Agricultural lime, often referred to as aglime or pulverized limestone, is a soil amendment derived from crushed limestone or chalk deposits. Its primary active component is calcium carbonate, though it may also contain magnesium carbonate if it is dolomitic lime. Applying lime is a strategy for managing soil chemistry, aiming to create a healthier environment for plant roots and beneficial soil organisms. This material is not a fertilizer, but it corrects a foundational soil problem—acidity—which allows plants to make better use of nutrients already present in the soil. The purpose of liming is to adjust the soil’s chemical balance to promote optimal growth and improve crop yields.
The Primary Role of pH Adjustment
The main function of agricultural lime is to raise the soil’s pH, effectively reducing its acidity. Acidity in soil is caused by an excess concentration of hydrogen ions (\(\text{H}^+\)) and aluminum ions (\(\text{Al}^{3+}\)), which naturally accumulate over time due to factors like heavy rainfall leaching basic minerals, the decomposition of organic matter, and the use of certain nitrogen fertilizers. The pH scale is logarithmic, meaning a small change in the number represents a large change in acidity.
When calcium carbonate (\(\text{CaCO}_3\)) from the lime dissolves in wet, acidic soil, it forms calcium ions (\(\text{Ca}^{2+}\)) and carbonate ions (\(\text{CO}_3^{2-}\)). The carbonate ions then react with the acidic hydrogen ions, neutralizing them to form water (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)). This consumption of \(\text{H}^+\) ions directly increases the soil’s pH.
Furthermore, the calcium ions released by the lime displace the toxic aluminum ions (\(\text{Al}^{3+}\)) and \(\text{H}^+\) ions from the soil’s negatively charged exchange sites. This process, called increasing base saturation, removes the acidity-causing elements from the soil solution and replaces them with calcium. The reduction in soluble aluminum is significant because high concentrations of aluminum in acidic soils can be toxic to plant roots, inhibiting their growth and function.
Impact on Existing Nutrient Availability
Correcting soil acidity with lime creates a more favorable chemical environment where nutrients are most soluble and accessible to plant roots. Most agricultural plants thrive in a “pH sweet spot,” generally considered to be in the range of 6.0 to 7.0, where the availability of many macronutrients and micronutrients is optimized. When the soil pH drops below this range, the chemical forms of several nutrients change, making them less available for plant uptake.
Raising the pH substantially increases the availability of phosphorus (P), which tends to become chemically locked up with iron and aluminum in highly acidic conditions. The availability of molybdenum (Mo), a micronutrient needed for nitrogen metabolism, is also significantly improved as pH increases. Conversely, the higher pH reduces the solubility of elements like manganese (Mn) and aluminum (Al), preventing them from reaching toxic levels that stunt plant growth.
Liming also indirectly supports the availability of nitrogen (N) by promoting the activity of beneficial soil microorganisms. The bacteria responsible for converting ammonium to plant-usable nitrate (nitrification) are most active in a near-neutral pH range. By creating a more hospitable environment, lime enhances the rate at which organic matter is decomposed, thereby releasing organic nitrogen, sulfur, and phosphorus back into the soil.
Supplying Essential Minerals
Beyond its role as a pH modifier, agricultural lime directly supplies two secondary macronutrients necessary for plant health: calcium (Ca) and magnesium (Mg). All forms of agricultural lime are a source of calcium, which is a structural component of plant cell walls. Calcium provides rigidity and stability to the plant tissue, which is important for overall plant structure and resilience against pathogens.
The specific type of lime determines the magnesium contribution. Calcitic lime is primarily calcium carbonate, while dolomitic lime contains a significant amount of magnesium carbonate. Magnesium is a fundamental atom at the center of the chlorophyll molecule, making it necessary for photosynthesis. Using dolomitic lime is an effective way to address a magnesium deficiency, particularly in sandy soils or areas where the mineral has been significantly leached out.
Determining When and What Type to Apply
The application of lime must be based on a comprehensive soil test, not a routine decision. A soil test provides the current pH level, which indicates the degree of acidity, and also measures the concentrations of existing nutrients like calcium and magnesium. The buffer pH measurement determines the specific “liming requirement,” which is the exact amount of lime needed to neutralize the soil’s acidity.
The soil test results also dictate the correct type of lime to use. If the soil is acidic but already has sufficient magnesium levels, calcitic lime (high in calcium) is the appropriate choice to avoid creating a nutrient imbalance. Conversely, if the test reveals both low pH and a deficiency in magnesium, dolomitic lime is recommended because it supplies both the acid-neutralizing capacity and the needed magnesium. Because lime is slow-acting—taking months or even a year to fully react—application is often recommended in the fall or dormant season, giving the material time to dissolve and adjust the soil chemistry before the next growing season.