Testing a Tomato 'Growth Booster'
A student wants to test a company's claim that 'Growth Boost' makes tomato plants produce more fruit. She has two tomato seedlings of the same variety, two pots, potting mix and a bottle of Growth Boost. She plants one seedling in a 15 cm pot and the other in a 20 cm pot, puts the 15 cm pot on the windowsill and the 20 cm pot on a shaded shelf, waters the windowsill plant with 200 mL of diluted Growth Boost every second day and the shelf plant with 100 mL of plain water every day. After eight weeks the windowsill plant has 14 tomatoes and the shelf plant has 6, and she concludes that Growth Boost works.
1. List the differences between the two plants other than the Growth Boost, and state for each whether it could plausibly affect fruit number.
Read the worked answer
| Difference | Could it affect fruit number? |
|---|---|
| Pot size, 15 cm vs 20 cm | Yes - root space and potting-mix volume limit growth and fruiting. |
| Position, windowsill (bright) vs shaded shelf | Yes, strongly - tomatoes need full sun; light drives photosynthesis and fruit set. |
| Water volume, 200 mL vs 100 mL | Yes - water stress reduces flowering and fruit size. |
| Watering frequency, every 2 days vs daily | Yes - affects soil moisture and root health. |
| One liquid is diluted booster, the other plain water | That is the intended variable, but it is tangled with the water differences above. |
Any one of these could produce a 14-versus-6 difference on its own. With all of them varying at once, the experiment cannot isolate the effect of the booster.
2. Rewrite the plan as a fair test. State what must be kept the same, what must be changed, and how many plants she needs.
Read the worked answer
Changed on purpose (independent variable): whether the plant receives Growth Boost or not - the booster diluted as instructed, versus the same volume of plain water.
Kept the same for every plant:
- Variety, age and starting size of seedling
- Pot size and type of potting mix, filled to the same level
- Position (all on the same bench by the window, rotated regularly so none is permanently shaded)
- Total liquid volume and watering frequency, applied at the same time of day
- Temperature and any feeding regime other than the booster
- Length of growing period and the date fruit is counted
Number of plants: at least 5 per group, ideally 8–10, so that natural plant-to-plant variation averages out. Two plants give one comparison; one unusually large or small plant could flip the result entirely.
Measured: total mass of ripe fruit per plant (a number count alone can be gamed by many tiny fruits).
3. Suppose she repeats the test with ten plants per group and still finds more fruit with the booster. What further check would make the conclusion stronger, and why does it matter?
Read the worked answer
She should run a control for the booster's other ingredients - for example a third group given a solution identical in everything except the active ingredient (or a 'blank' of the same colour and dilution). If the booster is mostly a fertiliser, the effect may simply be extra nutrients, not anything special about the product.
Why it matters: the claim being tested is about the product, and a company claim of the form 'makes plants produce more fruit' is only meaningful if the effect is not just 'we watered and fed the plants'. She should also repeat the whole experiment at a different time of year or in a different room - a single eight-week run in one spot may reflect that particular season. Only when the effect appears across repeats and against a proper control does the conclusion become strong enough to recommend the product.