Aluminium Sulphate is used to acidify the soil.
One use can be it brings out the blue colour in hydrangeas. The blue colour happens when the aluminium compound and aluminium ion are taken up by plants. The soils pH will determine the colour of the sepals (5.5 and lower will bring blue sepals)
Soil pH can be changed quickly by adding Aluminium Sulphate.
The acid pH can be maintained by the regularly using Aluminium Sulphate.