When you measure the pH of soil, you are determining its alkalinity or acidity levels based on a scale from 0 to 14, with 7 considered neutral. A soil pH above 7 is considered alkaline, while below 7 is acid. Rhododendrons require acid soil in order to survive and do best with a pH between 5.0 and 5.5. To make soil more acidic for rhododendrons, you often need to add amendments to the existing soil.
Take a soil pH test. Home testing kits are available at most large gardening centers, or you can have the soil professionally analyzed at your county cooperative extension office.
Dig up the planting bed, 18 inches deep and 30 inches wide.
Pour agricultural sulfur or iron sulfate onto the bed and blend it into the existing topsoil, using the gardening fork. The amount of material to apply depends upon the current pH of the soil and the rates listed on the amendment package. Once the materials are mixed in it is time to plant.
Post-Planting pH Adjustment
Check the soil pH in the rhododendron bed periodically (two or three times per year), especially if the plant is failing to thrive.
Lower the soil pH by adding 1/2 pound of sulfur for every 100 square feet of bed. This will lower the pH one point (for instance, from 7.0 to 6.0). Sprinkle it on the soil around the plant and spread out to the drip line (the soil at the widest part the plant). Scratch it in to the top inch or two of soil.
Raise the soil pH if it is below 5.0 by adding ground limestone around the plant. Mix it in to the top 2 inches of soil, using the gardening fork or a hoe.
Water the rhododendron as you usually do.
Check the pH of the soil around the rhododendron one month after adding the amendments. Make a second application if needed.