Soil pH measures the acidity or alkalinity of the soil on a scale from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, and values above 7 indicate alkalinity. This measurement is crucial for plant health because it controls the chemical forms of nutrients in the soil, which in turn governs their solubility and availability for root uptake.
A moderate pH range, typically between 6.0 and 7.5, is optimal for most plants. In this range, essential nutrients, including nitrogen, phosphorus, and most micronutrients, are readily accessible. When the pH deviates too far, nutrients can become “locked up,” meaning they are chemically present but unavailable to the plant, leading to deficiencies. For instance, highly acidic soil can cause aluminum toxicity, while highly alkaline soil makes micronutrients like iron and manganese insoluble. Adjusting the soil pH aims to create the ideal chemical environment for the specific plants being grown, not necessarily perfect neutrality.
Testing Soil pH
Accurate measurement is the necessary first step before attempting any soil adjustment. The pH scale is logarithmic, meaning small numerical changes represent large shifts in acidity or alkalinity, highlighting why precision in testing is valuable. The most accurate method for determining soil pH involves sending a soil sample to a professional agricultural laboratory. This provides a detailed report including the current pH, the “buffer pH,” and the precise amount of amendment needed.
For a quicker assessment, a gardener can use an at-home test kit (which uses a color-coded chemical reaction) or a digital pH meter. To take a representative sample, collect soil from several spots across the planting area, mix them together, and remove any debris like rocks or roots. Understanding your soil type is also important, as plants generally thrive in a slightly acidic to neutral window.
Correcting Acidic Soil
Correcting overly acidic soil involves raising the pH, or making it more alkaline, through the addition of liming materials. The most common and effective amendment is agricultural lime, also known as calcium carbonate, which works by neutralizing the excess hydrogen ions causing acidity. Dolomitic lime is another option that contains both calcium carbonate and magnesium carbonate, which is beneficial if the soil test also indicates a magnesium deficiency.
Liming products come in powdered or pelletized forms. Powdered lime reacts more quickly due to its finer particle size, though it is often more difficult to spread evenly. The amount of lime required depends entirely on the soil’s current pH, the desired target pH, and the soil texture. Clay soils have a higher buffering capacity and require significantly more lime to achieve the same pH change than sandy soils do.
To apply the material, spread the calculated amount evenly over the soil surface and then incorporate it thoroughly into the top six to eight inches of soil using a tiller or spade. Surface application without incorporation is much less effective because the material cannot easily move down into the root zone where the pH adjustment is needed. The chemical reaction that raises the pH is a gradual process, often taking several months to fully complete, so it is best to apply lime well in advance of planting.
Correcting Alkaline Soil
Lowering the pH of soil, or making it more acidic, is necessary for plants that prefer an acidic environment, such as blueberries, azaleas, or rhododendrons, or when the native soil is excessively alkaline. This process is generally slower and requires more patience than raising the pH. The most common and reliable material used to decrease soil pH is elemental sulfur, which is a slow-release acidifying agent.
Elemental sulfur works through a biological process: soil bacteria oxidize the sulfur into sulfuric acid. This microbial conversion can take several months and depends on warm soil temperatures and adequate moisture. The application rate is determined by the soil’s current pH and its buffering capacity, with heavier clay soils requiring higher amounts of sulfur than sandy soils.
For a temporary, milder adjustment, organic materials like sphagnum peat moss or composted pine needles can be incorporated. These materials decompose and release organic acids, slightly lowering the pH in the immediate area. However, these organic amendments are not a permanent solution, as their acidifying effect is less pronounced and shorter-lived than elemental sulfur. Retest the soil after a few months to track the pH change, as over-acidifying can lead to toxicity issues with elements like aluminum and manganese.
Long-Term Soil pH Management
Achieving the correct soil pH is not a one-time event; it is an ongoing maintenance process that requires regular monitoring. Natural processes, such as the decomposition of organic matter and the use of certain nitrogen fertilizers, tend to slowly drive soil pH downward. Irrigation with hard water, which contains dissolved calcium and magnesium carbonates, can also gradually increase the soil’s alkalinity. Periodic re-adjustments are usually necessary.
Gardeners should re-test their soil annually or biannually, particularly in the areas where pH-sensitive plants are grown. When making subsequent applications of amendments, whether lime or sulfur, apply them gradually and wait at least three to six months between major applications. This waiting period allows the chemical reactions to stabilize and prevents accidental over-correction, which can be difficult to reverse. Consistent, small-scale maintenance based on testing results is far more effective than large, infrequent amendments.