Designing and Carrying Out a Plant Growth Inquiry

From "I wonder" to evidence

You've now met many factors that affect plant growth: light, water, nutrients, temperature, soil, hormones. Knowing that they matter is a start. Scientists ask how much they matter, and they find out with carefully designed investigations. In this lesson you'll learn to design a fair, reliable plant growth inquiry, the way the curriculum asks you to, and then carry one out at home over about three weeks.

Step 1: a testable question

A good question names one factor to change and one thing to measure:

• Weak: "How does fertilizer affect plants?" (Which fertilizer? Which effect?)
• Strong: "How does the concentration of an all-purpose fertilizer (0, 1, 2 and 4 g/L) affect the height of radish seedlings after 21 days?"

Other good factors from the curriculum: nutrient quantity, light quantity (distance from a lamp, hours per day) or quality (coloured filters), temperature, and water retention or percolation (soil type, watering amount).

Step 2: a hypothesis with a reason

Use "If ..., then ..., because ...": If the fertilizer concentration increases, then the seedlings will grow taller, up to a point, because more nitrogen is available for proteins and chlorophyll; but at the highest concentration growth will decrease, because salty soil water makes it harder for roots to absorb water. A hypothesis must be testable and based on science you know.

Step 3: plan the variables

• Independent variable: fertilizer concentration (0, 1, 2, 4 g/L).
• Dependent variable, with an operational definition: height from the soil surface to the tip of the tallest leaf, measured with a ruler to the nearest 0.1 cm on day 21. (Even better: dry mass, which measures all growth, but it destroys the plants.)
• Controlled variables: seed variety and batch, pot size, type and amount of soil, volume and timing of watering, light (same window or lamp), temperature, planting depth, number of seeds.
• Control group: 0 g/L (plain water), for comparison.
• Replicates: at least 5 plants per group, so one unusual plant can't decide the result. Randomly assign pots to positions, and rotate them, so no group always gets the best spot.

Step 4: carry it out safely and record carefully

Write a numbered procedure someone else could follow. Label everything. Record raw data in a table as you go, with units in the headings. Note anything unusual (a broken stem, a pot that dried out) in an observations column.

Safety: wash hands after handling soil and fertilizer; keep fertilizer away from children and pets; follow the label; don't use pesticides.

Step 5: analyse and conclude

• Describe the trend: height increased from 6.1 cm (0 g/L) to 9.1 cm (2 g/L), then fell to 7.1 cm at 4 g/L.
• Compare with the hypothesis: the results support it: growth rose with nutrients up to an optimum, then dropped at a high concentration.
• Calculate a percent change to show the size of an effect: from 0 to 2 g/L, (9.1 − 6.1) ÷ 6.1 × 100% ≈ 49% taller.
• Be careful with claims: you only tested four concentrations, so the true optimum could be anywhere between about 1 and 4 g/L. One species, one soil and one season limit how far you can generalize.

Step 6: evaluate errors and bias (A1.8)

• Random errors (vary from plant to plant): reading the ruler at a slight angle, natural variation between seeds. Reduce them with more replicates.
• Systematic errors (push all results one way): a ruler that starts 2 mm from its end; always measuring the "best-looking" plant first. Reduce them with careful technique and checking equipment.
• Bias: if you expect the fertilizer to work, you might water the fertilized plants a little more without meaning to. Using measured volumes, random positions and, if possible, having someone else measure without knowing which group is which (a blind measurement) reduces bias.

Step 7: communicate (A1.11)

A lab report usually has: title, question, hypothesis, materials, procedure, data (tables and graphs), analysis, conclusion, evaluation (errors and improvements) and references.