Soil acidity limits plant growth and crop yield in many agricultural regions. Soil pH measures acidity or alkalinity on a scale of 0 to 14, with values below 7.0 indicating acidity. Most cultivated plants thrive in a slightly acidic to neutral range, typically between pH 6.0 and 6.8. Applying lime, a calcium or magnesium carbonate material, neutralizes this acidity and raises the soil pH to an optimal level. Determining the precise amount of lime required per acre is complex because it depends entirely on the specific characteristics of the field’s soil.
The Critical Role of Soil pH
Soil acidity develops naturally over time, often accelerated by rainfall and the use of ammonium-based nitrogen fertilizers. The chemical basis of acidity is the presence of high concentrations of hydrogen ions (H+) and aluminum ions (Al3+). When soil pH drops below 5.5, aluminum becomes soluble in the soil water, which is toxic to plant roots. This toxicity inhibits root elongation, limiting the plant’s ability to absorb water and nutrients.
Low soil pH also reduces the availability of several essential nutrients. For example, phosphorus is easily bound by iron and aluminum oxides in acidic soils, making it inaccessible to plant roots. Furthermore, the activity of beneficial soil microbes that convert nitrogen and sulfur into plant-available forms is restricted when the pH falls below 6.0. Liming addresses these issues by neutralizing the H+ and Al3+ ions, detoxifying the soil and optimizing nutrient accessibility.
Variables Influencing Lime Requirement
There is no universal application rate for lime because the required amount depends on the soil’s resistance to pH change. The primary variable is the difference between the current soil pH and the target pH required for the specific crop. While many crops prefer a pH near 6.5, acid-tolerant plants like blueberries require a much lower target (pH 4.5 to 5.5). The greater this gap, the more lime is necessary to achieve the desired result.
The soil’s texture and organic matter content are key factors because they determine the Cation Exchange Capacity (CEC), which dictates the soil’s buffer capacity. Buffer capacity is the soil’s ability to resist pH change, which is related to the “reserve acidity” held on clay and organic matter particles. Soils with high clay content and high organic matter have a large CEC, meaning they hold a greater reservoir of acidity. Therefore, they require substantially more lime to raise the pH. For instance, raising the pH by one unit in heavy clay soil might require nearly double the amount of lime compared to light, sandy soil.
The most accurate measurement of this resistance is the “buffer pH,” determined only by a professional soil test. Standard pH measures the “active acidity” in the soil solution. In contrast, the buffer pH measures the “reserve acidity” that replenishes the soil solution’s H+ ions as they are neutralized. This buffer reading indicates how much lime is needed to overcome the soil’s natural buffering capacity and achieve the target pH.
Calculating and Applying Lime Per Acre
The recommendation provided by a soil testing laboratory is the most reliable guide for application, typically given as a specific amount of lime per acre. This recommendation is often expressed in pounds of Effective Calcium Carbonate Equivalent (ECCE). ECCE is a standardized measure that accounts for both the purity (neutralizing value) and the fineness (particle size) of the specific liming product. This ensures the applied amount provides the necessary neutralizing power. Coarser or lower-purity lime materials require a heavier application rate to match the neutralizing effect of a finely ground, high-purity product.
Liming materials generally fall into two main chemical types: calcitic lime (primarily calcium carbonate) and dolomitic lime (containing both calcium and magnesium carbonates). Dolomitic lime is often preferred when the soil test indicates a deficiency in magnesium. Lime is available as pulverized agricultural lime (ag-lime) or as pelletized lime.
Typical application rates to correct moderate acidity generally range from 1 to 4 tons of ag-lime per acre. For example, a common recommendation for a soil with 60% ECCE is 1,000 to 1,200 pounds per acre for every 0.1 unit the buffer pH is below the target of 7.0. For best results, the lime should be spread uniformly across the field and thoroughly mixed into the top 6 to 8 inches of the soil profile, ideally by tilling. If the required rate is very high, such as over 4 tons per acre, it is recommended to apply the total amount in two split dressings over a period of time.