Certain common plants require acidic soil (pH under 7) including:

Azaleas and rhododendrons, blueberries, butterfly bush, blue-flowered hydrangeas,
camellias, dogwoods, heathers, magnolias, pines, spruces, and yews. While these plants may survive in soils with a higher pH, they grow and bloom best in soil with a pH of 4 to 5.5.