The blue colour of flowers of Hydron macrophylla as opposed regular pink colour is associated with the presence of which element in soil?

The blue colour of flowers of Hydron macrophylla as opposed to the regular pink colour is associated with the presence of Aluminium (Al) element in soil.
Hydrangea macrophylla is a species of flowering plant in the family Hydrangeaceae, native to Japan. It is a deciduous shrub growing to 2 m tall by 2.5 m broad with large heads of pink or blue flowers in summer and autumn.
Colour variation in hydrangeas is due to the presence or absence of aluminum compounds in the flowers. If aluminum is present, the colour is blue. In general, more acidity makes hydrangeas turn blue, less acidity promotes pink. This is caused by a colour change of the flower pigments in the presence of aluminium ions which can be taken up into hyperaccumulating plants.