Most horticultural species grow favorably in soils at or close to neutrality. However, ferns, azalea, rhododendron, and camellia, for example, require an acidic pH between 4.5 and 5.5, whereas asparagus, spinach, and cacti and other succulent species prefer mildly alkaline soils, to about 7.5 on the pH scale. Hydrangea tolerates a wide pH range, but the flower colors indicate the soil pH—the flowers become blue in acidic soils, pink in alkaline.
