Effect of soil acidity. Flowers of hydrangea plants (Hydrangea macrophylla) that have been grown in soils with different acidity. At left is a flower from an acidic soil (pH less than 7) with a characteristic pink colour. At right the flower was grown in alkaline soil (pH more than 7) resulting in blue flowers. At centre the flower was grown in neutral soil, so both colours are present combining to make purple. The difference is due to the way aluminium ions in the soil react with the plant's pigments.