Final answer:
The best control for an experiment determining the effect of fertilizer on plants would be plants with no fertilizer. This allows for a baseline comparison of plant growth. Consistency in variables such as water, sunlight, and soil type across groups is vital for accurate results.
Step-by-step explanation:
Control Group in Botany Experiments
To determine the effect of fertilizer on plant growth, implementing a control group is essential. In botany experiments like the ones described, the best control would be plants with no fertilizer (Option A), as they serve as a baseline to compare the effects of the fertilizer on plant growth. Keeping other factors such as the amount of water, the amount of sunlight, and the type of soil consistent across all experimental groups ensures that the independent variable (the presence or absence of fertilizer) is the only factor affecting the dependent variable (plant growth).
In experiments such as the one comparing the growth rate of plants with and without detergent in the soil, the inclusion of a control group is done correctly, but the experiment is flawed because the control pots are placed in shade, a variable that could significantly impact plant growth. To improve this experimental design, the placement regarding sunlight should be consistent for both test and control groups.
Variables that should be controlled in plant growth experiments can include the amount and quality of water, soil type, sunshine, and environmental temperature. It is critical to manage potential factors that could influence the outcome, like soil temperature, water loss due to evaporation, and the effect of insects.