Effect of soil acidity. The flower of a hydrangea (Hydrangea macrophylla) that has been grown in neutral soil (pH approximately 7). This is shown by the purple colour of the flower. In acidic soils the flowers are blue and in alkaline soil they are pink. In neutral soil both colours are seen together, making them look purple. The difference in colour is due to the way aluminium ions in the soil react with the plant's pigments.