Testing a Claim About Fertiliser and Bean Growth
A Form 3 class at a junior secondary school in Serowe wants to find out whether adding a locally sold fertiliser increases bean plant growth. They have 24 bean seeds of the same variety, 24 identical plastic cups, a bag of garden soil from one spot, a packet of fertiliser, a ruler, a measuring jug, and a balance. The teacher suggests dividing the cups into two groups of 12: Group A gets plain soil and Group B gets soil mixed with 5 g of fertiliser per cup. Both groups are placed on the same windowsill and watered with 50 cm³ of water every second day. After three weeks the students measure the height of each plant. One student says the results will 'prove' the fertiliser works, while another wants to change the experiment so that every plant gets a different amount of fertiliser.
1. Identify the independent, dependent and two controlled variables in the students' plan, and explain why each controlled variable matters.
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The independent variable is whether fertiliser is added (and its amount, 5 g per cup). The dependent variable is the height of the bean plants after three weeks, measured with the ruler.
Controlled variables include:
- Amount of water and watering interval (50 cm³ every second day). If one group got more water, any height difference could be due to water, not fertiliser.
- Type and amount of soil from the same spot. Different soils differ in nutrients and texture, which affects growth.
- Light and temperature (same windowsill). Plants near a brighter or warmer part would grow differently.
- Seed type, number of seeds per cup, and cup size. Different varieties or crowding change growth.
Controlling these means the only planned difference between groups is the fertiliser, so a height difference can reasonably be attributed to it. Note that 12 plants per group is a reasonable sample size, and repeating the whole setup would strengthen the conclusion further.
2. The Group B plants are, on average, 2 cm taller. One student claims this 'proves' the fertiliser works. Evaluate this claim and describe one improvement to make the conclusion more reliable.
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The word 'proves' is too strong. The result is evidence, not proof, for several reasons:
- Natural variation. Individual plants differ in height even under identical conditions. A 2 cm average difference might fall within the normal spread of heights, so the students should look at the range and whether the groups overlap.
- Single trial. One experiment at one time in one place is not enough. The result could be due to chance, e.g. one group's seeds happening to be slightly larger or one cup getting more light.
- Only one fertiliser amount tested. We cannot say whether 5 g is the best amount, or whether more fertiliser might harm plants.
- Measurement limits. Heights measured to the nearest cm have uncertainty; the students should state how they measured (e.g. soil surface to growing tip).
An improvement is to repeat the whole experiment several times (or use more plants per group and randomise cup positions on the windowsill), then compare averages and ranges. This reduces the chance that a single unusual result drives the conclusion.
3. The second student proposes giving every plant a different amount of fertiliser. Explain whether this is a good idea, and how you would modify the design to find the best fertiliser amount.
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Giving every plant a different amount is not a good design for the original question, because there would be no untreated comparison group and no replication at each amount. With only one plant per amount, any difference could be due to that individual plant rather than the fertiliser.
A better design keeps the comparison fair and still tests amounts:
- Use several groups, e.g. 0 g, 2 g, 5 g, 10 g and 20 g of fertiliser per cup.
- Put several plants (e.g. 6–12) in each group, so each amount is replicated and averages can be compared.
- Keep water, soil, light, seed type and cup size the same for all groups.
- Measure height (and perhaps number of leaves) after the same time.
- Plot a graph of average height against fertiliser amount. The best amount is where average growth is highest without signs of damage (yellowing, wilting).
This design still allows a fair test and gives more useful information than a single 5 g comparison.