The majority of garden plants prefer a soil pH of 6 to 7, but certain common plants require acidic soil (pH under 7). They include:
- Azaleas and rhododendrons
- Butterfly bush
- Blue-flowered hydrangeas
While these plants may survive in soils with a higher pH, they grow and bloom best in soil with a pH of 4 to 5.5.
You can test your soil's pH yourself with an inexpensive kit—look for one at a good nursery or hardware store. Your extension agency will also measure pH in a soil test. Some garden centers will also test a sample for you.
If you find your soil is not acidic enough for these plants, you can lower the pH with some common amendments. Changing the soil pH is best done gradually over several seasons, and you'll need to continue to monitor it. The pH may creep back up over time.
Elemental sulfur can be used to lower soil pH. It reacts slowly, lowering the pH over 3 to 6 months. Another common recommendation, iron sulfate, works in about a month, but in total more of it is needed. The amount of either material depends on the soil type (sandy, clay or loam) and the number of points the pH must be lowered.
For application rates plus a guide to fertilizing acid-loving plants, see this article from Missouri State University.
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