Several cultivars of flowering hydrangea shrubs have the capacity to produce blooms in either pink or blue, based entirely on the pH level of the soil surrounding their roots. Blue blooms will come from an acidic soil with a low pH, between 5.0 and 5.5, according to the University of Rhode Island. Maintaining the production of blue flower heads over time in a naturally alkaline, neutral or even mildly acidic soil will likely require the ongoing application of acidic soil amendments and careful use of low-phosphorus fertilizers.
Conduct an analysis of your garden soil to determine its baseline pH level. Test kits can often be purchased at your local garden center or online. Knowing how much you will need to lower the pH level for blue hydrangea flowers will help you determine how much iron sulfate to use and how to apply it.
Determine your soil texture. Fast-draining sand and loam soils will require considerably less iron sulfate to lower the pH than the same volume of loamy and silt-laden soil. Use 4.8 lbs. of iron sulfate for every 100 square feet of sandy soil to lower the pH by one point, from 6.5 to 5.5 for example. Use 19.2 lbs. of iron sulfate to lower the pH by one point for the same 100 square feet of loamy or silty soil.
Till the appropriate amount of iron sulfate into the planting soil to a depth of at least 6 inches before planting your hydrangea shrubs. Alternatively, top dress established plantings, casting the iron sulfate over the soil surrounding the shrubs and till in lightly to mix with the top few inches of soil, being careful not to disturb any larger roots.
Water the soil immediately after applying the iron sulfate to make the soil fully wet at least 6 inches down, but not so much that the soil remains soupy wet for more than a few minutes after watering. Water regularly and deeply to leech out any excess salts left as a trace from the soil amendment or fertilizer.
Fertilize your blue hydrangea shrubs with a low- or no-phosphorous formulation to prevent the phosphorous from breaking down into aluminum and increasing the acidity of the soil too much for the plants' health. Look for a fertilizer product with a guaranteed analysis of 15-0-15 or 13-0-20 if you are forcing your hydrangeas into the blue spectrum by using iron sulfate.
Monitor the effectiveness of the soil pH alteration by the color of resulting hydrangea blooms produced from spring through early fall. Alternatively, repeat the soil test to determine the need for and amount of, repeat applications of iron sulfate.