How to Acidify Potting Soil
A number of plants---blueberries, evergreens, azaleas, camellias and rhododendrons, just to name a few---need acidic soil. Acidifying potting soil can be accomplished relatively easily with an appropriate acidic additive. But because different plants need different levels of acidity, it is important to test the pH of the soil before adding any substances to it. Most of the aforementioned plants, and ericaceous plants in general, require a pH between 5.0 and 6.0. Does this Spark an idea?
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Instructions
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Test the pH of your soil and determine how much change in pH level your soil will require to grow the desired plant. In your local garden center you should be able to find pH testers, as well as instructions on how to use them.
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Decide on a soil acidifying substance. There are fertilizer options, and there are more drastic ones. Many fertilizer products contain ammonia, either in ammonium sulfate or urea form. These act as chemical sources of nitrogen for most acid-loving plants.
Raw elemental sulfur is an effective drastic option, but tends to work far slower than aluminum or ferrous sulfate, requiring several weeks to even months in colder climates and seasons to properly acidify the soil. However, the speed you get from the sulfates is countered by its tendency to interfere with the soil's phosphorous levels, if over-applied, according to the Royal Horticultural Society. Still, a Clemson University study says that the sulfates are probably the best to use for home gardeners and hobbyists because it is more difficult to over-apply. This is because the sulfates produce chemical reactions, while sulfur induces biological reactions. Aluminum sulfate will likely be easier to find at your local garden center, as it is becoming more common.
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Mix the substance according to the directions listed on the product packaging. Follow it closely, as too much of any acidifying substance can alter soil nutrients, even something as difficult to over-apply as aluminum or ferrous sulfate. Start with a small dose if you're unsure of the correct quantity to apply---you can always add more later.
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