News release: Preferred Soil pH for Azaleas and Blueberries

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640
660-663-3232, bakert@missouri.edu

Release Date: October 15, 2015

Headline: Preferred Soil pH for Azaleas and Blueberries

Most of the crop and ornamental plants that we deal with prefer a slightly acid soil.  We measure this with a pH meter, when a soil test is performed.  On the pH scale, 7.0 is neutral.  A pH higher than 7.0 means that you have an alkaline soil, and less than 7.0 indicates an acid soil.  Commonly, the ideal range for most crop and ornamental plants is around 6.0 to 6.5.

If the pH is too far out of the preferred range for a given plant, the plant will be stressed.  Nutrients will not be taken up out of the soil properly, even if they are present.  If the pH is too far out of range, the plant may not survive.

That is one reason why soil tests are important.  They not only tell you the nutrient status of the soil, they tell you what the pH is.

I have occasionally run across some folks who indiscriminately add lime every year to “sweeten” the soil.  This is fine, if the soil is acid, but if your soil is alkaline, you will get yourself into trouble by adding additional lime.  A soil test will tell you if you need to add lime or not.

Some plants are acid-loving, and do not follow the preference of other plants in needing a soil pH of 6.0 to 6.5.  Two common acid-loving plants are azaleas and blueberries.

Both of these plants really like an acid soil environment.  Azaleas prefer a soil pH between 5.0 and 5.5.  Blueberries like it even more acid, from 4.7 to 5.2, although they may tolerate it a little higher.

It’s best to get a soil test performed at least six months to a year before planting azaleas or blueberries, so that you can adjust your soil pH, if needed.  This is done by adding elemental sulfur.  As rain falls on the sulfur, a small amount of sulfuric acid is created which lowers the soil pH.  Because this takes time to happen, we really do like to see sulfur applied well before planting.  I’ve seen too many azaleas and blueberries planted before the soil pH was adjusted, and the plants were very stressed and not doing well.

You will need to test your soil frequently after planting as well.  Established plantings can be stressed by high pH, and with time the soil pH will probably rise, and you will need to add more sulfur. 

I have seen several instances of stressed azaleas through the years.  In one case, lace bugs had severely attacked an azalea plant.  In addition to controlling the lace bugs, I told the homeowner that she should get a soil test.  The test came back with a pH of 7.3, which is far too high for azaleas.  I believe that the stressed plants were more attractive to the lace bugs.

I have also seen many examples of stress in blueberries due to high soil pH.  Often, the planting had been established for several years by the time I saw them. The plants were struggling, and some were dying. When soil tests were performed, they almost always showed a high pH.  I always stress to the grower the importance of keeping up with soil tests, to make sure that the pH is low enough for blueberries.

If you would like additional information on growing azaleas or blueberries, please give me a call and I will be glad to help.