Agricultural lime, often called aglime, is a soil amendment composed of pulverized limestone or chalk. Its primary active ingredient is calcium carbonate (\(\text{CaCO}_3\)). Lime’s most important function is to modify the soil’s potential of hydrogen (pH), which measures acidity or alkalinity. Adjusting soil pH is a foundational step in soil management because this chemical property dictates nutrient solubility, microorganism activity, and the potential for toxicity. By adjusting the pH, lime creates a healthier chemical environment, leading to improved plant growth and overall soil productivity.
Neutralizing Soil Acidity
Soil acidity is a natural process accelerated by rainfall leaching basic elements, organic matter decomposition, and the use of nitrogen fertilizers. The pH scale measures this acidity; values below 7.0 are acidic. When soil becomes too acidic, it accumulates hydrogen ions (\(\text{H}^+\)) and dissolved aluminum ions (\(\text{Al}^{3+}\)), which compete with essential nutrients.
Lime works by introducing a base to neutralize these acidic components. Calcium carbonate reacts with water to release bicarbonate ions (\(\text{HCO}_3^-\)). The bicarbonate then combines with the hydrogen ions, neutralizing them to form water (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)).
This process consumes the hydrogen ions, gradually raising the soil pH. Because lime must dissolve, neutralization is not immediate and takes time. Effectiveness relies on the purity and fineness of the grind, as smaller particles react more quickly.
Impact on Essential Nutrient Availability
Soil pH controls the availability of most plant nutrients. When soil pH drops below approximately 5.5, the chemical environment changes, leading to nutrient deficiencies and toxicities. The most significant issue in highly acidic soil is the increased solubility of aluminum, which is toxic to plants. Dissolved aluminum ions inhibit root growth and function, preventing the plant from absorbing water and nutrients.
Raising the pH through liming alleviates aluminum toxicity by converting the soluble, toxic aluminum into insoluble, non-toxic forms. This detoxification allows plant roots to grow normally and efficiently absorb resources. Liming also impacts the availability of other nutrients that become “locked up” in acidic conditions.
For instance, phosphorus (P) is often chemically fixed by iron and aluminum compounds in acidic soil, making it unavailable. By raising the pH, lime reduces the concentration of these fixing agents, which increases phosphorus solubility and uptake. Lime also supplies two essential secondary nutrients: calcium (Ca) and, if dolomitic lime is used, magnesium (Mg). The optimal pH range for most plants, where nutrient availability is maximized, is between 6.0 and 7.0.
Determining When and How to Apply Lime
Lime should be applied only after a comprehensive soil test. This test provides two crucial measurements: the current soil pH (active acidity) and the buffer pH (resistance to pH change). Laboratories use the buffer pH to calculate the precise amount of lime required to reach the desired target pH. Soils rich in clay or organic matter have a higher buffering capacity and require a larger quantity of lime than sandy soils for the same pH adjustment.
There are two primary types of agricultural lime: calcitic and dolomitic. Calcitic lime is mostly calcium carbonate, while dolomitic lime contains both calcium carbonate and magnesium carbonate. The choice between the two is determined by the existing magnesium level shown in the soil test results. If the soil is deficient in magnesium, dolomitic lime is the appropriate choice to supply both the neutralizing agent and the nutrient.
For application, lime is most effective when incorporated into the soil, helping it mix and react more quickly throughout the root zone. Since lime is slow-acting, it is best applied well in advance of planting to allow time for dissolution and neutralization. Finely ground lime reacts faster, but materials with varied particle sizes provide both an immediate effect and a longer-lasting benefit.
Improving Soil Structure and Microbial Activity
Beyond pH adjustment, the calcium supplied by lime plays a physical role in improving soil structure. Calcium promotes the flocculation of clay particles, binding small particles into larger, stable aggregates. This aggregation creates better soil tilth, leading to improved water infiltration and aeration, which benefits root development.
Liming also supports a robust community of beneficial soil microorganisms. Most bacteria responsible for essential processes like organic matter decomposition and nutrient cycling, including nitrogen fixation, prefer a near-neutral pH environment. In highly acidic soils, the activity of these bacteria is hindered, slowing the breakdown of organic material. By raising the soil pH, lime stimulates the activity of these microbes, enhancing the natural fertility and health of the soil.