When acid-loving plants, such as blueberries, azaleas, or rhododendrons, require a more acidic soil environment, gardeners often use ammonium sulfate (\(\text{NH}_4\text{SO}_4\)). This compound is a common, fast-acting solution favored for its dual role as a nitrogen fertilizer and a powerful acidifying agent. It promotes soil acidification more quickly and effectively than many other nitrogen sources. Understanding the precise quantity and correct application method is necessary to safely adjust the soil to the optimal pH range.
How Ammonium Sulfate Lowers Soil pH
The primary mechanism by which ammonium sulfate reduces soil pH involves a microbial process called nitrification. When applied to the soil, the ammonium ion (\(\text{NH}_4^+\)) component is taken up by specialized soil bacteria. These bacteria then convert the ammonium first into nitrite (\(\text{NO}_2^-\)) and subsequently into nitrate (\(\text{NO}_3^-\)). This conversion process releases hydrogen ions (\(\text{H}^+\)) into the soil solution, directly increasing the soil’s acidity and lowering the pH reading. Ammonium sulfate is two to three times more acid-forming per pound of nitrogen than other common ammonium fertilizers, making it highly effective.
The sulfate (\(\text{SO}_4^{2-}\)) portion of the compound also contributes to the soil’s acidity over time. The dual action of the ammonium and the sulfate makes ammonium sulfate a stronger and quicker acidifier compared to elemental sulfur.
Determining Current Soil Needs
Accurate calculation of the required ammonium sulfate is impossible without reliable, quantitative data about the current soil conditions. The first step involves obtaining a professional soil test to determine the starting pH level and the target pH level for the specific plants being grown. Guessing the current pH can lead to incorrect application rates, potentially damaging plants through over-acidification or nutrient toxicity.
Soil texture, such as sand, loam, or clay, determines the soil’s buffering capacity—its ability to resist changes in pH. Heavy clay soils, which have a high buffering capacity, require significantly more ammonium sulfate to achieve the same pH drop compared to lighter, sandy soils. Sandy soils have a low buffering capacity and require less material, making them more susceptible to rapid changes. Conversely, soils with high organic matter or clay content are highly buffered and will require higher rates of the amendment. Adjusting the application rate based on soil texture is necessary for safety and effectiveness.
Calculating the Required Amount
Since the amount of ammonium sulfate needed varies widely based on the starting pH and the soil texture, general guidelines must be used with specific soil test results. To lower the pH by a certain amount, a heavier soil type will always require a greater quantity of the amendment than a lighter soil.
Elemental Sulfur Equivalents
To lower the pH by 1.0 unit (e.g., from 7.5 to 6.5), the following approximate amounts of elemental sulfur are needed per 100 square feet:
Sandy soil: 1 to 1.5 pounds.
Loamy soil: 1.5 to 2 pounds.
Clayey soil: 2 to 2.5 pounds.
These figures represent elemental sulfur, which must be converted to the equivalent amount of ammonium sulfate. Ammonium sulfate is a less concentrated acidifier, meaning a larger quantity is needed to match the acidifying power of pure sulfur. One pound of elemental sulfur is roughly equivalent to 2.83 pounds of ammonium sulfate in acidifying potential. Therefore, if a calculation suggests 1 pound of sulfur is needed, the actual application of ammonium sulfate would be close to 2.83 pounds for that area. Large pH changes should be done gradually over multiple applications, split across several months or seasons, to avoid shocking the plants and prevent excessive nitrogen buildup.
Safe Application Techniques
The calculated amount of ammonium sulfate should be applied evenly across the soil surface using a broadcast spreader. After broadcasting, the material should be lightly mixed into the top few inches of the soil, typically the top six inches, to incorporate the amendment into the root zone. Proper incorporation is necessary because the acidifying reaction occurs within the soil profile where the roots are located.
Immediately after application, the area must be thoroughly watered. This step dissolves the granular material and washes the fertilizer into the soil, which activates the acidifying process and prevents “fertilizer burn.” Direct contact between concentrated ammonium sulfate granules and plant foliage or roots can cause tissue damage. To prevent plant damage, a maximum application rate of approximately 5 pounds of elemental sulfur equivalent per 100 square feet should not be exceeded in a single application. Since ammonium sulfate is also a nitrogen fertilizer, over-application can lead to excessive nitrogen levels, negatively affecting plant growth and health.
Monitoring pH and Follow-Up Adjustments
The acidification process initiated by ammonium sulfate is not immediate, as it relies on the activity of soil microorganisms. Noticeable changes in pH often occur over a period of weeks to months. Because of this lag time, re-testing the soil pH too soon can provide inaccurate results.
It is recommended to conduct a follow-up soil test six to twelve months after the initial application to accurately assess the resulting pH change. This allows sufficient time for the chemical reactions to stabilize and for the full acidifying effect to be realized. The results of this second test will determine if any further adjustments are necessary to reach the target pH. Soil pH naturally tends to revert to its original state over time, especially in highly buffered soils, meaning maintenance applications may be required. Maintenance rates are generally lower than the initial corrective application. Continuing to monitor the pH every one to three years will help maintain the soil in the optimal range for acid-loving plants.