Hydrangeas

The thing you're probably thinking of is an acid feed to keep blue hydrangeas blue instead of mixed pinkish-lilac, because when hydrangeas are grown in alkaline soil, the blue colour is lost. So depending on the colour of your hydrangeas, you may or may not want to use this. Otherwise, just something like Growmore or phostrogen or miracle gro general purpose.

Bury a few old nails in the soil round the Hydrangea. The iron is supposed to help the blue colouring. It seems to work.

but there are some cultivars that are true blue or true pink no matter what the soil. so it depends on which one you are growing.

for the ones that are pH sensitive you can get sequestine which is an iron tonic that you water on to the soil. you use it one camelias and rhododendrons too.