Garden lime is a naturally occurring rock powder, typically derived from crushed limestone, used as a soil amendment to promote healthier plant growth. The active ingredient is primarily calcium carbonate, which works to neutralize soil acidity. When soil becomes too acidic, essential plant nutrients become less available for root uptake. Applying lime, known as liming, is a chemical method of raising the soil’s pH level to balance soil chemistry.
The Foundation: Determining Soil Needs
Applying any soil amendment without first understanding the current soil condition is often wasteful and potentially harmful. Therefore, the first step before using garden lime is a professional soil test, which measures the soil’s acidity level, or pH. The pH scale ranges from 0 (most acidic) to 14 (most alkaline), with a pH of 7 being neutral. Most plants thrive in a slightly acidic to neutral range, typically between 6.0 and 7.0.
Soils registering below 6.0 can inhibit the plant’s ability to absorb nutrients like phosphorus, potassium, and magnesium. Conversely, soils that are too alkaline (above 7.5) can cause deficiencies in micronutrients such as iron and manganese. A comprehensive soil test provides a precise pH reading and a specific recommendation for the amount of lime needed. This recommendation accounts for the soil’s texture, as clay soils require more lime to change the pH than sandy soils.
The soil test also measures the soil’s buffering capacity, which is its ability to resist a change in pH. This measurement is factored into the final lime recommendation, ensuring the prescribed amount is effective for your particular soil composition. Following the laboratory’s exact recommendation prevents the mistake of raising the pH too quickly or too high.
Types and Selection of Garden Lime
Garden lime comes in several forms, each offering a distinct chemical composition and physical structure. The two main types are differentiated by their mineral content: calcitic and dolomitic limestone. Calcitic limestone is composed almost entirely of calcium carbonate, providing a calcium boost in addition to raising the pH. This form is a good choice if your soil test indicates a calcium deficiency without a concurrent magnesium deficiency.
Dolomitic limestone contains a mixture of calcium carbonate and magnesium carbonate. Selecting this type is advisable when the soil test reveals low pH and a deficiency in magnesium, an element plants need for photosynthesis. While both types are effective at neutralizing acidity, the choice between them should be based directly on the specific nutrient requirements identified in your soil analysis.
The physical form of the lime also influences its performance and ease of application. Pulverized or powdered lime features a very fine particle size, allowing it to react more quickly with the soil to adjust the pH. However, this fine powder can be dusty and challenging to spread evenly, requiring careful handling to avoid inhalation.
Pelletized or granulated lime is essentially powdered lime compressed into small, easy-to-handle granules. These pellets are less dusty and can be spread more uniformly using a standard fertilizer spreader. Although the pellets must first dissolve back into a fine powder to begin working, the convenience and uniformity of application often make pelletized lime the preferred choice for home gardeners.
Application Techniques and Timing
The most effective time to apply garden lime is in the fall or very early spring, well before the start of the main growing season. Liming materials do not immediately alter the soil’s chemistry; they require time and moisture to dissolve and react with the soil particles. Applying lime in the fall allows winter precipitation and freeze-thaw cycles to help integrate the material, with the pH adjustment typically becoming noticeable several months later.
The first step in application is accurately measuring the area to be treated and calculating the amount of lime needed based on the lab recommendation. For large areas, a broadcast or drop spreader ensures the most uniform distribution across the soil surface. This prevents pockets of overly-limed or under-limed soil, which can lead to patchy plant growth.
When applying powdered lime, it is necessary to wear protective gear, including a mask or respirator and gloves, to avoid inhaling the dust and irritating the skin. Once spread, the lime must be incorporated into the soil, ideally to a depth of four to six inches in garden beds. Tilling or raking the lime into the root zone maximizes contact between the amendment and the acidic soil particles, promoting a faster reaction.
If applying lime to an established lawn or perennial bed where tilling is not possible, the material should be watered in lightly immediately after spreading. This watering helps the pellets or powder settle into the topsoil and prevents it from being blown away. Since the reaction time is slow, a single application is usually effective for several years, making the initial effort a long-term investment in soil health.
Common Mistakes to Avoid When Liming
The most common error in liming is applying the material without a soil test, which often results in over-liming. If the soil pH is raised too high, typically above 7.5, it can induce nutrient deficiencies by causing micronutrients like iron, zinc, and manganese to chemically lock up in the soil. This nutrient unavailability can cause chlorosis, where plant leaves turn yellow while the veins remain green, leading to stunted growth.
A second mistake is mixing lime directly with certain nitrogen fertilizers or organic materials like manure. Lime is alkaline, and when combined with materials containing ammonium compounds, it can cause a chemical reaction called ammonia volatilization. This reaction releases nitrogen gas into the atmosphere, leading to a loss of the fertilizer’s intended nutrient benefit.
To prevent this loss, it is recommended to separate the application of lime and nitrogen sources by at least one to two months. This time lag allows the lime to react with the soil and minimize the risk of nutrient dissipation. Finally, re-testing the soil every one to three years is necessary for proper soil management. Since soil acidity naturally returns over a period of years, re-testing determines if another, often smaller, lime application is needed.