Soil pH measures acidity or alkalinity on a logarithmic scale from 0 to 14, where 7.0 is neutral. Numbers below 7.0 indicate increasing acidity, and numbers above indicate increasing alkalinity. This measurement dictates which nutrients are soluble and accessible to plant roots, controlling nutrient availability and uptake. Most common garden plants thrive in a slightly acidic to neutral range, typically between 6.0 and 7.0. Specialized plants, such as blueberries, prefer highly acidic soil, while others, such as lavender, are better suited to alkaline conditions.
Determining Your Current Soil pH
The most reliable method for determining pH is a professional soil test conducted by a certified laboratory. This provides a highly accurate pH reading, often including data on nutrient content and buffering capacity. While this method is precise, results may take several weeks, and the sample must be collected consistently, typically as a composite sample taken from the top six inches of soil across the desired area.
The first is at-home testing options, including chemical kits and electronic meters. Chemical kits use color-coded reagents to estimate the pH, but subjective color matching and potential user error make them less dependable than lab analysis. Portable electronic pH meters offer a quick reading directly in the soil or in a soil-water slurry. However, these meters require frequent calibration and careful maintenance to ensure accurate readings.
Methods for Lowering Soil pH
The most common amendment for lowering soil pH is elemental sulfur. Elemental sulfur is a slow-release material that relies on naturally occurring soil microbes to oxidize it into sulfuric acid. This biological process is temperature-dependent and can take several months, or even a year, to achieve the full pH reduction. Application rates vary significantly, but generally, one to two pounds of elemental sulfur per 100 square feet may be required to lower the pH by one full unit.
A much faster alternative is aluminum sulfate, which chemically reacts in the soil almost immediately to produce acidity. Because it bypasses the slow microbial oxidation step, a pH reduction can be seen within weeks, making it suitable for quick adjustments. However, aluminum sulfate is required at a significantly higher rate—approximately six times the amount of elemental sulfur—to achieve the same result. Over-application is a serious risk with this compound, as excess aluminum can reach toxic levels for plants and also negatively impact the availability of phosphorus.
For minor or temporary adjustments, incorporating organic materials such as sphagnum peat moss into the planting area can also help lower the pH. When applying acidifying material, mix the product thoroughly into the topsoil, ideally six inches deep, rather than leaving it as a surface topdressing. If a large quantity of sulfur is needed, divide it into multiple smaller applications spread out over several months to prevent shocking the soil and avoid potential toxicity.
Methods for Raising Soil pH
The preferred amendment for raising soil pH is agricultural limestone. Limestone works by neutralizing the hydrogen ions responsible for soil acidity. The two main forms are calcitic and dolomitic limestone, both effective in neutralizing acidity. Calcitic limestone is primarily composed of calcium carbonate and is the preferred choice if your soil already has sufficient magnesium.
Dolomitic limestone contains both calcium carbonate and magnesium carbonate, making it an excellent amendment for soils that are deficient in magnesium. Magnesium is an essential nutrient for plants, playing a role in photosynthesis, so using the dolomitic form provides the dual benefit of pH adjustment and nutrient supply. Limestone is a slow-acting material, and it typically takes three to six months for the full pH-raising effect to be realized, meaning it should be applied well in advance of planting.
Limestone is often sold in powdered or pelletized forms, with the finer, powdered material reacting more quickly due to its greater surface area. Pelletized lime is easier to spread and handle but may take slightly longer to dissolve and begin working. For a quicker, though less precise, boost, finely ground wood ash can be used as an alkaline amendment, but its composition and neutralizing power are highly variable and less predictable than agricultural lime.
Managing pH Changes Over Time
The full effect of amendments like limestone or elemental sulfur may not be visible for six to twelve months. Clay-heavy soils and those with high organic matter are especially resistant to change and require higher application rates to shift the pH. Conversely, sandy soils have a low buffering capacity and will change pH more easily, but they may also revert to their original state more quickly.
A follow-up soil test is necessary six to twelve months after the initial application to accurately gauge the success of the adjustment and determine if more material is needed. If the total quantity of amendment is large, it should be split into multiple applications over successive years. Over-application can lead to nutrient deficiencies; for example, a pH that is too high can render micronutrients like iron and zinc insoluble, while a pH that is too low can lead to aluminum toxicity.