Soil pH indicates the acidity or alkalinity of soil. All plants grow best within a specific range, but the ideal range varies greatly from one plant to another. Peat moss is one of the many tools gardeners have at their disposal to alter the pH of their soil.
A pH between 6.6 and 7.3 is neutral. Anything higher is alkaline, and anything lower is acidic. Most garden vegetables grow best in neutral to slightly acidic soil. Peat moss has a pH of 3.0 to 3.5.
Because peat moss is so acidic, adding it to soil will lower the soil's pH. This is useful in areas where the soil is too alkaline.
Peat moss is sometimes used as a growing medium by itself for container plants. Be aware of peat's pH, and add nitrogen fertilizer of elemental sulfur to your potting mix to raise the pH if necessary.