"Jim S" wrote in message
...
The colours are the opposite way to litmus paper so you are
looking towards an acid soil.

Acid turns pink litmus blue, alkali turns blue litmus pink.

I am trying a mixture of iron and aluminium roofing nails, but I did put
them there before the winter.

That's one way of reducing the acid content of the soil perhaps for acid
plus metal = salt plus water plus hydrogen and the salt will be leached out
by rain.

Before you do the next thing, check the ph of your soil. If it's good and
acid, do not bother any more.

If you have an alkaline soil, a young pink hydrangea and you would like it
blue, dig it up. dig a deepish and wide hole, put plenty of horse manure
compost, peat and ericacious soil in the hole get rid of as much soil from
the plants roots as is reasonable and carefully replant it. Watering with
the dregs of your teapot and mulching with all your neighbours' spent tea
leaves would not hurt.

The plant might stay blue for a few years.

By the way, it's not a good environmental practice to bury aluminium
especially if it's compounds might get into a pond with fish in it.

Geoff