Soil pH measures the acidity or alkalinity of the soil on a logarithmic scale from 0 to 14. Values below seven indicate acidity, while values above seven indicate alkalinity. Maintaining the correct pH is fundamental because it governs the availability of essential plant nutrients. If the pH is too low, nutrients like phosphorus, nitrogen, and potassium become chemically bound and unavailable for plant uptake. Adjusting acidic soil to a neutral or slightly alkaline range is accomplished by adding alkaline materials, a process known as liming, which allows roots to absorb the necessary elements for vigorous growth.
Essential Pre-Requisite: Testing Soil pH
Adjusting soil pH without a current test is inefficient and risks over-liming, which is detrimental to plants. While home kits provide a general estimate, professional laboratory testing is the only accurate way to determine the factors necessary for successful liming. Lab results provide the current pH and the soil’s buffering capacity—its resistance to pH change.
The buffering capacity is used to calculate the “lime requirement,” which is the precise quantity of material needed to reach a target pH. Soils high in clay or organic matter have a greater buffering capacity and require more material than sandy soil to achieve the same change. Sending a sample to a certified lab, often through an agricultural extension office, ensures the recommendation is tailored to the specific soil type. This prevents material waste and the risk of creating a new nutrient imbalance.
Choosing the Right pH-Raising Material
Agricultural limestone, primarily calcium carbonate, is the standard material for raising soil alkalinity. The choice between the two main types depends on the soil test results, specifically the magnesium level. Calcitic limestone is used when the soil contains sufficient magnesium, as it adds only calcium to neutralize acidity. Dolomitic limestone contains both calcium carbonate and magnesium carbonate, making it the appropriate choice for soils deficient in magnesium.
Faster-Acting Alternatives
Agricultural limestone works slowly, providing a sustained effect, but faster-acting materials exist for quick corrections. Hydrated lime (calcium hydroxide) is highly reactive and provides rapid pH adjustment, but it is caustic and requires extreme caution and protective equipment. Wood ash is another option containing calcium and potassium, offering a quicker release than ground limestone. However, its effects are shorter-lived, and its higher salt content can harm young plants. The optimal material should be selected based on elemental needs, desired speed of change, and safety considerations.
Effective Application Methods and Timeline
The timing of the application significantly influences the material’s effectiveness. Applying the amendment in the fall or early spring is ideal, providing several months for the material to break down before the main growing season. Fall application is particularly beneficial because winter moisture and freeze-thaw cycles help incorporate the material and speed up its chemical reaction. For gardens, the material must be incorporated into the top six to eight inches of soil for the fastest results. In established areas like lawns, the material is typically surface-applied using a broadcast spreader.
Raising soil pH is a gradual process that can take three to six months for measurable change and up to a year for the full effect. Due to the fine dust created during spreading, wearing a dust mask, protective eyewear, and gloves is strongly recommended for safety. Retesting should be performed every two to three years to maintain the desired soil pH.