To determine which group is the control group in the experiment described, we need to identify the group that is used as a standard of comparison and is not subjected to the independent variable (fertilizer).
Based on the information provided, Harriet grew four groups of plants in identical conditions, but only one variable was changed, which is the application of different types of fertilizer. The table describing the groups is not provided in the question, so we cannot determine the exact treatments used.
However, the control group is typically the group that receives no treatment or a neutral treatment. In this case, the control group would be the group that does not receive any fertilizer or receives a standard or neutral fertilizer treatment. This group is used as a reference point for comparison with the other groups that receive different types or amounts of fertilizer.
Therefore, based on the information provided, we cannot determine which group is the control group without additional information about the treatments used in the experiment.