Gardeners often wonder if bone meal should be used on hydrangeas to boost blooms. Bone meal is a popular organic amendment, traditionally used to support strong root systems and prolific flowering. However, the unique nutritional requirements and pH sensitivity of hydrangeas mean that the recommendation for this product is not straightforward. Understanding bone meal’s chemical makeup and how it interacts with the soil is necessary before application.
Understanding Bone Meal’s Composition
Bone meal is an organic byproduct created by steaming and grinding animal bones into a fine powder. It is not considered a complete fertilizer because its nutrient profile is heavily skewed toward phosphorus (P) and calcium (Ca). The typical N-P-K ratio is low in nitrogen and contains no potassium, often ranging from 1-13-0 to 4-12-0. The high phosphorus content is essential for robust root development, cell division, and flower formation, as it aids in energy transfer within the plant. Bone meal is also a rich source of calcium, which strengthens plant cell walls and promotes sturdy growth.
Hydrangeas’ Primary Nutritional Needs
Hydrangeas require a balanced diet of macronutrients to thrive. Nitrogen is necessary for healthy leaf and stem growth, contributing to a lush, green appearance. Phosphorus supports energy transfer and is crucial for developing strong roots and abundant blooms. Potassium helps regulate water movement and enhances the plant’s resistance to disease and stress.
The distinguishing factor for bigleaf hydrangeas (Hydrangea macrophylla) is the direct influence of soil pH on their flower color. This effect depends on the availability of aluminum ions in the soil. In acidic soil (pH 5.2 to 6.0), aluminum is readily available, resulting in blue flowers. If the soil is alkaline (pH 7.0 or higher), the aluminum becomes chemically bound and unavailable, causing the blooms to appear pink.
The Direct Answer: Bone Meal’s Effect on Hydrangeas
Bone meal is generally not the best choice for hydrangeas, especially if the goal is to maintain blue flowers. Although the high phosphorus content is beneficial for flowering, the high calcium content is the main concern. Calcium acts as a liming agent, raising the soil’s pH level and making it more alkaline. This shift toward alkalinity directly counteracts the conditions required for blue hydrangeas, pushing the flower color toward pink.
An increase in soil pH causes the aluminum needed for blue coloration to become chemically locked up and inaccessible to the plant. High phosphorus levels can also interfere with the plant’s ability to absorb aluminum, even if the pH is borderline. Regularly applying bone meal to a blue hydrangea will likely cause the blooms to turn pink and may lead to deficiencies in other micronutrients less available in alkaline soil.
Bone meal should only be considered as a supplemental source of phosphorus after a professional soil test confirms a severe deficiency. It is appropriate only if the gardener is actively trying to grow pink blooms or if the hydrangea variety is not pH-sensitive, such as Hydrangea paniculata. For bigleaf hydrangeas, a balanced fertilizer with a neutral effect on pH is a safer choice.