Make Garden Soil More Acidic With An Affordable And Effective Addition

A rotten egg-like smell lingers in the air after you pluck a crop of onions or garlic from your beds. The oils responsible for that distinctive allium odor are generated by sulfur — an important horticultural macronutrient and an essential component in healthy garden soils. While sulfur plays many other roles in plant growth, from amino acid production and photosynthesis to aiding nitrogen fixation in peas and beans, adding elemental sulfur also happens to be one of the most practical and cheapest ways to make your soil more acidic.

Most plants like a slightly acidic soil, somewhere in the range of 6.5 to 6.8 pH, because that's where the highest rates of available nutrients exist. Others — think blueberries, rhododendrons, azaleas, hollies, hydrangeas, dogwood, and conifers — thrive with even greater acidity. Acidic soils can even mitigate disease. For example, maintaining a soil pH of 5.0 to 5.2 can help reduce potato scab. The problem is that much of the western United States has naturally alkaline soil, with readings above 7.0 on the pH scale. Likewise, typically, the drier the climate and the more naturally occurring limestone in the ground, the more alkaline the soil. One sign of too much alkalinity is chlorosis, or yellowing, of leaves. When you apply elemental sulfur (sometimes also called "garden" or "soil sulfur") to your dirt, it induces a biological reaction in soil microbes. These teeny critters work hard to break down the compound into hydrogen sulfate (or sulfuric acid), a process dubbed mineralization in horticultural science, and this is what lowers the soil pH, changing it from alkaline to acidic.

Elemental sulfur isn't the only way to fix soil acidity imbalances. Iron sulfate, acidifying nitrogen fertilizers, peat moss, and compost are among the other options. Magnesium sulfate (Epsom salts) and garden gypsum also contain sulfur (the latter in the form of calcium sulfate), but they're used as fertilizers and won't meaningfully alter soil pH. Soil sulfur is a go-to product because it's easily accessible, safe, and cheap. What's more, while it takes a little bit longer to work than other sulfur-based soil amendments (like aluminum sulfate), it keeps the soil acidic longer, too. Look for elemental sulfur in the 90% to 99% purity range at garden centers, nurseries, or online. A 4 pound bag of 90% Soil Mender Elemental Sulfur costs just under $28. IMUSTGARDEN Soil Sulfur Pellets come in a handy 5-pound shaker jar for about $22. Alternatively, get 25 pounds of Hi-Yield (32189) Dusting Wettable Sulfur for about $19. 

Before you attempt fixing imbalances with sulfur, you need to run a test to measure the soil's pH level. Ideally, this should be done right before spring planting, when knowing the condition of your soil is most vital. Stick a pH meter into your garden bed or contact your local extension office to see if they offer soil testing and analysis services.

The amount of elemental sulfur you need to reduce pH levels depends on your soil type, acidity, and the element's concentration. For example, to drop soil pH levels by one unit (say, from 7.5 to 6.5) in 100 square feet of sandy soil, you'd need to apply between 1 and 1.5 pounds of 95% elemental sulfur. In clay soils, you'd have to add more — 2 to 2.5 pounds. Another rule-of-thumb guideline is a ¼ pound of sulfur for every square yard of sandy soil and a ½ pound per square yard for heavy clay soils. Use an online calculator to determine how much elemental sulfur to apply, like this one offered by The National Gardening Association. Don't exceed 20 pounds of sulfur per 1,000 square feet at one time for unplanted areas and 5 pounds in planted areas or you risk burning your plants.

Apply elemental sulfur directly to the soil when the weather is warm to take advantage of biological activity-boosting soil temperatures. You can also apply elemental sulfur directly to planting holes when you're preparing the soil in the early spring or late fall. Apply this soil amendment according to the package instructions and water it in before mulching. Elemental sulfur works slowly; your soil pH levels may take up to six months to change. Test your soil every six months and amend with more sulfur as needed.