High-pH or alkaline soils will not allow most plants to grow well, as high pH can cause nutritional deficiencies. In addition, some species -- such as rhododendrons and blueberries -- require a low-pH or acidic soil to grow properly. Gardeners often use aluminum sulfate -- a chemical commonly referred to as alum -- to lower the pH of garden soils.
Although several products will lower the pH of your garden soil, aluminum sulfate offers immediate results rather than taking months to work like sulfur, which is also used to lower soil pH. However, with this benefit come risks: Aluminum sulfate can create excessive levels of aluminum that can damage plant roots. In particular, gardeners use aluminum sulfate to produce blue blossoms in hydrangeas.
Always test your soil before applying any amendments. Your local extension office can perform a basic soil pH test for you or refer you to a private testing lab. Because you need large amounts of aluminum sulfate to produce only small changes in pH, if you need to make drastic pH changes, you may want to consider using sulfur or another product. Furthermore, aluminum sulfate creates only temporary pH changes, so you will likely need to apply it annually to maintain pH. Test your soil regularly to apply the correct amounts.
In general, it takes 1 lb. of aluminum sulfate per 100 square feet to lower the pH 0.1 units, reports the University of Illinois Extension. You will need to modify application rates based on the texture of your soil. To determine soil texture, squeeze a handful of moistened soil into a ball. If it forms a hard ball that looks shiny when you rub it, then you have a clay soil. If the soil won't form a ball and you can see individual soil particles, you have a sandy soil. Apply one-third less than the calculated amount for sandy soils, and add one-half the calculated amount for clay soils. Work the aluminum sulfate into the soil with a tiller or garden fork.
Do not apply more than 5 lbs. of aluminum sulfate per 100 square feet of garden area at any single time. Over application of aluminum sulfate can create toxic conditions that harm or kill your plants. In addition, aluminum sulfate is toxic to blueberries. Rinse off any leaves that came into contact with the aluminum sulfate, as the chemical can burn plant foliage if left in contact with it for too long.
If aluminum sulfate isn't the best choice for your garden, elemental sulfur often provides a viable alternative. Although it works more slowly than aluminum sulfate, it requires smaller amounts, making it safer to use and better able to make the drastic pH changes many gardeners desire. Some fertilizers also lower pH, including ammonium sulfate, diammonium phosphate, monoammonium phosphate, urea and ammonium nitrate.
- Oregon State University Extension: Acidifying Soil
- Iowa State University Extension: How to Change Your Soil's pH
- University of Illinois Extension: How to Lower Soil pH
- University of Florida IFAS Extension: Soil pH and the Home Landscape or Garden
- Clemson University Extension: Changing the pH of Your Soil
- NASA Soil Science Education: Soil Texture