In scientific experiments, a control group is essential as it allows researchers to isolate the effects of the variable being tested—in this case, the type of fertilizer. By comparing the growth of tomato plants using natural compost against those using artificial fertilizer and those grown without any fertilizer, the student can determine the effectiveness of each fertilizer. Keeping other factors constant ensures that any observed differences in plant growth are due to the fertilizers alone, thus providing a clearer understanding of their impact on plant growth.