Work backwards: from the symptom, recall what the mineral builds, and decide which shortage would cause it. Support the answer with numbers and compare with a control.
This lesson is part of plant nutrition. The roles come from explaining why plants need mineral ions.
How do I reason from a symptom?
- Describe the symptom and where it appears.
- Recall which minerals build the affected part.
- Choose the most likely mineral and give the reason.
- Compare with the control.
Worked example: a culture-solution experiment
An original experiment: seedlings grew for 3 weeks in culture solutions with different minerals missing.
| Solution | Height gain (cm) | Leaf colour |
|---|---|---|
| Complete (control) | 12.0 | Dark green |
| Without nitrate | 4.5 | Pale yellow, older leaves first |
| Without magnesium | 9.0 | Yellow between the veins |
| Without phosphate | 5.0 | Dark green, small leaves |
Without nitrate. Claim: nitrate is needed for growth and chlorophyll. Evidence: height gain is 4.5 cm against 12.0 cm in the control, and leaves are pale yellow. Reason: nitrate is used to make proteins and chlorophyll, so growth and colour are reduced.
Without magnesium. Evidence: the leaves are yellow between the veins, but height gain is 9.0 cm. Reason: magnesium is needed for chlorophyll, so the leaves lose colour and less photosynthesis takes place.
Why is the control needed?
Without the control at 12.0 cm, a student cannot tell whether 4.5 cm is a large loss or normal growth. The control gives the comparison.
The mistake that over-claims
A student sees yellow leaves and writes “The plant lacks nitrate.” The data for the magnesium solution also shows yellow leaves, so colour alone does not identify the mineral.
The improved answer uses two pieces of evidence: the pattern of yellowing (older leaves first or between veins) and the height gain compared with the control.
A self-check question
A plant in a solution without phosphate has a height gain of 5.0 cm against 12.0 cm in the control, with small dark green leaves. Which mineral role explains the stunted growth?
Answer
Phosphate is used to make DNA, ATP and membranes, which are needed for new cells and energy transfer. Without phosphate, fewer new cells form, so growth is stunted. The leaves stay green because chlorophyll is still made, as nitrate and magnesium are present.
Practise the answer frame with the claim evidence revision desk, and record slips in the mistake log tool. Next, see comparing nutritional adaptations.
If symptom questions still feel like guessing, SPM Biology one-to-one tuition lets a teacher rehearse the frame with you.