Azaleas prefer acidic soil with a pH of 5 to 6.5.
Soil pH is a very important factor in plant health – if the soil is too acidic or too alkaline, plants will be unable to absorb nutrients properly, and your garden won’t grow. The degree of acidity and alkalinity is measured on a scale of 0-14, with a pH of 7 neutral, 0-7 acidic, and 7-14 alkaline.
It’s important to identify the plants in your yard before attempting to adjust the pH level of your soil, since some flowers and shrubs thrive in a slightly higher or lower pH soil. If the results of a soil test indicate that the pH of your soil needs adjusting, here’s how to go about adjusting it.
Acidifying fertilizers can help lower soil pH over time to make soil more acidic.
Tips for Soil pH Correction
- Read Label: No matter which product you choose, it’s important to follow the instructions on the package to the letter, even if you have to buy a special spreader or applicator to get it right. For example, one brand of sulfur may be more finely ground than another, and over application could damage your plants. While your soil test results will provide general guidelines about how much amendment is needed, follow the label on the particular product.
- Proceed Slowly: Make one application of the product, wait at least three months, retest the pH of the soil, and reapply again if needed. It might take a year or more to get your soil on track, and overdosing can cause more harm than good.
- Fall Application: For best results, incorporate pH-correcting amendments in the fall, to give them plenty of time to break down for spring planting. Many gardeners make soil testing and pH-fixing an annual fall ritual.
- Plant Selection: You’ll have much better luck if you choose plants well suited to the soil you already have. It’s OK to tweak the pH a little to optimize the growing conditions, but your overall soil makeup pretty much is what it is.
Lime is the most popular additive for acidic soils to raise the pH.
How to Raise the pH in Acidic Soil
- Lime: Limestone is the most common soil additive for raising pH of your soil to make it less acidic. You’ll generally see two types: calcitic limestone (which is mostly calcium carbonate), and dolomitic limestone (which also adds magnesium to the soil). Both work equally well at raising soil pH. Liming products come in granular, hydrated, pelletized, or pulverized forms. Pulverized lime is a fine powder that is faster-acting, but it tends to clog spreaders. The granular or pelletized types of limestone spread more easily and take longer to break down. Hydrated lime is the fastest-acting but is very easy to overdose. All lime products will work much better if they can be worked down into the soil, rather than left on top. This is why applying lime to lawns is often paired with core aeration and fall watering.
- Wood Ash: For an organic way to make your soil less acidic, sprinkle about 1/2″ of wood ash over your soil and mix it into the soil about a foot deep. This method takes small applications over several years, but it can be very effective, as well as a great way to recycle fireplace ashes!
Sulfur is commonly applied to alkaline soils to make them more acidic.
How to Lower the pH in Alkaline Soil
- Sulfur: Plain elemental sulfur (or sulphur) is probably the easiest and most common way to make soil more acidic, since it’s cheap, relatively safe, and can be spread on top of the soil. Since sulfur is pretty slow-acting, you shouldn’t apply more than 2 pounds per 100 square feet at a time.
- Sphagnum Peat: This is a great organic solution, since sphagnum peat also adds organic matter to your soil and increases water retention. Simply work a 2” layer of sphagnum peat into your soil at least a foot deep. Larger areas will probably require a tiller.
- Aluminum Sulfate and Iron Sulfate: These two products are very fast-acting, but they can also be the most damaging by adding salts and elements that can build up in the soil. Be sure not to apply more than about 5 pounds per 100 square feet.
- Acidifying Fertilizer: Fertilizers that contain ammonia (such as ammonium nitrate), urea, or amino acids can, over time, have an acidifying effect on the soil in your yard.
- Mulches and Compost: As organic matter breaks down, it tends to make soil more acidic. Regular use of organic compost and mulches will, over time, bring the soil pH closer to the desired neutral to slightly acidic level. The easiest way to lower your soil pH is just to keep heaping on the rotten stuff. Mother nature sure is smart!
- Changing the pH of your Soil (Clemson University)
- Soil pH and Fertilizers (Mississippi State University Extension)
- Changing pH in Soil (University of California Extension, PDF/151 Kb)
- Soil Acidity and Liming (Clemson University)