Using aluminum sulfate is a common method to decrease the pH of alkaline soil, making it more suitable for acid-loving plants.
The Process: Applying Aluminum Sulfate
To effectively lower your soil's pH using aluminum sulfate, follow these steps:
- Determine How Much to Apply: The amount needed depends on your soil's current pH and your target pH. According to general guidelines, you'll need about 1 lb (0.45 kg) of aluminum sulfate per 10 square feet (1 square meter) of soil for every 0.5 pH point you want to lower. For example, if you have a 100 sq ft area and want to lower the pH by 1.0 point, you'd typically need about 2 lbs (0.9 kg) of aluminum sulfate (2 lbs for the 10 pH point drop x 10 units of 10 sq ft in 100 sq ft).
- Prioritize Safety: Always wear protective gear, including gloves and goggles, when handling aluminum sulfate. Avoid inhaling the dust.
- Prepare the Area: Clear the soil surface of any debris.
- Apply Evenly: Evenly spread the calculated amount of aluminum sulfate over the soil surface. This helps ensure consistent acidification across the area.
- Incorporate (Optional but Recommended): Gently mix the aluminum sulfate into the top few inches of soil using a rake or tiller. This helps it dissolve and react more quickly.
- Water Thoroughly: Water the treated area well after application. Water helps dissolve the aluminum sulfate and move it into the soil profile where it can begin to react and lower the pH.
- Allow Time to Work: The acidification process takes time. Wait several weeks or even months before re-testing the soil pH.
- Re-test Soil pH: Use a reliable soil pH test kit or meter to check the new pH level. If the desired pH hasn't been reached, you may need to apply more, but always do so incrementally and re-test.
Factors Affecting Application Rate
- Soil Type: Sandy soils require less aluminum sulfate than clay soils to achieve the same pH change.
- Organic Matter: Soil high in organic matter may require more aluminum sulfate.
- Current vs. Target pH: The greater the difference, the more aluminum sulfate is needed.
Example Application Rate Guide (Based on 1 lb/0.45 kg per 10 sq ft/1 sq m per 0.5 pH drop)
| Target pH Drop | Amount per 10 sq ft (1 sq m) |
|---|---|
| 0.5 points | 1 lb (0.45 kg) |
| 1.0 point | 2 lbs (0.9 kg) |
| 1.5 points | 3 lbs (1.35 kg) |
| 2.0 points | 4 lbs (1.8 kg) |
Note: These are general guidelines. Always test your soil first and follow product-specific instructions if available.
Aluminum sulfate works by reacting with water in the soil to form sulfuric acid, which lowers the pH, and aluminum hydroxide. While effective, it's potent, so careful measurement and application are crucial to avoid over-acidification or aluminum toxicity, which can harm plants. For less drastic or more gradual pH adjustments, consider using elemental sulfur, which works more slowly but provides longer-lasting results and avoids the potential for aluminum toxicity.
Remember to always test your soil pH before adding any amendments to understand your starting point and calculate the appropriate amount needed. Follow safety guidelines and apply evenly for best results.
