To add acidity to rhododendrons' soil, apply Holly-Tone during the initial planting, prior to the flowering season in the spring and again in autumn before the plant enters its winter dormancy. Home Guides explains that one determines the amount of Holly-Tone to use based upon the size of the plant: Use 1 cup for the first 3 feet of branches and 2 additional cups for each subsequent foot.
Rhododendrons, like other species in its genus, require acidic soil with a pH between 5 and 5.5, which is more acidic than the natural soil in many areas. When initially transplanting a rhododendron, dig a whole around three times as large as the plant's root ball. Transplant the rhododendron to the hole, and surround it with a mixture of Holly-Tone, the soil and humus. Gardeners working in an area with poor soil can use purchased soil. After ensuring that the soil mixture offers proper drainage, cover the newly planted area with mulch.
In the spring, remove the mulch, and apply the Holly-Tone in a circle around the plant. Determine the radius to cover based on the farthest-reaching branches. Use at least 1 cup of Holly-Tone. Reapply the mulch and water thoroughly. Repeat the process in the fall, but use half the amount of Holly-Tone that is used in the spring.