Drainage:  Rhododendrons and azaleas thrive in moist, well-drained soils high in organic matter.  Rhododendrons and azaleas have shallow fine hair-like roots.  These roots do not tolerate water-saturated soil conditions but do require moist soils.  Poor drainage and wet soils are problems often associated with heavy clay and compacted soil.  To test drainage, dig a hole about 10 to 12 inches deep and fill it with water. Then after it drains, fill it with water again and see how long it takes to drain.  If the hole drains within an hour you have good drainage.  If the water has not drained out of the hole within one hour, the soil is poorly drained and you must correct the drainage problem before planting.  Planting in raised beds is the best solution in heavy soils.  Raised beds are built on top of the native soil to a depth of 12" to 18" and held in place with timbers or stones.  Raised beds may require watering during the summer as they dry out quickly.

Aeration: Aeration is important for healthy growth of rhododendrons and azaleas.  Beneficial microorganisms in soils require air for respiration and metabolism.  Vital microbial and fugal activity, such as decomposition of organic matter that make nutrients available for plants, nitrification and beneficial mycorrhizal associations, depend on the oxygen present in soil.  Poor aeration results in the development of toxins in soil.  Plants in heavy soils with poor aeration often become chlorotic from malnutrition.  To improve soil aeration the best amendment is organic matter, with compost being an excellent choice.  Soil bacteria acting on compost produces humus that binds with soil particles - that forces tightly packed particles apart; improves drainage and allows the fine roots of rhododendrons to more easily penetrate through the soil.  In coarse sandy soil, it lodges in the large pore spaces and acts as a sponge, so the soil stays moist longer.  Amending heavy clay soils is not recommended.  Eventually the organic amendments break down and the soil reverts to its original condition.  As mentioned above, in gardens with heavy clay soil the best approach for growing rhododendrons and azaleas is using a raised bed atop the native soil.

Acidity:  Rhododendrons and azaleas prefer acidic soils having a pH between 4.5 to 6.0.  Rhododendrons and azaleas will let you know if the pH is not correct.  If the leaves turn yellow between green veins then you most likely have a pH problem.  If this occurs, a soil test is suggested for exact recommendations on adding a soil amendment to the soil in order to adjust the pH.  Materials commonly used to lower soil pH are wettable sulfur or ferrous sulfate.  Do not use aluminum sulfate to acidify the soil; it is toxic to rhododendron and azalea roots.  Avoid planting azaleas near concrete sidewalks, driveways or foundations that may leach out lime which raises the pH.  In rare cases, the pH may be too low.  This is equally serious and must be rectified.  The recommendation is usually to use dolomitic limestone.

Soil Mix:  About half of the planting medium should be organic material.  Combinations of sphagnum peat moss, pine or fir bark fines, compost, and aged, chopped leaves should be worked into the soil to a depth of about 12".  Oak leaves are excellent.  Make sure there are no walnut tree roots or leaves in the soil.  All parts of walnut trees are toxic to rhododendrons and azaleas.  Pine bark is particularly good because substances in the pine bark are thought to inhibit fungi that cause root rot.  Adding a large amount of organic matter will raise the bed, which will improve the drainage and aeration of the soil.  Inorganic materials that may also be added to soil include perlite, vermiculite or small diameter lava rock.  Time-tested mixes for growing rhododendrons and azaleas in raised beds include the following:
• Rhododendron Species Botanical Garden mix: two-thirds coarse sand mixed with one-third medium bark, no soil
• Everett Farwell rhododendron mix: 80% medium fir bark mixed with 20% small (⅛"-¼" diam.) crushed lava rock, no soil
• Holden Arboretum mix: five parts coarse sand and one part medium pine bark to four parts loamy soil.
Up to ¼ loamy soil can be added to ¾ RSBG or Farwell mixtures.

Keep Moist:  Rhododendrons and azaleas will not survive in wet, poorly-drained soil.  Although too much water can injure the roots it is important that they receive adequate moisture, especially during the first year after transplanting.  A two to three inch layer of organic mulch surrounding the base of the plants helps retain moisture and helps control weeds.

More information about soils can be found in the following Journal ARS articles:

  Tips For Beginners: Good Soil Promises Rhody Success by Harold Greer
  Tips for Beginners: How To Adjust Acidity Levels in Your Soil by Fred C. Galle
  Experiences With Rhododendrons In Southern Red Clay, More About Rhododendrons In Southern Red Clay by C.A. Dewey, Jr.