Aluminium Sulphate (ALUM) is used to acidify the soil under hydrangeas whose flowers must be blue. The blue colour is induced by aluminium compound and aluminium ion will be taken up by plants only under acidic conditions. Hence 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.

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2.5kg