Osmocote contains a balance of nitrate and ammoniacal nitrogen which will have a slight acidifying effect to the soil. Here are a couple hints to keep your hydrangeas blue. It is actually the availability of aluminum to the plant that makes hydrangea flowers blue. Aluminum is more available in acidic soil. Avoid applying excessive phosphorus from fertilizer (high middle number in the analysis) because phosphorus can bind with aluminum and create an insoluble form. Keep growing, Marianne Binetti.