The Benefits of Alum in Gardening

What is Alum?

Alum refers to several chemical compounds known as hydrated double salts that are composed of aluminum sulfate, potassium sulfate, and water. Alum is commonly available in two forms: powder and crystal. It is widely used for various purposes, such as water purification, leather tanning, papermaking, and dyeing, due to its ability to form large, sticky flocs that attract impurities and settle them out of water or other liquids.In gardening, alum has been found to have several benefits, primarily related to improving soil pH, which is a crucial factor in determining plant growth. Soil pH is a measure of the acidity or alkalinity of soil and ranges from 0 to 14, with 7 being neutral. Soil pH below 7.0 is acidic, while pH above 7.0 is alkaline. Most plants prefer soil with pH between 6.0 and 7.2, which is slightly acidic.Alum powder can be used to lower soil pH, making it more acidic, which is beneficial for plants that prefer acidic soil. Acid-loving plants like hydrangeas, grapes, strawberries, blueberries, and hibiscus grow best in soil with a pH range of 4.5 to 5.5. Alum powder works better than diluted vinegar as a soil amendment because it keeps the soil acidic for a longer time.Aside from adjusting soil pH, alum can also be used in water purification and insect control in gardening. It can settle suspended particles in water, making it crystal clear, and prevent the formation of larvae, including mosquito larvae. Alum is also believed to keep slugs, snails, and some other insects away, including cockroaches.It is important to note that alum is not a fertilizer and should not be used as a substitute for it. The application of alum in your garden should be done judiciously and in conjunction with the appropriate amount of fertilizers to ensure that your plants get the nutrients they require for healthy growth.In conclusion, alum is a versatile chemical compound that has several benefits in gardening. Its ability to adjust soil pH, purify water, and control insect growth makes it an essential tool for gardeners who want to create a healthy growing environment for their plants. Alum is readily available and easy to use, making it an ideal solution for gardening.

Using Alum to Improve Soil pH

Alum can be an effective and affordable solution to improve soil pH for acid-loving plants. Most gardeners struggle with soil pH, which can affect the growth and health of their plants. Soil pH is a measure of its acid or alkaline content on a scale from 0 to 14. A pH value below 7 indicates an acidic soil, while above 7 indicates an alkaline soil. A neutral pH of 7 is ideal for most plants, but some prefer more acidic soil conditions.Acidic soil is common in areas with high rainfall, and it can lead to poor plant growth and yield. If your soil is too acidic, you can use alum to increase the pH level gradually. Alum powder is made of potassium aluminum sulfate and is easy to find at your local supermarket or gardening store.Before adding alum to your soil, it’s essential to test the pH level to determine the right amount needed. A pH tester is a simple tool that gives you accurate readings of your soil’s pH. Once you’ve tested the soil, you can determine the right amount of alum to use to raise the pH level. In general, one pound of alum powder per square yard can raise the pH level by one unit.When adding alum to your soil, make sure to dig it in and mix it well with the soil. You can also sprinkle it on top of the soil, but it will take longer to lower the pH. Alum works by releasing positively charged aluminum ions, which attract negatively charged acidic ions in soil. When alum reacts with water, it releases acid, which slowly lowers the pH level in soil.It’s important to note that alum can take several months to lower the pH level, so it’s best to add it in the fall before planting or in early spring. It’s also important to keep adding alum every six months to maintain the desired pH level. Once you have reached the desired pH level, you can stop using alum.Alum is not suitable for all plants, so it’s essential to research which plants prefer acidic soil. Plants like azaleas, rhododendrons, blueberries, and strawberries thrive in acidic soil conditions. However, other plants like tomato, potato, and spinach prefer neutral to slightly alkaline soil.In conclusion, using alum to improve soil pH is an effective and affordable solution for acid-loving plants. It takes time and patience to see the results, but with the right amount of alum and proper mixing, you can create a healthy environment for your plants to grow. Remember to test your soil before adding alum and to research which plants prefer acidic soil before using it.

Alum for Water Purification and Insect Control

In addition to its soil pH adjusting properties, alum can also be used for water purification and insect control in gardening.Dirty and cloudy water is not ideal for plants as it can cause their roots to suffocate, resulting in stunted growth or even death. Fortunately, alum can help to clear up dirty water in ponds, fountains, and other water features and make it crystal clear. When added to water, alum forms positively charged particles that attract negatively charged dirt particles, causing them to clump together and sink to the bottom, leaving clear water on top. This process is called coagulation and it is an effective way to remove impurities from water. The amount of alum needed will depend on the size of the water feature and the level of dirt particles in the water.Moreover, alum can also control mosquito growth in water. Mosquitos need water to lay their eggs, and if the water is treated with alum, it will kill the mosquito larvae, preventing them from growing and laying more eggs. This can help to reduce the mosquito population in your garden, which is especially important if you live in an area where mosquito-borne diseases are prevalent.In addition, alum is believed to have insecticidal properties that repel or kill some insects like slugs, snails, and cockroaches. When sprayed on plants, it creates an alkaline surface that repels insects, and if the insects come in contact with alum, it can disrupt their cell walls and cause them to die. However, it is important to note that alum is not a broad-spectrum insecticide, and it may only work on certain types of insects.It is also important to use alum in moderation in water features as it can harm fish and other aquatic animals. It should be added gradually and in small quantities to avoid over-treating the water.In summary, alum has several benefits in gardening beyond soil pH adjustment. It can be used to purify and clarify water features, control mosquito growth, and repel or kill some insects. When used correctly and in moderation, alum can help to maintain a healthy and thriving garden ecosystem.


In conclusion, alum is a versatile chemical compound that has several benefits in gardening. It can help improve soil pH, thus promoting better plant growth and providing a healthier environment for acid-loving plants. It is readily available and affordable, making it a convenient option for gardeners.Apart from soil amendment, alum can also be used in water purification and insect control. By settling down dirty water and killing mosquito larvae, alum can help promote a healthy aquatic environment. Additionally, it can keep slugs, snails, cockroaches, and other insects at bay, reducing pest infestation.It is important to note that alum is not a fertilizer and should not be used as a substitute for fertilizers. It is primarily used to adjust soil pH levels and improve soil quality. Gardeners should use alum in moderation and follow the recommended application instructions to avoid any adverse effects on the plants.To test the soil pH, gardeners can purchase a soil pH tester online or at their local garden center. This will help them determine the soil’s current pH level and decide on the appropriate amount of alum needed to achieve the desired pH.Overall, using alum in gardening can have several benefits, but it is crucial to use it appropriately and in moderation. Gardeners should do their research and understand how alum works before using it in their garden. By doing so, they can create a better environment for their plants and promote healthier growth.