Plants feed from the elements dissolved in the water they absorb from the soil. Soil pH controls which elements are available to the plant. Getting to know your soil is an important factor in becoming a good gardener.
Chicago soil is slightly alkaline (7.5), so if you want to grow plants like Rhododendron, Blueberries, Astilbe, Evergreens or Hydrangeas, you can add sphagnum moss, cotton bur compost or sulfur to the root zones of the plants to lower the pH over time. It is a good idea to add peat moss to the backfill when you install these plants. A soil test will give you more information about your soil.