Dolomite lime is a naturally occurring rock powder used as a soil amendment, primarily in acidic environments. Composed of calcium carbonate and magnesium carbonate, it supplies two essential nutrients for plant growth. Its main function is to neutralize soil acidity, raising the soil’s pH level to a range where most garden plants can efficiently absorb available nutrients. This process corrects soil imbalances and promotes vigorous plant growth.
The Necessary Precursor: Soil Testing
Determining the correct amount of dolomite lime requires a comprehensive soil test. Guessing the application rate without this foundational data often leads to ineffective use or unintended harm. The test results provide two fundamental pieces of information: the current soil pH and the soil’s buffer pH.
The current pH level indicates the existing degree of acidity or alkalinity, showing how far the soil is from the target range (typically pH 6.0 to 7.0 for most plants). The buffer pH, sometimes called the lime requirement index, measures the soil’s resistance to a change in pH, known as its buffering capacity.
Soils rich in clay and organic matter have a high buffering capacity and require significantly more lime for a small pH adjustment than sandy soils. This is because the lime must neutralize both the active acidity in the soil water and the reserve acidity held by the soil particles. Without the buffer pH reading, calculating the required amount of lime is based on incomplete information and will likely result in an incorrect rate.
Determining the Application Rate
The exact amount of dolomite lime needed integrates three factors: the difference between the current and target pH, the soil’s texture, and the planned depth of incorporation. Laboratories use the buffer pH to provide a precise recommendation, often expressed in pounds of lime per 100 or 1,000 square feet, to reach a target pH of 6.5. This recommendation accounts for the starting acidity and the soil’s inherent resistance to change.
The target pH for most garden plants is generally 6.0 to 6.5, optimizing the availability of most plant nutrients. If a soil test shows a current pH of 5.0, the goal is to raise the pH by 1.0 to 1.5 units. For example, a medium loam soil may require approximately 7 pounds of ground limestone per 100 square feet to raise the pH by one full point.
Soil texture significantly affects the required rate. Clay particles and organic matter hold onto acid-forming ions, demanding more neutralizing agent. Sandy soils require substantially less lime for the same pH change than heavy clay soils. To achieve a one-point pH increase, a sandy loam might require about 5 pounds per 100 square feet, while a heavy clay soil could require 8 to 10 pounds.
The intended depth of incorporation also influences the quantity of lime required. Recommendations are often based on mixing the lime into the top 6 to 8 inches of soil, the typical rooting zone. If the lime is only surface-applied or incorporated to a shallower depth, the application rate should be proportionally reduced. If tilling is planned for a depth greater than 8 inches, the rate may need to be adjusted upward to treat the larger volume of soil.
Methods for Applying Dolomite Lime
Once the precise rate is determined, the application of dolomite lime must ensure even distribution. The two primary methods are full incorporation or surface application. Full incorporation is preferred for unplanted garden beds, as it provides the fastest and most uniform pH adjustment.
Full Incorporation
Spread the calculated amount evenly over the soil surface using a rotary or drop spreader. Then, thoroughly mix the lime into the top 4 to 6 inches of soil using a tiller, spade, or heavy rake. Mixing the lime into the root zone allows it to react quickly with the acidic soil particles.
Surface Application
For established areas like lawns or no-till garden beds, surface application is necessary, though results are slower. The lime is broadcast over the surface and allowed to wash into the soil by rain or irrigation. Apply lime during the fall or winter months, allowing time for the material to react before the spring planting season.
Wear a dust mask and gloves when handling powdered dolomite lime, as the fine particles can irritate the respiratory system and skin. Applying the lime to a dry soil surface helps minimize dust. Watering immediately after application helps move the particles into the soil profile.
Consequences of Incorrect Application
Applying an incorrect amount of dolomite lime can lead to problems that negatively affect plant health. When too little lime is applied, soil acidity persists, and the application is ineffective. The pH will not rise sufficiently to overcome the toxicity of elements like aluminum or optimize nutrient availability, leaving plants struggling.
The more serious consequence is over-liming, where excessive dolomite lime causes the soil pH to rise into the alkaline range. An overly high pH severely limits the availability of several micronutrients, including iron, manganese, and zinc, effectively “locking them up.” This deficiency often manifests as chlorosis, causing the plant’s leaves to turn yellow.
Applying dolomite lime when the soil already has sufficient magnesium can create a nutrient imbalance by disrupting the optimal calcium-to-magnesium ratio. Correcting an over-limed soil is a slow process, sometimes requiring acidic amendments like elemental sulfur to lower the pH. Often, the most practical measure is to wait, as the effects of lime are long-lasting, and the soil will naturally acidify over several years.