Soil pH measures the acidity or alkalinity of the soil, a fundamental factor governing plant health. The pH scale runs from 0 to 14; 7 is neutral, below 7 is acidic, and above 7 is alkaline. This measurement directly controls the chemical forms of nutrients in the soil solution, influencing their solubility and availability for plant uptake.
When soil pH is too high or too low, essential minerals become chemically “locked up,” preventing roots from absorbing them. In highly acidic soil, the availability of macronutrients like phosphorus and calcium decreases, and elements such as aluminum and manganese can become toxic. Conversely, in alkaline soils, micronutrients like iron, zinc, and manganese often become less available, leading to deficiency symptoms like leaf yellowing. Optimizing soil pH ensures plants access the full spectrum of nutrients needed for vigorous growth.
Understanding Your Soil’s Current pH
Determining the current pH level is the necessary first step before any adjustment is made. The pH scale is logarithmic, meaning a single unit change represents a tenfold increase in acidity. Most garden plants thrive in a slightly acidic to neutral range, typically between 6.0 and 7.5.
Two primary methods exist for testing soil: inexpensive home kits and professional laboratory analysis. Home testing kits provide quick, affordable results, often relying on color-matching techniques to estimate the pH. However, these kits can be inaccurate due to the subjective nature of color interpretation and the variable quality of the chemical reagents used.
Professional soil testing laboratories offer a more accurate measurement using standardized procedures and sophisticated equipment. Lab tests also provide a buffer pH, which indicates the soil’s resistance to change and informs how much amendment is needed. Although a lab test costs more and requires a waiting period, it provides the precise data necessary for effective and targeted soil amendments.
Raising Soil pH to Reduce Acidity
When soil is too acidic (below pH 6.0 for most plants), the goal is to increase the pH by adding a liming material. The most common amendments are ground agricultural limestone (calcium carbonate) and dolomitic lime (which contains magnesium carbonate). Choosing between them depends on existing magnesium levels, as dolomitic lime should be avoided if magnesium is already high.
The speed and effectiveness of the liming material are determined by its fineness and chemical form. Finely ground pulverized lime reacts faster than pelletized or granular forms because it has a greater surface area exposed to the soil solution. Hydrated lime (calcium hydroxide) is the fastest-acting form, but it is caustic and is not recommended for home gardeners due to the risk of over-application and root burn.
Application rates must be based on the soil’s texture, which affects its buffering capacity, or resistance to pH change. Clay soils and soils high in organic matter require significantly more lime than sandy soils to achieve the same pH shift. For example, a sandy soil may require 2 pounds of ground limestone per 100 square feet, while a clay soil might need 5 pounds for the same effect. Wood ash is a faster-acting, less potent alternative that also raises pH, but it must be used sparingly to avoid excessively high pH levels.
Lowering Soil pH to Reduce Alkalinity
For soils that are too alkaline (above pH 7.5), the objective is to decrease the pH, a process known as acidification. The most common and cost-effective material used is elemental sulfur. Elemental sulfur does not immediately change the pH but relies on soil microorganisms to slowly convert it into sulfuric acid. This oxidation process releases hydrogen ions that neutralize alkalinity, making it a safe, long-lasting, but slow method that can take several months or up to a year to fully react.
Sulfur’s reaction rate depends on factors like soil temperature, moisture, and the population of necessary bacteria. When a quicker reduction is desired, iron sulfate or aluminum sulfate are used because they acidify the soil as soon as they dissolve, bypassing microbial action. However, these sulfates require higher application rates—up to six to eight times the amount of elemental sulfur—and carry a risk of introducing toxic levels of iron or aluminum if misapplied.
Organic matter also contributes to lowering pH over time; materials like sphagnum peat moss or composted leaves can be incorporated. Sphagnum peat is effective, but massive amounts are needed for a significant, lasting change across a large area. Ammonium-based fertilizers, such as ammonium sulfate, are also acid-forming because soil bacteria convert the ammonium into acidic compounds, offering the dual benefit of fertilization and gradual acidification.
Application Timing and Monitoring Results
The effectiveness of any pH adjustment is influenced by the timing and method of application. The best time to apply amendments, whether lime or sulfur, is in the fall or early spring. This allows the materials maximum time to react with the soil before the next planting season. Soil chemical reactions are not instantaneous, and a waiting period is necessary for the pH to stabilize.
For both liming and acidifying materials, thoroughly incorporate the amendment into the top 6 to 8 inches of soil. Scattering the material on the surface only affects the top layer, preventing the desired change from reaching the root zone. Moisture is required for the chemical reactions to occur, so apply when the soil is moist or water the area well after application.
Since the pH change process is slow, especially with elemental sulfur, patience and re-testing are essential. Gardeners should re-test the soil approximately six to twelve months after the initial application to measure the actual shift in pH. This confirms the success of the adjustment and prevents the mistake of over-correcting the soil before the first application has fully reacted.