I am about to plant some new hydrangeas. What should I use to amend the soil with to make the hydrangeas blue?
To obtain a blue flowering hydrangea, the soil around the roots of the plant must contain aluminium.
For this aluminium to be available as a nutrient, the pH of the soil must be very low / acidic (around 4.5-5.5).
Note that an acidic soil may be detrimental to the growth of surrounding plants that may prefer a more neutral soil pH. It is therefore sensible to group acid loving plants together.
When added to the soil, aluminium sulphate can serve to provide a source of aluminium and create a more acidic soil. Aluminium sulfate is a water soluble chemical compound with the formula Al 2 (SO 4 ) 3 .
Creating suitable soil chemistry with aluminium sulphate may be a process that takes two or three or more years to achieve success.
Additionally, there are some important considerations:
- aluminium sulphate used as an agricultural soil amendment, has the potential at stronger doses to burn the roots of young plants;
- A fertiliser low in phosphorus and high in potassium is helpful in producing a strong blue color (superphosphates and bone meal should be avoided when trying to produce blue);
- Rich compost will contribute to creating an acidic soil, however once the compost has broken down the immediate effects are significantly reduced.