Westland Hydrangea Colourant is a salt which changes pink hydrangea varieties to blue, and darker pink varieties to mauve blue. This colourant easily absorbs nutrients and nitrogen for greener leaves. For best results, use with plants growing in acid soils. If the soil is naturally alkaline, use ericaceous compost or double the application rate. This product is a combination of iron and aluminium salts that acidifies the soil to produce blue flowers and contains potassium to boost flowering.