Agricultural lime is a soil amendment, usually composed of pulverized limestone or chalk, primarily consisting of calcium carbonate. It is not a fertilizer that directly feeds plants but rather a material used to modify the growing environment. Lime can be extremely beneficial to plants, but only when specific soil conditions indicate a need for its application. Using lime without confirmation of soil necessity can cause significant harm, so it should be treated as a targeted adjustment tool.
The Primary Role of Lime in Soil
The primary function of agricultural lime is to neutralize soil acidity and increase the soil’s pH level. Soil pH is a measure of the concentration of hydrogen ions, with values below 7.0 considered acidic and values around 7.0 being neutral. When lime is applied, the carbonate component reacts chemically with these acidic hydrogen ions, effectively neutralizing them to form water and carbon dioxide. This chemical reaction raises the pH, creating a more favorable environment for most agricultural crops that prefer a slightly acidic to neutral range (6.0 to 7.0).
In highly acidic soils, certain beneficial macronutrients, such as phosphorus, become chemically bound and unavailable for plant uptake. Low pH can also increase the solubility of elements like aluminum and manganese to toxic levels, which can inhibit root growth and nutrient absorption. Raising the pH through liming also stimulates the activity of beneficial soil microbes responsible for decomposing organic matter and cycling nutrients like nitrogen.
Understanding Different Lime Varieties
The two main types of agricultural lime, calcitic and dolomitic, differ based on their mineral composition. Calcitic lime is composed predominantly of calcium carbonate and is used when the soil is only deficient in calcium or when magnesium levels are already sufficient. Dolomitic lime is a mixture of calcium carbonate and magnesium carbonate, supplying both calcium and magnesium. This variety is the appropriate choice when soil tests reveal that the soil is acidic and also low in magnesium. Choosing the correct type depends entirely on the current nutrient balance of the soil.
Practical Steps for Safe Application
Applying lime safely and effectively begins with a comprehensive soil test. This test is the only way to accurately determine the current pH and the required application rate. A lab test provides a “buffer pH” reading, which helps calculate the precise amount of lime needed to reach the target pH level. Applying lime based on guesswork is strongly discouraged, as the wrong dosage can lead to severe soil imbalances.
For best results, lime is typically applied in the fall or very early spring, allowing ample time for the material to react before the planting season. Since lime is not highly water-soluble, it must be thoroughly incorporated into the soil profile, usually by tilling or discing it into the top six to eight inches. Surface-applied lime reacts much slower and only corrects the pH in the very top layer, which is less effective for established root systems. The fineness of the lime particles also affects reaction speed.
Risks Associated with Overliming
Applying too much lime and raising the soil pH beyond the optimal range is known as overliming. Overliming carries significant risks, primarily the phenomenon called nutrient lockup. When the soil becomes excessively alkaline, typically above a pH of 7.0 to 7.5, the solubility of several essential micronutrients is severely reduced.
Metals such as iron (Fe), manganese (Mn), and zinc (Zn) precipitate out of the soil solution, becoming chemically unavailable to plants. This unavailability causes plants to display deficiency symptoms, most commonly interveinal chlorosis, where the leaves yellow while the veins remain green. This paradox starves the plant of necessary micronutrients even though they are technically present in the soil.