Alum (Aluminium Sulphate)
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Always check your soil pH before applying Aluminium Sulphate using a pH soil test kit or probe.

Aluminium Sulphate (Alum) is used to acidify the soil and under hydrangeas where the blue flower is desired. The blue colour is induced by aluminium. As the soil pH determines the colour of sepals, it is important that the pH of the soil or potting media is determined and adjusted prior to the growing period. Any pH between 5.5 and 6.0 is acceptable during plant growth, but when forcing begins the soil pH for a crop of blue sepals must be 5.5 or lower

Directions for use: Dissolve 30 grams (approximately 2 heaped tablespoons) in 5 litres of water. Apply over 5 square metres of soil, or to the soil surrounding four full size Hydrangeas
Commence applying immediately after Winter pruning, repeat applications every two weeks until flowering is finished

Note: Soil pH can be quickly lowered by adding Sulphur. The acid pH can be maintained by regular use of Alum.

Analysis
Aluminium (Al) present as Sulphate 8.7%
Sulphur (S) present as Sulphates 15.5%