Duckweed, Fertilizer, and a Shady Corner
Priya wants to know whether fertilizer runoff changes how quickly duckweed, a tiny floating plant, grows. She fills six identical 500 mL jars with 200 mL of pond water and adds 20 duckweed fronds to each. Three jars receive 5 drops of liquid fertilizer; three receive none. All six jars sit in a row on the same windowsill for 10 days. Final frond counts are: fertilized 61, 58, 64; unfertilized 55, 22, 49. Later Priya realizes the jar that ended at 22 sat at the end of the row, partly behind a poster that shades it every afternoon. She also remembers filling the jars from two different pond-water containers on the first day.
1. Her teacher says this experiment cannot support a conclusion about fertilizer. Identify which result is most likely not caused by the fertilizer treatment, and explain what is probably responsible.
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The 22-frond jar is the problem. Look at the spread within each group:
- Fertilized: 61, 58, 64 - a range of only 6 fronds.
- Unfertilized: 55, 49, 22 - a range of 33 fronds, driven entirely by one jar.
A treatment effect would shift a whole group, not scatter one jar far below its own groupmates. The shaded jar received less light, so its duckweed photosynthesized less and grew more slowly. That makes light, not fertilizer, the best explanation for the 22.
The two pond-water sources are a second uncontrolled difference: different starting nutrients or microorganisms could also shift growth. Because light and water source varied alongside the treatment, any difference between groups could be caused by either one.
2. Name one specific change that would most improve the design, and state exactly what alternate explanation it removes.
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Move all jars to a position with identical light exposure (or rotate jar positions daily), and draw all the water from a single, well-mixed container.
- Equal light removes the explanation that shading, not fertilizer, caused the slow jar. If one jar must sit at the row's end, rotating positions spreads any light gradient evenly across both groups instead of concentrating it in one.
- One water source removes the explanation that different starting water caused the difference.
Adding more replicates per group (say 8 rather than 3) would also help, because a single odd jar then carries less weight in the group average. The key idea is that only variables deliberately changed should differ between groups.
3. Suppose the redesigned experiment gives fertilized counts of 60, 62, and 59 and unfertilized counts of 40, 44, and 41. State what Priya may now claim and one thing she still may not claim.
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May claim: Under these conditions - this pond water, these jars, this fertilizer, 10 days, this light level - fertilizer addition was associated with faster duckweed growth. The groups no longer overlap (59–62 versus 40–44), and the confounds have been controlled, so a causal link within the experiment is reasonable.
May not claim:
- That the same result holds in a real pond, where dilution, flowing water, other plants, and grazing animals all operate.
- That the effect holds at the concentrations found in actual runoff - 5 drops in 200 mL may be far more concentrated than field conditions.
- That fertilizer is the mechanism because of any particular nutrient; the experiment shows an effect, not which ingredient caused it.