants grow larger leaves, photosynthesise more and produce more crop. Adding fertiliser only increases yield if a mineral is limiting.
Natural and artificial fertilisers
natural (organic) fertilisersFertilisers made from animal manure or compost that release mineral ions slowly as they are decomposed. such as manure and compost release minerals slowly as they decompose and improve soil structure. Their mineral content varies.
artificial (inorganic) fertilisersManufactured fertilisers that contain known amounts of soluble mineral ions, such as nitrate, phosphate and potassium. contain known quantities of soluble minerals that act quickly, but are made using energy and are easily leached if overused, which can cause eutrophication (5.17).
Natural and artificial fertilisers
Both supply minerals but behave differently.
| Feature | Natural (manure, compost) | Artificial (NPK) |
|---|---|---|
| Mineral content | Variable and unknown. | Precise and known. |
| Speed of release | Slow, as it decomposes. | Rapid, as soluble ions. |
| Soil structure | Adds organic matter and improves structure. | No organic matter added. |
| Leaching risk | Lower but still possible. | Higher if overused. |
Language that earns precision
- fertiliser
- A substance added to soil to supply mineral ions for plant growth.
- NPK fertiliser
- Fertiliser containing nitrogen, phosphorus and potassium.
- mineral ion
- An inorganic ion, such as nitrate, needed in small amounts for plant growth.
Common misconceptions
Claim: Fertiliser is plant food.
Correction: Plants make their own food by photosynthesis; fertilisers supply mineral ions.
Claim: More fertiliser always means more yield.
Correction: Above the amount needed, yield plateaus and excess is wasted or leached.
Claim: Natural fertilisers cannot cause pollution.
Correction: Manure can also leach minerals and cause eutrophication if overused.
Match the answer to the command word
Name a mineral and its use (nitrate → protein; magnesium → chlorophyll) and link to growth.
Compare yield benefits with costs and leaching risk.
Recall, apply, then justify
Explain how adding nitrate fertiliser can increase the yield of a wheat crop.
Hint
Absorption, protein, growth.
Suggested marking points
- Nitrate ions are absorbed by the roots (by active transport).
- Nitrate is used to make amino acids / proteins.
- Proteins are needed for growth, so the plants grow more and yield increases.
Give one advantage and one disadvantage of using manure instead of artificial fertiliser.
Hint
Soil structure; known content.
Suggested marking points
- Advantage: improves soil structure / releases minerals slowly / cheap where available.
- Disadvantage: mineral content unknown or variable / slower acting.
A farmer doubled the nitrate fertiliser applied, but yield increased by only 3% (illustrative data). Explain this result and its possible environmental consequence.
Planning hint
Limiting factors and leaching.
What a strong design includes
- Nitrate was probably no longer the limiting factor.
- Another factor, such as light, water or another mineral, limited growth.
- Plants could not use all the extra nitrate.
- Excess soluble nitrate can be leached into rivers.
- This can cause eutrophication.
- The extra cost was not justified by the yield gain.