If you are attempting to grow plants that require soil with a pH more on the acidic end of the spectrum, then you may need to take steps to acidify your soil. In general, most plants can do fine with a mid-level pH that leans slightly toward the acidic side of the scale (somewhere around 6.0 to 7.5), however there are certain plants that require a high acidity in order to thrive. For these types of plants, a soil that is too alkaline will limit their ability to make use of certain nutrients in the soil, which in turn will limit their growth.
The pH scale is a way of measuring how alkaline or acidic soil is. It runs from 1 to 14, with 7 considered a neutral pH. Anything below 7 is acidic, while anything above 7 is alkaline. If you are unsure what the pH level of your soil is, you can do a soil test in order to find out. There are many plants that prefer acidic soil, which means they will simply do better with a lower pH. However there are also many plants that will not grow at all without acidic soil. A few common examples of types of plants that need soil with a low pH are blueberries, pin oaks, rhododendrons, and azaleas.
Ideally, it is best to acidify soil before planting. There are a number of ways to go about this. If your soil is well draining, you can simply add organic matter that is known to acidify as it breaks down, like peat moss or oak leaves. However, if your soil is clay based, this method is not advisable. The alternative is to use elemental sulfur or iron sulfate. Another possibility is cottonseed meal, which can fertilize your soil as well as acidify it.