Agricultural lime, primarily composed of calcium carbonate, is a common soil amendment used to manage soil acidity. Whether applying lime is beneficial for a tree depends entirely on the existing chemical conditions of the soil and the specific biological needs of the tree species. Using it without first understanding these factors can cause more harm than good, potentially inducing nutrient deficiencies. Therefore, the decision to apply lime must be an informed one, guided by scientific analysis.
How Lime Alters Soil Chemistry
The main function of agricultural lime is to neutralize soil acidity by raising the soil’s potential of hydrogen (pH) level. Soil acidity is caused by an excess of hydrogen ions in the soil solution, which lime directly counteracts. The active ingredient, calcium carbonate, dissolves and reacts with these hydrogen ions, converting them into water and carbon dioxide. This reaction effectively increases the soil pH, making it less acidic.
This adjustment is important because highly acidic soil, typically below a pH of 5.5, can “lock up” essential plant nutrients. Nutrients like phosphorus (P) and magnesium (Mg) become chemically bound and insoluble in strongly acidic conditions, making them unavailable for root uptake. Furthermore, low pH can increase the solubility of elements like aluminum and manganese to toxic levels, which can damage tree roots and inhibit nutrient absorption.
By raising the pH into the optimal range, generally between 6.0 and 7.0 for most plant systems, lime releases these otherwise-fixed nutrients, improving their availability. The addition of lime also supplies calcium and sometimes magnesium, which are micronutrients required for healthy growth. Raising the pH too high, however, can cause a separate set of problems, emphasizing the need for precise application.
Identifying When Trees Need Liming
The only accurate way to determine if a tree needs lime and how much to apply is through a professional soil test. Misapplication can be detrimental to a tree’s health. This test provides the current “water pH,” which measures the active acidity of the soil solution.
The soil test also measures the “buffer pH,” which indicates the soil’s capacity to resist a change in pH. Soils with high clay or organic matter content have a high buffering capacity and require significantly more lime to achieve the desired pH change than sandy soils. The buffer pH value is used by the laboratory to calculate the exact amount of lime required to reach a specific target pH, often 6.5.
While visual cues like general yellowing of leaves (chlorosis) or stunted growth may be observed, they are ambiguous signs. These symptoms are often caused by nutrient deficiencies, but applying lime based on symptoms alone is risky. For example, chlorosis is frequently a result of iron or manganese deficiency, which is commonly caused by soil that is already too alkaline (high pH), making lime the wrong solution.
Tree Species Sensitivity to Soil pH
The question of whether lime is “good” depends entirely on the tree’s genetic preference for soil pH. Trees can be broadly categorized into acid-loving and alkaline-tolerant species, and changing the pH dramatically away from a tree’s preference will cause health problems.
Acid-loving species, which thrive in a lower pH environment (typically 4.5 to 6.0), include varieties like Pin Oak, Red Maple, River Birch, Rhododendron, and Pine. If lime is applied to the soil around these trees, raising the pH above 6.5, they often suffer from nutrient deficiencies. The most common ailment is iron or manganese chlorosis, where these micronutrients become insoluble and unusable in alkaline soil.
Conversely, a large number of trees are naturally alkaline-tolerant and perform well in neutral or slightly alkaline conditions, up to a pH of about 8.2. Examples of these species include American Elm, Hackberry, Honey Locust, and Chinkapin Oak. These trees are well-suited to urban environments where concrete and building materials often leach alkaline substances into the surrounding soil. For these species, lime is only beneficial if the soil is found to be excessively acidic.
Safe Application Guidelines
Once a soil test confirms a low pH and recommends a specific amount of lime, the correct application technique is necessary. Lime is available in two main forms: agricultural lime (calcitic lime), which primarily contains calcium carbonate, and dolomitic lime, which also contains magnesium carbonate. The choice between the two should be based on whether the soil test reveals a magnesium deficiency alongside the low pH.
For ease of spreading and more even distribution, granular or pelletized lime is preferred over fine powdered lime. For established trees, the lime should be spread uniformly across the root zone, which extends out to the tree’s drip line and sometimes beyond. It is important to avoid piling the lime directly against the tree trunk, as this can cause bark damage.
Lime moves very slowly through the soil profile, often penetrating only a few centimeters per year when applied to the surface. For this reason, surface application to established trees is considered a long-term soil maintenance strategy, relying on rainfall or irrigation to gradually wash the material into the root zone. If the tree is newly planted, the lime can be lightly incorporated into the top few inches of soil to speed up the reaction time.