Science Fair: Fertilizer

Question: My son has designed a science fair experiment to compare plant growth using different amounts of fertilizer. He plans to water one plant with plain water, one with fertilizer at half strength, one with fertilizer at full strength, and one with fertilizer at double strength. Do you have any suggestions for improving the design?

Answer: Comparing plant growth at different fertilizer rates is a great idea for an experiment! However, scientists generally prefer to use several plants per treatment, rather than just one. This helps them see if the results they get show some consistency.

I suggest your son use 4 to 6 plants per treatment; if he can't accommodate 16 or 24 plants, have him scale back the number of different treatments to two or three. Then if, for example, five out of six of the fertilized plants grow faster than the plants watered with plain water, he can draw some meaningful conclusions about the effect of fertilizer on plant growth. If he uses only one plant, results could be called a fluke, or he might never know that some other variable, such as planting depth, influenced the results.