Agricultural lime is a soil amendment composed primarily of calcium carbonate or a combination of calcium and magnesium carbonate. Its purpose is to correct soil acidity, which is measured on the pH scale. There is no standard application rate for lime, as the required poundage per acre varies significantly depending on the unique chemical and physical conditions of the soil. The amount needed is calculated to adjust the soil to a more neutral level, optimizing conditions for plant health and nutrient uptake.
The Function of Agricultural Lime in Soil
The primary challenge in acidic soils is that a low pH hinders the availability of necessary plant nutrients. When soil pH drops below approximately 5.5, the solubility of toxic elements, particularly aluminum, increases sharply, which can severely restrict root growth and function. Aluminum toxicity prevents the absorption of water and other nutrients, leading to stunted plant development.
Lime works by introducing a source of alkalinity, typically carbonate ions. These carbonate ions react chemically with the free hydrogen ions that cause acidity, neutralizing them to form water and carbon dioxide gas. This neutralization raises the soil pH, reducing the concentration of harmful aluminum and manganese ions.
Neutralizing acidity improves the uptake of essential nutrients. For instance, in highly acidic conditions, phosphorus often becomes chemically bound to iron or aluminum, making it unavailable to plants. Raising the pH through liming releases this fixed phosphorus and other positively charged nutrients, such as calcium and magnesium, allowing them to be absorbed efficiently by the plant roots.
Determining Your Soil’s Specific Needs
The only dependable method to determine the exact pounds of lime required per acre is through a professional soil test. A simple pH test only reveals the active acidity—the hydrogen ions currently in the soil solution—which does not reflect the total acidity that must be neutralized. The true measure of the required lime is found by assessing the soil’s buffering capacity, or reserve acidity.
To obtain an accurate recommendation, a representative soil sample must be collected from multiple random spots across the area being treated and sent to a certified laboratory. The analysis provides several readings, but the most important for calculating lime application is the Buffer pH reading, sometimes referred to as the SMP or Mehlich Buffer.
The Buffer pH is measured after the lab adds a neutral buffer solution to the soil sample. The resulting drop in the buffer solution’s pH indicates the soil’s resistance to a pH change, or its buffering capacity. Soils with high buffering capacity require substantially more lime to achieve the desired pH target than weakly buffered soils, even if their initial active pH reading is the same. The final soil test report uses the Buffer pH, the target pH for the crop being grown, and the tillage depth to generate a precise recommendation, typically provided in pounds or tons of lime per acre.
Variables that Dictate the Required Poundage
The recommended poundage of lime per acre is highly dependent on the interplay of three specific soil and material factors.
Soil Texture and Buffering Capacity
The first factor is the soil’s texture, which directly influences its Cation Exchange Capacity (CEC). Soils with high clay content and high organic matter have a greater number of negative charge sites, giving them a high CEC and a strong buffering capacity. These soils resist pH change significantly. They require a much greater volume of lime to raise the pH one unit compared to sandy soils with low CEC.
Current vs. Target pH
The second variable is the distance between the soil’s current pH and the desired target pH for the intended crop. A soil needing to move from a pH of 5.0 to 6.5 requires a considerably higher application rate than a soil moving from 5.8 to 6.5 because of the exponential nature of the pH scale. A larger pH adjustment requires neutralizing a greater amount of acidity, directly increasing the pounds-per-acre recommendation.
Quality of Liming Material
The third factor is the quality of the liming material itself, measured by its Calcium Carbonate Equivalent (CCE) and its fineness. The CCE reflects the neutralizing power relative to pure calcium carbonate; a higher CCE means a greater acid-neutralizing capability per pound of material. Furthermore, the fineness of the grind is important because smaller lime particles have a greater surface area, which allows them to react more quickly with the soil acidity.
Methods for Effective Lime Application
Once the required poundage is determined, applying lime during the fall or winter is often recommended. This timing allows several months for the material to react with the soil before the next planting season. Lime is a slow-acting amendment, and its full effect may take six months to a year to be realized.
Incorporation is the most efficient method for rapid pH adjustment, typically achieved by tilling the material into the top six to eight inches of the soil profile. This mixing ensures the lime is distributed throughout the root zone. Surface application is necessary for no-till systems, relying on natural processes like earthworm activity and rainfall to slowly move the lime down into the soil.
When selecting a product, the choice between calcitic and dolomitic lime should be guided by the soil test results. Calcitic lime is primarily calcium carbonate, while dolomitic lime contains magnesium carbonate. If the soil test indicates a magnesium deficiency, dolomitic lime corrects both the acidity and the nutrient imbalance.