Soil acidity or alkalinity is measured on the pH scale, which runs from 0 to 14. A reading of 7.0 is neutral, while values below 7.0 are acidic and those above are alkaline. Most garden plants thrive in a slightly acidic to neutral range, typically between 6.0 and 7.0. However, many ornamental and fruit-bearing plants, such as blueberries, azaleas, and rhododendrons, are acid-loving and require a much lower pH. When soil is highly alkaline, essential micronutrients like iron, manganese, and zinc become chemically bound to soil particles. This process, known as nutrient lockout, renders the nutrients unavailable to plant roots, leading to deficiency symptoms like yellowing leaves and stunted growth. Adjusting a high soil pH downward is necessary to unlock these nutrients and ensure the health of acid-loving species.
Measuring Current Soil pH
Accurately measuring the current soil pH is the necessary first step before applying any amendments. The pH scale is logarithmic; a soil with a pH of 8.0 is ten times more alkaline than one at 7.0, profoundly affecting the required quantity of acidifying material. Professional laboratory analysis is the most precise method, providing an exact pH number and measuring the soil’s buffering capacity. Buffering capacity indicates the soil’s resistance to pH change, explaining why clay soils require more amendment than sandy soils. Inexpensive at-home test kits offer a general pH range but lack the accuracy of a lab report. Interpreting results involves comparing the current pH to the target range required by the desired plants. Without a precise starting point and target goal, attempting to acidify the soil risks wasting product or harming plants through over-application.
Chemical Methods for Quick Acidification
When rapid pH reduction is required, chemical compounds such as aluminum sulfate or iron sulfate offer an immediate solution. These materials bypass slow biological processes by reacting chemically with water to release acidity instantly. This action is often preferable for container plants or temporarily correcting nutrient deficiencies in established plantings.
Aluminum sulfate is a fast-acting acidifier, hydrolyzing to form aluminum hydroxide and a dilute solution of sulfuric acid. However, this method carries risks, primarily the potential for aluminum toxicity if the soil pH drops below 5.0. Additionally, excess sulfate can increase the soil’s salt content, potentially causing osmotic stress and root damage.
Iron sulfate (ferrous sulfate) is a slightly safer alternative that also acts quickly to lower pH. It works by displacing hydrogen ions, with sulfate ions forming a weak sulfuric acid. Iron sulfate supplies a readily available source of iron, which can immediately correct iron chlorosis, a common problem in alkaline soils. Both sulfate-based products must be applied carefully, as large quantities can interfere with phosphorus availability.
Using Elemental Sulfur for Sustained Change
Elemental sulfur is the preferred amendment for achieving a long-term, stable reduction in soil pH. This method relies on a biological process, making it a gradual and sustainable way to acidify a garden bed. Specialized soil microorganisms, primarily Acidithiobacillus bacteria, convert elemental sulfur (S⁰) into plant-available sulfate.
This oxidation reaction releases hydrogen ions, which combine to form sulfuric acid, the compound that ultimately lowers the soil pH. Since the bacteria are most active in warm, moist, and well-aerated soil, the process is slow, often taking several weeks to months to show a significant effect. Spring or summer applications will work faster than those made in cold winter soil when microbial activity is low.
The correct application rate is determined by the soil’s texture and buffering capacity, which a professional soil test measures. Clay-heavy soils require a substantially greater amount of elemental sulfur than sandy soils to achieve the same pH drop. A single application should not exceed 5 to 15 pounds per 1,000 square feet to avoid over-acidification.
Supplementary organic amendments can be incorporated alongside elemental sulfur, though their acidifying effect is mild and temporary. Peat moss, which has a naturally acidic pH of 3.0 to 4.0, can provide an initial, minor drop in pH while adding organic matter. Materials like pine needles and coffee grounds are often rumored to lower soil pH, but their effect is negligible in buffered alkaline soils, though they provide good mulch and soil conditioning benefits.
Maintaining pH and Safety Precautions
Soil acidification is not permanent because the natural alkalinity of the soil tends to cause the pH to revert over time. Re-testing the soil every six to twelve months after the initial application is required to plan for subsequent treatments. If the pH begins to creep back up toward the alkaline range, smaller, follow-up applications of elemental sulfur can maintain the desired level.
For established plants, it is often more practical to apply the acidifying material only to the soil surrounding the root zone. This localized application conserves product and focuses the pH adjustment where acid-loving plants need it most. When handling elemental sulfur and sulfate compounds, proper safety precautions are required. Gardeners should wear gloves and avoid inhaling the dust, particularly fine-grade elemental sulfur, which can irritate the respiratory system. All acidifying materials must be thoroughly watered into the soil immediately after application to dissolve the chemicals and minimize runoff risk.