Bursting with huge, colorful petals, azaleas (Rhododendron spp.) offer brilliant flowers, along with deep green shrubbery. Enjoying filtered light, these decorative plants need specific soil conditions to produce the large flowers and glowing leaves they are known for; their growth can be stunted or lack blooms if the soil pH level is not within a certain range. Acidic soil is the azaleas’ prime need for healthy growth.
The best pH range for an azalea is between 4.5 and 6.0. This range is just below the neutral level of 7.0, making the soil only slightly acidic. One way to keep the azalea within the peak range is to use a pH meter. Periodically check the soil’s pH level so that you can amend the soil as needed. In general, rich and fertile soil is needed to create acidic conditions. For example, a wooded area with constant organic matter decomposing onto the soil creates acidic conditions that allow the azalea to flourish.
If soil has an overly low pH level, such as 3.5, azaleas will show a stark difference in growth compared to previous years. Although the plant will not lose any color or blooming capabilities, the shrub’s spread will be stunted until the pH level is amended. Ground limestone spread across the soil’s surface is a safe and effective way to balance the pH level within the optimum range. However, azaleas are known for their shallow root systems; using hydrated limestone, instead of ground, can damage the roots by burning the sensitive tips.
The azalea’s deep green foliage is due to the chlorophyll working diligently for photosynthesis processes. However, if the azalea’s soil conditions lean toward alkaline, or above 7.0, the foliage will begin to turn yellow. As it becomes more alkaline, lack of iron within the soil impedes chlorophyll production. Ferrous sulfate works well to lower the soil pH level. However, it is imperative to keep any aluminum-based soil amendments away from the azalea, because this element is harmful to the shrub.
Other Soil Amendments
Alkaline soils can be amended with other substances for a healthy pH range. In particular, pine bark offers a safe way to lower the pH level while supplying a moisture retaining mulch. Fertilizing azaleas can also help the pH level. Choosing an acid-forming fertilizer with a slow-release mixture helps retain acidity while adding key nutrients. However, amending soil through constant fertilizing is not the best choice; azaleas do not need excess fertilizer in their environment. As an alternative, tilling ammonium sulfate or iron sulfate into the soil helps the pH remain acidic.