Hydrangeas need well-draining, consistently moist soil. Soil for hydrangeas is a balancing act--it should be moist, but not wet, nutrient-rich but not heavy. The best way to achieve good quality bedding for hydrangea plants is to enrich the soil with loam or hummus. If possible, choose a planting site that has shade in the afternoon, or dappled shade throughout the day, to keep the soil from becoming dry and hot.
Hydrangeas can thrive in acidic or slightly alkaline soil, and some bigleaf or panicle hydrangeas actually change color when their soil pH is changed. The color-changing varieties typically grow in shades of pink to blue-white, green, and many pink hydrangeas do not change color. In those plants that do change bloom color, soil pH affects the amount of aluminum absorbed by the plant, which results in a shift from pink to blue. Hydrangeas planted in alkaline soil are more likely to remain pink or lavender, while hydrangeas planted in acidic soil and provided with aluminum may turn blue or lilac.