A soil acidifier is a product used to lower the pH of soil and make it more acidic. Lowering the pH of soil is needed in regions where the soil pH is too high, usually from a high lime, calcium, or magnesium content in the soil or water. Soil acidifiers are also used for plants such as blueberries or azaleas that have unusually low pH requirements.
Soil pH affects the bioavailability of nutrients in the soil. It also affects plants' ability to take in and process nutrients. Most plants prefer pH neutral soil, or around pH 7. Many plants, such as broccoli, cabbage, and kale prefer slightly alkaline soils. In regions where soil is very acidic, lime is added yearly to “sweeten” the soil and make it more hospitable to plant and microbiotic life.
Rhododendron, heather, and camellia are all ericaceous plants that need acidic soil to grow well. If soil pH is 6 or above, these plants will have stunted growth, yellow leaves, and poor flower and fruit production. This is usually due to an iron deficiency, because the plants can't effectively access iron at a higher pH. Most soils require some type of soil acidifier before these plants go into the ground, or they will not be successful.
A common soil acidifier is elemental sulfur, which is a type of rock dust. Sulfur is a slow-release mineral that lowers pH over time. It generally needs to be added yearly because soil tends to go back to its original pH. Elemental sulfur is not immediately effective as a soil acidifier. Instead, it must be applied at a depth of six inches up to a year before planting.
Fast-acting soil acidifiers include aluminum sulfate and iron sulfate. These are generally effective within four weeks, but the results don't last as long and there is a risk of poisoning plants with too much iron or aluminum. Additionally, the salts released as these acidifiers break down can be harmful.
Concerns about elemental sulfur turning into sulfuric acid when it interacts in water have led some gardeners to use organic means of acidifying the soil. Peat or sphagnum moss placed in the planting hole is helpful for lower pH right around the plant, but these materials are in increasingly short supply. Other slow-release organic soil acidifiers include pine bark chips and pine needles, coffee grounds, and aged sawdust. Composted leaves from chestnut, oak, or beech trees may also be used to acidify soil organically.