A pH scale is used to measure the acidic and alkaline level of soil. The acidic or alkaline level can indicate what will grow well in the soil if the soil is not amended. Plants like azalea and rhododendron thrive in acidic soil. Sometimes, you may want to adjust pH level to create a specific outcome. For instance, if you want your hydrangeas to produce blue blooms and the soil pH level is at 7.0, then you will need to make the soil more acidic because blue hydrangeas require an acidic pH level of approximately 5.5.
Get your soil tested to see if the soil is currently acidic or alkaline. You can contact your local Cooperative Extension Office for places in your area where you can get your soil tested. See "Resources" (below) to locate the Cooperative Extension Office in your area. You may also find a soil pH test kit at your local home and garden center or nursery. The soil pH test kits are not as accurate a laboratory-evaluated soil test, but can still give you a good idea of your soil's pH level.
Dig the area where you will be amending the soil down to a depth of 8 to 12 inches.
Mix in 1 to 2 inches sphagnum peat. Because of the expense of sphagnum peat (around $4 for 8 quarts; $10 for 2 cubic feet), you will want to reserve this process for a small bed. For a large area, use granular sulfur (around $8 for a 5-pound bag) at a rate not to exceed 2 pounds per hundred square feet. As an example, to reduce a pH of 7.0 in loamy soil down to a pH of 5.0, you would apply 4/10 of a pound of sulfur for 10 square feet of soil. Closely read the manufacturer's instructions for the proper amount of sulfur to mix with your soil to achieve the desired pH level.
Test the soil pH again in six months (or according to the product manufacturer's instructions) to see if additional amendment is needed.