It is important your potting soil has the correct pH for your plants. Different plants prefer different pH levels. Some prefer acidic soils (lower pH), and some prefer alkaline soils (higher pH). If the pH level of your soil isn't in the appropriate range for the plants you are growing, they may die. If your soil pH is too low, you can add lime to raise the pH level.
Test the pH level of your soil with a pH meter. If the pH is already high, you don't want to add any lime to the soil.
Figure out how much lime you need to add to your soil to raise the pH level to the appropriate level. See chart in the first resource.
Mix the lime with the soil and water the soil until it is moist.
Check the pH level of the soil to verify it is now in the appropriate range. If the pH is still too low, add a little more lime. If the pH went too high, lower it with sulfur.
Monitor the pH of the soil periodically to ensure it stays in the correct range. Continue to add lime as necessary if the pH level falls too low.