Agricultural lime, typically composed of ground limestone rock containing calcium carbonate or dolomite, is a soil amendment used to manage soil acidity. Its function is to raise the soil’s pH level, reducing acidity and creating a more favorable environment for plant growth. This adjustment is necessary only when the soil has become too acidic for desired plants to thrive.
How Lime Affects Soil Chemistry
Lime reverses soil acidity through a chemical process. Soil acidity is caused by an abundance of positively charged hydrogen ions and aluminum ions binding to soil particles. When lime dissolves, it releases base ions, such as carbonate, which neutralize these acidic ions by forming water and carbon dioxide.
This neutralization raises the soil’s pH toward a neutral range, typically between 6.0 and 7.0, optimal for most cultivated plants. High acidity causes essential nutrients, such as phosphorus, to become chemically bound to soil particles, making them unavailable for plant uptake. By raising the pH, lime frees these nutrients, improving overall plant nutrition. Liming also reduces aluminum toxicity, which can stunt root growth under highly acidic conditions.
Specific Plants That Require Liming
Many common garden vegetables and turf varieties prefer a neutral to slightly alkaline soil environment and benefit from lime application if the pH is low. The Brassica family, including cabbage, broccoli, cauliflower, and Brussels sprouts, thrives when the soil pH is closer to 6.5 or 7.0. Liming is beneficial for Brassicas because it helps suppress clubroot, a disease that flourishes in acidic conditions.
Legumes like peas, beans, and clover respond well to liming, as a higher pH encourages the growth of bacteria that fix atmospheric nitrogen into a usable form. Other plants that perform best in neutral soil and may require lime include:
- Onions
- Garlic
- Spinach
- Parsnips
- Kentucky Bluegrass
- Tall Fescue
Lime should be avoided for plants that require acidic soil, as raising the pH will cause them harm. Acid-loving plants, often called ericaceous plants, include blueberries, rhododendrons, azaleas, and hydrangeas. For these varieties, a high pH can induce nutrient deficiencies, such as iron and manganese, leading to chlorosis or yellowing of the leaves. Potatoes are an exception, as they tolerate acidic conditions, and liming can promote potato scab disease.
Determining If Your Soil Needs Lime
Lime should never be applied without first testing the soil, as over-liming can make the soil too alkaline and create new nutrient deficiencies. The most reliable assessment method is to send a soil sample to a cooperative extension office or a private laboratory for analysis. These professional tests provide an accurate pH reading and offer a specific recommendation, called the lime requirement, which indicates how much lime is needed to reach the target pH.
While DIY testing kits offer a quick estimate of pH, they do not provide the detailed application rate or account for the soil type, which affects buffering capacity. Clay soils have a high buffering capacity and require more lime than sandy soils to achieve the same pH change. The optimal pH for most lime-responsive vegetables and lawns falls between 6.0 and 7.0.
Methods for Lime Application
Once the soil test confirms the need for lime and provides an application rate, the next steps involve choosing the correct material and technique. Agricultural lime is most commonly sold as calcitic lime (primarily calcium carbonate) or dolomitic lime (which includes magnesium carbonate and is useful for magnesium-deficient soils). These come in powdered or granular (pelletized) forms.
Powdered lime reacts quickly but is dusty and difficult to spread evenly. Pelletized lime is easier to handle and apply with a standard spreader, making it the preferred choice for established lawns, though it takes longer to dissolve. The best time to apply lime is in the fall or early winter, allowing the material several months to react before the spring planting season. For maximum effectiveness, the lime should be spread uniformly and worked into the top four to six inches of soil, especially in garden beds.