Azaleas have shallow roots, and prefer moist, well-drained soil with a pH between 4.5 and 6.0. If in doubt, you can submit soil samples through your County Extension Service to have the pH and nutrient levels tested, usually for little or no cost. The test results will also have information about changing the pH if needed, which will be appropriate for your soil type and climate.
Check how well your soil drains by filling a hole with water. If it is empty in a few hours, it drains well. If not, you have so-called "heavy" soil. In heavy soil, don't plant azaleas into small holes filled with amended soil, since the holes will hold water and be death traps for the plants. Instead, you can build raised beds, or amend a large area, or "plant high", ranging from an inch or so above ground level to planting on top of the ground, and mounding amended soil around the root balls.
Azalea roots need access to both moisture and oxygen. In wet soil, the roots will grow closer to the surface to get oxygen. In dry soil, the roots will go deeper to get moisture. In reasonably well drained soil, the roots of evergreen azaleas tend to stay in a well defined mass of fine feeder roots from the surface down to around 12 inches deep. They seldom extend beyond the width of the plant, and usually stay within a foot or so of the trunk. Roots of deciduous azaleas may range deeper and further from the trunk in their search for water.
Sand and clay are the soil extremes, and neither are suitable for growing azaleas. Sand has large particles with large spaces between them. Gravity quickly drains excess water from it, there is rather little surface area to hold a film of moisture for roots, and there is plenty of space for oxygen. Clay has small particles with a lot of surface area. Gravity takes a long time to drain excess water from clay, there is a lot of surface area to hold moisture, and almost no space for oxygen. The ideal soil, called loam, has a mixture of large and small particles. That mix allows excess water to quickly drain out by gravity, and has a lot of surface area moisture and spaces for oxygen. Sand and clay can be improved by mixing them together, to provide the desirable mix of particle sizes. They can also be amended by adding organic matter and humus (decomposed organic matter), as much as half by volume, to improve their fertility.
About an inch of rainfall each week is ideal for azaleas. Supplemental water may be needed if the rainfall is much less than that, especially if there is no rainfall for extended periods. Especially watch for signs of dryness on recent transplants, on azaleas which have been planted high, and on azaleas which are in full sun or locations exposed to drying winds. Fortunately, drooping leaves show the need for water well before the plant dries out completely, and watering it slowly and deeply usually restores it within hours.
Azalea Society of America
