The pH level of the soil determines what nutrients a plant can take up with its roots. Most plants enjoy slightly acidic to neutral soil. Some plants, such as blueberries (Vaccinium spp.), hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8, require acidic soil. If you have an alkaline environment or wish to grow an acid-loving plant, you'll need to bring down the pH of the dirt to raise the acidity.
Applications of sulfur to the soil, in a variety of forms, can bring up the acidity level. Elemental sulfur works slowly but can keep the pH level low for years. Iron sulfate and aluminum sulfate work more quickly to lower the pH, but they can leave toxic deposits of iron and aluminum, if you add too much. Elemental sulfur, the least expensive but slowest acting amendment, can take months to change the soil to the necessary pH. The amount you will need to apply depends on the texture of the soil; the more clay present, the more sulfur you will need to change the acidity.
If you only need to gradually and minimally lower the pH of the soil, nitrogen fertilizers including acidifying elements such as urea, ammonium nitrate and ammonium sulfate can do the trick. With these, you can simultaneously fertilize your plants as well as acidify the soil. These only work for small pH changes, however. If you apply too much, you risk overfertilization and the decline of your plants. Because acidifying fertilizers work more gradually than sulfur, you run less of a risk of overacidification when applying it to the soil.
Sphagnum peat and pine bark mulch can effectively lower pH but tend to cost more than sulfur and fertilizer. The cost makes these organic elements prohibitive for large gardens, so most only use them for small plots if at all. If you have clay or sandy soil, these acidifying elements have the added benefit of increasing the quality of the soil composition, water retention and drainage.
Ideally, you should bring down the pH of the soil over the two years before planting. If you have to plant sooner, however, you can cautiously lower the pH without injury to your plants. Apply your soil amendment, mixing it into the top 6 to 10 inches of the soil before planting for the best results. If you already have plants in the ground, avoid injuring them by applying no more than 2 pounds of sulfur or 5 pounds of aluminum or iron sulfate per 100 square feet every three months.
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