The answer to whether lime raises soil pH is a definitive yes, which is why it is a widely used soil amendment in agriculture and gardening. Soil pH measures the acidity or alkalinity of the soil on a scale from 0 to 14, with 7.0 being neutral; values below 7.0 are acidic and values above 7.0 are alkaline. Soil acidity is a common problem, especially in areas with high rainfall or where certain nitrogen-based fertilizers have been used. Raising the soil pH is necessary because the availability of many essential plant nutrients is reduced in acidic conditions. Lime works by neutralizing the free hydrogen ions that cause acidity, creating a more favorable environment for most plants to thrive.
The Mechanism of Alkalinity
Lime, typically in the form of ground limestone, acts chemically to neutralize the acidity in soil, thereby increasing the pH. The active component in agricultural lime is the carbonate ion, which is part of the compounds calcium carbonate or magnesium carbonate. When lime is introduced into acidic soil, it dissolves slowly in the soil water. The carbonate component then reacts with the hydrogen ions that are responsible for the soil’s acidity.
The key neutralization reaction involves the carbonate ion combining with two hydrogen ions to form water and carbon dioxide. This reaction effectively removes the acid-causing hydrogen ions from the soil solution, which is the direct cause of the pH rising. The calcium or magnesium ions released from the lime also contribute to the process by displacing hydrogen and aluminum ions from the negatively charged exchange sites on soil particles, promoting a higher pH.
The process of neutralization also reduces the solubility of aluminum and manganese in the soil. In highly acidic soils, these elements can become toxic to plants, stunting root growth and limiting nutrient uptake. By raising the pH, the lime converts these toxic ions into non-toxic, insoluble forms, which improves crop yields on acidic land.
Types of Liming Materials
Liming materials are primarily categorized based on their mineral composition, which determines their nutrient contribution and rate of reaction. The two most common forms of agricultural lime are Calcitic and Dolomitic lime, both derived from crushed limestone rock. Calcitic lime is composed mainly of calcium carbonate, making it a source of calcium for the soil while neutralizing acidity.
Dolomitic lime contains a blend of both calcium carbonate and magnesium carbonate. This type is beneficial when soil tests indicate a deficiency in magnesium, as it supplies this essential nutrient along with calcium. Although both types are effective at neutralizing soil acidity, the choice between them should be guided by a soil test that reveals the existing calcium-to-magnesium ratio.
Other materials, such as quicklime (calcium oxide) and hydrated lime (calcium hydroxide), are also highly effective at raising pH due to their concentrated alkalinity. However, these forms are rarely used in general gardening or large-scale agriculture. They react much more rapidly and are caustic, meaning they can be hazardous to handle. Ground agricultural limestone is preferred for its slower, safer, and more sustained action.
Practical Application and Measurement
Accurately determining the amount of lime needed requires a professional soil test, which is the most important step before application. The test measures the active pH of the soil, but more importantly, it determines the soil’s buffer pH, which is used to calculate the lime requirement. The buffer pH test assesses the soil’s capacity to resist a change in pH, known as its buffering capacity, because it accounts for the “reserve acidity” held by the soil particles.
Application rates are highly dependent on the soil type because different soil textures have varying buffering capacities. Clay soils and those high in organic matter tend to have a higher buffering capacity, meaning they require a larger quantity of lime to achieve the same pH change compared to sandy soils. The lime recommendation is calculated based on the difference between the current pH and the desired target pH for the specific crop being grown, as well as the soil’s texture.
For maximum effectiveness, the lime should be uniformly mixed or incorporated into the top six to eight inches of the soil, which allows for optimal contact with the acidic soil particles. When incorporation is not possible, such as on established lawns or no-till fields, lime can be surface-applied, but the reaction will be much slower, sometimes taking several years to affect the pH below the top inch or two. Since agricultural lime is slow-acting, often taking several months to a year to fully neutralize acidity, it is best applied well in advance of planting, typically in the fall or winter.