Aluminum sulfate is a highly effective agent for lowering soil pH, a process often necessary to support the health of specific garden plants. This compound, frequently referred to as “alum,” is a powerful soil amendment used to rapidly increase soil acidity. Its primary use is in horticulture, where it quickly creates the acidic conditions that certain plants require to thrive, making it a valuable tool for gardeners dealing with alkaline or high-pH soils.
How Aluminum Sulfate Creates Acidity
Aluminum sulfate (Al2(SO4)3) is a soluble salt. When introduced to moist soil, it quickly dissolves and dissociates into aluminum ions and sulfate ions. The resulting acidity is not caused by the sulfate, but rather by the aluminum ions reacting with water molecules in a process called hydrolysis.
The aluminum ion (Al3+) is highly reactive with water (H2O), effectively stripping a hydroxyl group (OH-) from the water molecule. This reaction generates an aluminum hydroxide complex and, more importantly, releases a free hydrogen ion (H+) into the soil solution. The concentration of these free hydrogen ions directly determines the soil’s acidity level, which is measured as pH. This chemical process is fast-acting, distinguishing aluminum sulfate from elemental sulfur, which requires a much slower, microbe-mediated conversion into sulfuric acid to lower the pH.
Using Aluminum Sulfate Safely in Soil
The safe and effective use of aluminum sulfate begins with conducting a professional soil test. A soil test determines the existing pH level, the soil’s buffering capacity (its resistance to pH change), and the overall soil type. These factors are necessary for calculating the correct application rate. Clay-heavy soils have a higher buffering capacity and typically require a significantly greater quantity of aluminum sulfate to achieve the same pH drop compared to lighter, sandy soils.
Once the required dosage is determined, safety precautions must be followed. Handling the powdered or granular material necessitates wearing protective gear, including gloves and eye protection, and avoiding inhalation of the dust. The application should be precise; a general guideline is that approximately one pound of aluminum sulfate per ten square feet of soil is needed to lower the pH by about 0.5 units in a typical loamy soil.
The compound can be applied by spreading the calculated amount evenly over the soil surface and thoroughly mixing it into the top six inches of the soil. Alternatively, it can be dissolved in water and applied as a liquid drench, which helps it disperse quickly into the root zone. Watering immediately after application is essential, as it initiates the hydrolysis reaction that releases the acidifying hydrogen ions.
It is important to avoid over-application. After the initial application, gardeners should monitor the soil pH regularly. Subsequent, smaller applications should only be made as needed to maintain the desired acidity, preventing chemical imbalances that are difficult to correct.
Targeting Soil pH for Specific Plants
Lowering the soil pH is often a necessary step to ensure that certain plants can properly absorb the nutrients they need to grow. Many plants, such as azaleas, rhododendrons, camellias, and blueberries, are classified as acid-loving and naturally thrive in soil with a pH range typically between 4.5 and 5.5. These species require this acidic environment because it affects the solubility and availability of various micronutrients.
In alkaline soils (pH above 7.0), essential micronutrients like iron (Fe) and manganese (Mn) become chemically bound to soil particles, making them insoluble and inaccessible to plant roots. When the soil pH is lowered by the addition of aluminum sulfate, these micronutrients are released from their insoluble forms and become readily available for plant uptake. A lack of available iron, for example, often results in chlorosis, where the leaves turn yellow while the veins remain green, a condition that severely limits a plant’s ability to photosynthesize.
Beyond nutrient availability, soil acidity can also affect the aesthetic qualities of certain plants, such as the bloom color of bigleaf hydrangeas. In these plants, a lower soil pH makes aluminum ions available, which are then absorbed and cause the flowers to turn blue. Conversely, pushing the soil pH too low (below 4.5) can have negative consequences, potentially leading to the toxic buildup of soluble aluminum and manganese, which can damage roots and hinder the uptake of other essential nutrients like phosphorus and calcium.