Soil pH measures the acidity or alkalinity of the soil on a logarithmic scale from 0 to 14. A reading of 7.0 is neutral; numbers below 7.0 indicate acidity, and numbers above 7.0 denote alkalinity. This measurement is important for plant health because it dictates the solubility of mineral nutrients in the soil solution. When the pH is outside a plant’s preferred range, essential elements like iron or phosphorus can become chemically locked up, making them unavailable for root absorption.
Determining Current Soil pH and Target Range
The first step before making any adjustment is to accurately determine the current soil pH. The most reliable method is sending a sample to a professional soil laboratory, often through a local agricultural extension office. These lab tests offer high precision and typically provide detailed recommendations for amendment application rates. At-home test kits or meters are a quicker alternative, but their accuracy can vary widely, sometimes yielding results that are off by as much as one full pH unit.
Knowing the current pH allows a gardener to establish a precise target range. Different plants have distinct preferences, meaning a single garden may require separate zones for optimal growth. Most common vegetables thrive in a slightly acidic to neutral range of pH 6.0 to 7.0, where nutrient availability is maximized. Acid-loving plants like blueberries prefer a much lower range of pH 4.5 to 5.5, while alkaline-tolerant plants such as asparagus prefer a slightly higher range of pH 6.5 to 7.5.
Raising Soil pH for Alkaline Needs
To raise the pH (make acidic soil less acidic), the primary material used is agricultural lime. This is ground limestone, which comes in two main types: calcitic lime (calcium carbonate) and dolomitic lime (which also contains magnesium carbonate). A soil test indicates whether dolomitic lime is needed to remedy a magnesium deficiency while correcting acidity.
Lime works by dissolving and releasing basic compounds that neutralize acidic hydrogen ions in the soil solution. Application is best done in the fall or early spring, as the material is slow-acting and requires time to fully react. For best results, the lime should be thoroughly incorporated 4 to 8 inches deep, rather than just spread on the surface. Full pH correction can take up to six months or a year.
A minor alternative is wood ash, which acts more quickly than lime and is rich in potassium and calcium. Wood ash must be used sparingly, as it is highly soluble and can rapidly cause the soil to become overly alkaline, leading to nutrient imbalances. Due to its high alkalinity, wood ash should only be applied to clearly acidic soil, and never around plants that prefer low pH.
Lowering Soil pH for Acidic Needs
To decrease the soil pH (make the soil more acidic), gardeners most commonly turn to elemental sulfur. This amendment relies on a biological process: soil microbes oxidize the sulfur into sulfuric acid. Because this is a microbial process, the speed of the pH change depends on warm soil temperatures and adequate moisture.
Elemental sulfur is slow-acting, often taking several months to a year to show its full effect. Due to the risk of toxicity to plants, do not apply more than 5 to 10 pounds per 1,000 square feet at any one time. If a large pH drop is necessary, the application must be split into multiple smaller doses over succeeding years to avoid damaging plant roots.
Sphagnum peat moss is another option, which has a naturally low pH (3.0 to 4.5). It provides a faster, though temporary, acidifying effect while boosting organic matter. Aluminum sulfate also lowers pH quickly through a chemical reaction, but its use is discouraged for general gardening due to the significant risk of aluminum toxicity if over-applied.
Long-Term Maintenance of pH Levels
Soil pH is not permanent and naturally shifts over time due to environmental and management factors. Regular re-testing, ideally every two to three years, is necessary to monitor these fluctuations and maintain the target range. Sandy soils, which have a low buffering capacity, require more frequent re-testing than clay-rich soils.
Ammonium-based fertilizers contribute to soil acidification over time as nitrogen undergoes nitrification. Conversely, irrigation water, particularly hard water rich in bicarbonates, can gradually increase the soil’s alkalinity. To counteract these ongoing changes, gardeners should plan for light, supplemental applications of the appropriate amendment, ensuring gradual adjustments rather than large, sudden changes.