Any adjustment to pH should be based on a soil test; do not indiscriminately add sulfur or lime to the soil without testing. In general, it is easier to raise the pH to make the soil more alkaline than it is to lower the pH to make it more acidic.
To raise pH and make the soil more alkaline, add some form of lime. Ground agricultural lime is most often used; the finer the particles, the faster it will change the pH. Lime is usually applied to garden areas in the fall; it should be in the soil two to three months prior to planting. Lime must be incorporated into moist soil in order for it to work well.
To lower soil pH and make the soil more acidic, use either aluminum sulfate or sulfur. Aluminum sulfate will change the pH almost instantly, whereas sulfur may require several months to work as it takes time for soil bacteria to convert the sulfur to sulfuric acid. The rate at which sulfur is converted depends on the fineness of the sulfur, moisture in the soil, soil temperature, and the presence of bacteria. Aluminum sulfate can be applied any time, while sulfur can be applied in the fall in order to affect a pH change for the following season. Either product should be worked into the soil after application. Do not overapply either product. Note that aluminum can be toxic to plants.
