Alum Uses In Gardens: Aluminum Soil Amendment Tips

By Mary H. Dyer, Master Naturalist and Master Gardener

Alum powder (Potassium aluminum sulfate) is typically found in the spice department of supermarkets, as well as most garden centers. But what exactly is it and how is it employed in gardens? Read on to learn more about alum uses in gardens.

What is Alum Used for?

Alum is implemented in water treatment and other industrial applications, but food-grade alum, approved by the FDA, is safe for household use in small quantities (less than one ounce). Although alum powder has a variety of purposes around the house, the most common is to add crispness to pickles. For other applications, you can also purchase liquid forms of aluminum sulfate.

Although alum isn’t a fertilizer, many people apply alum in the garden as a way to improve soil pH. Read on to see how it works.

Aluminum Soil Amendment

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Soils vary widely in their level of acidity or alkalinity. This measurement is known as soil pH. A pH level of 7.0 is neutral and soil with a pH below 7.0 is acidic, while soil with a pH above 7.0 is alkaline. Dry, arid climates often have alkaline soil, while climates with higher rainfall typically have acidic soil.

Soil pH is important in the gardening world because unbalanced soil makes it more difficult for plants to absorb nutrients in the soil. Most plants do well with soil pH between 6.0 and 7.2 – either slightly acidic or slightly alkaline. However, some plants, including hydrangeas, azaleas, grapes, strawberries and blueberries, require more acidic soil.

This is where alum comes in – aluminum sulfate can be used to lower soil pH, thus making the soil suitable for acid-loving plants.

If your acidic plants aren’t thriving, take a soil test before you attempt to adjust the pH level. Some Cooperative Extension offices perform soil tests, or you can purchase an inexpensive tester at a garden center. If you determine your soil is too alkaline, you may want to adjust it by adding aluminum sulfate. Clemson University Extension provides in-depth information on adjusting soil pH.

Using Alum in the Garden

Wear gardening gloves when working with alum in the garden, as the chemicals may cause irritation when it comes in contact with the skin. If you are using the powdered form, wear a dust mask or respirator to protect your throat and lungs. Alum that comes in contact with the skin should be washed off immediately.

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