Even though we here at eNature encourage students to do their own research, I can tell you that the general thinking is that salts found in Miracle Gro disrupt the biological activity of the soil. Search for information on the importance of soil health as a basis for sustaining vigorous plant growth. Explore why worms and micro-organisms are important to soil health and what conditions they like. Look up the ingredients in Miracle Gro and learn about their effect on living organisms. All this should lead you to developing some interesting experiments. Good luck!