There are a number of macro and micronutrients needed to stimulate and ensure proper plant growth, most of which are found in soil. A few of these include nitrogen, potassium, phosphorous, and sulfur. Sulfur is necessary for protein and chlorophyll production, enzyme and vitamin activity, and root and seed production and growth. Sulfur is largely supplied to the plant through rainwater.
There are other nutrients that can affect a plant’s growth as well. Aluminum, for example, is not one of the micro or macronutrients required by plants, but still has the ability to affect hydrangeas. When aluminum is present in the soil, hydrangeas tend to grow blue flowers, while an absence of aluminum will often result in pink hydrangea blooms. Aluminum in soil typically indicates lower pH levels or more acidic soil. A soil with a pH between 5 and 5.5 will typically yield blue flowers.
True Blue Aluminum Sulfate is one method of adding aluminum to the soil and lowering pH levels. By providing both sulfur and aluminum, gardeners can ensure their plants have high chlorophyll levels and acidic soil, leading to deep green leaves and brilliant blue flowers. Aluminum sulfate should be applied to hydrangeas shortly before they go into dormancy, and potentially applied again during the winter and early spring. It should be applied on the plants’ drip line, away from the base of the plant, to have the greatest effect