Acid Stomach and Growing Azaleas: What Does pH Have to Do with Soil?

George Lindley Taber azalea flowers

George Lindley Taber azalea flowers

What does ‘acid stomach’ have to do with growing azaleas? Both the human stomach and soil rely on a small but critical number somewhere in the range of zero to fourteen!

This number is recognized as the pH of a material, or its hydrogen ion concentration. The human stomach acid is around 4.0 which explains our discomfort when experiencing a condition called “acid reflux.” That’s even on the acid side when growing blueberries or azaleas, as they prefer a pH of 4.8 – 5.5 or 4.5 – 6.0 respectively. To put another way, soil with a pH of the human stomach would not be a good growing medium for blueberries!

The point? Most growing things, including humans and azaleas, have a “preferred pH” or number indicating whether an acid or alkaline environment supports their growth habits.

So what does this have to do with gardens and landscapes? It’s fall, the optimum time for planting shrubs, trees, some perennials and bulbs. So before sinking time and money into several trees, a nice shrub arrangement and some native perennials, do yourself and the plants a favor – test your soil. If you’ve accessed Extension publications, programs, and workshops, you may recognize a common mantra “don’t guess, soil test”! You can’t count on what your neighbor has in her soil being the same composition as yours, either. Send samples of your site’s native soil to the Soil Testing Lab at Auburn University to find out!

What if the results of a soil analysis are above 7.0 (or neutral) on the pH scale? That soil is considered alkaline or “sweet” and is usually more common in areas with less rainfall (as in desert areas of the southwest). It also means phosphorus, iron, zinc and manganese are less available, resulting in conditions such as iron chlorosis.

Many of the species cherished in Southern gardens prefer soil on the neutral to acid end of the pH spectrum, meaning pH will be 7 or less, becoming more acid the lower the number. However, if your property sits on or near a limestone outcropping, pH could be 7 or above. Maybe azaleas won’t thrive in that environment, but there are many lovely species that can. So if ‘we can’t beat ‘em, join ‘em’ as a politician was quoted as saying in 1932, planting vegetation tolerant of ‘sweet’ soil!

ph soil

Reeves Bridalwreath Spirea. Photo courtesy of Wilson Bros Garden

A few examples perhaps familiar to us: Meadow Sage (Salvia nemorosa), Bridalwreath spirea (Spiraea prunifolia), Skyline Honey Locust (Gleditsia triacanthos ‘Skyline’, Burr Oak (Quercus macrocarpa), Eastern redbud (Cercis canadensis). Boxwood, Sweet peas, Dianthus, salvia, phlox, zinnias are just a few trees, perennials, and shrubs that adapt to higher pHs. Beware though that not all varieties of the same species behave equally, so check specific requirements or tolerances before purchasing and installing plants.

A couple of helpful resources in the search for ‘sweet’ soil plants include Lois Trigg Chaplin’s “The Southern Gardener’s Book of Lists” and online links such as

By Sallie Lee, Urban Regional Extension Agent, Alabama Cooperative Extension System.  Contact Sallie at

The Alabama Cooperative Extension System (ACES) operates as the primary outreach organization for the land-grant functions of Alabama A&M and Auburn Universities. ACES is committed to affirmative action, equal opportunity and the diversity of its workforce.  Educational programs of ACES serve all people regardless of race, color, age, sex, religion, disability, sexual orientation, or national origin.

More About Soil Testing

Alabama Extension

The Alabama Cooperative Extension System operates as the primary outreach organization for the land-grant functions of Alabama A&M and Auburn Universities and answers home-gardeners' questions each week on Birmingham Gardening Today.

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