Azaleas are one of the plants that need acidic soil.
Alkaline soil, also known as sweet soil, is 7.1 to 14 on the ph scale; 7 is neutral, while acidic or sour soils run from 1 to 6.9, so when soil is too alkaline, you have to lower the pH. Before you lower or raise the pH, however, you'll need to test the soil to determine the pH to be sure you aren't going in the wrong direction, especially since there's a tenfold increase between each number. It is also important to know what the best pH is for what you are planting. Some plants, including azaleas, thrive in acidic soil. Others don't.
Instructions
Additives:
1. Add elemental sulfur to lower the pH, using chelated iron sulfate or aluminum sulfate. Granular sulfur is safe, inexpensive, but slow. Aluminum sulfate and iron sulfate are quicker, but you'll need to apply them at a much higher rate, and too much will injure the plants. Iowa State University Extension recommends not exceeding 2 pounds of sulfur per 100 square feet per application, and then waiting at least three months before repeating.
2. Rhododendrons also prefer acidic soil.
You can add sphagnum peat or acidifying nitrogen. An organic mulch is another choice, but it's much slower. Sphagnum peat is best for small areas; it would be too expensive to use in large areas. Before you plant, add 1 to 2 inches of the peat to the soil and mix it into the soil to a depth of about 8 to 12 inches.
3. Hydrangeas don't care so much; they'll just change color.
Acidic fertilizers are recommended by Iowa State University Extension include ammonium sulfate, diammonium phosphate, monoammonium phosphate, urea and ammonium nitrate, but check the fertilizer label to be sure.
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