Agricultural lime, which is derived from pulverized limestone rock, is fundamentally an alkaline material. Its primary function is to neutralize soil acidity, effectively raising the soil’s pH level towards the neutral or alkaline range. This adjustment is necessary because most commercial crops and garden plants thrive in slightly acidic to neutral soil, typically within a pH range of 6.0 to 7.0.
The Chemical Effect of Agricultural Lime
Soil acidity is created by an abundance of free hydrogen ions (\(\text{H}^+\)) in the soil solution. To counteract this, agricultural lime introduces a base, usually in the form of carbonate (\(\text{CO}_3^{2-}\)), oxide (\(\text{O}^{2-}\)), or hydroxide (\(\text{OH}^-\)) ions. When lime dissolves in moist soil, the carbonate ions react with the hydrogen ions, effectively removing them from the soil solution.
This chemical reaction neutralizes the acid, producing water (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)). The consumption of \(\text{H}^+\) ions raises the pH, making the soil less acidic and more alkaline. The liming process also helps to mitigate the toxicity of elements like aluminum and manganese, which can become soluble and harmful to plant roots in highly acidic conditions.
Types of Liming Materials
The term “lime” in the context of soil amendment encompasses several calcium and magnesium compounds. The most common form is ground limestone, which is primarily calcium carbonate (\(\text{CaCO}_3\)), often called calcitic lime. Another widely used option is dolomitic limestone, which contains both calcium carbonate and a significant amount of magnesium carbonate (\(\text{MgCO}_3\)). These materials are natural, pulverized rock and are slower-acting but safer to handle.
Processed Limes
Processed forms, such as quicklime (calcium oxide, \(\text{CaO}\)) and hydrated lime (calcium hydroxide, \(\text{Ca(OH)}_2\)), are also available. These processed limes are highly caustic and much more reactive, meaning they change the soil pH very quickly. They require extremely careful handling and are generally not recommended for home gardeners.
Why the Confusion About Acidity Exists
The confusion regarding lime’s effect often stems from the shared name with the acidic citrus fruit. The fruit, a lime, is highly acidic due to its citric acid content, which is the opposite chemical property of the soil amendment. Agricultural lime is a pulverized mineral rock that works to reduce acidity.
The misconception is sometimes reinforced by other soil amendments that increase acidity, such as elemental sulfur or aluminum sulfate. These substances are used to lower the pH for acid-loving plants like blueberries and azaleas.
Application and Measurement for Soil Health
Applying lime without first testing the soil is strongly discouraged, as over-liming is detrimental to plant health. An overly alkaline soil can lock up essential micronutrients like iron and zinc, causing plant deficiencies. A professional soil test will determine the current pH level and the soil’s reserve acidity, often measured using a buffer method like the SMP test.
The required lime application rate is calculated based on the current pH, the target pH for the intended crop, and the soil type. Clay soils, for example, have a higher cation exchange capacity and require significantly more lime to change the pH than do sandy soils. Incorporating the lime into the top six to eight inches of the soil allows for the best and fastest reaction.
Since agricultural lime is slow to dissolve and react, it is best applied in the fall or winter to allow three to six months for the pH adjustment to occur before the spring planting season. Regular testing every three to four years will help determine the need for maintenance applications, as soil naturally tends to revert to an acidic state over time.