On 10/10/2011 08:28, Jake wrote:
On Mon, 10 Oct 2011 07:30:04 +0100, Chris wrote:

I'm never clear on whether it's iron or aluminium that makes
hydrangeas blue. I know that an acid soil is necessary, and in such
soils both aluminium and iron are available. Adding either iron
sulphate or aluminium sulphate to the soil will acidify it, releasing
both aluminium and iron, so it's not clear which actually does the
biz.

AIUI, sequestrine doesn't actually acidify the soil, but provides iron
(and manganese) in a form that's soluble at high pH and that the plant
can absorb. But sequestrine doesn't provide soluble aluminium. If
sequestrine is sufficient to turn hydrangeas blue, then it suggests
the key requirement is iron. But does anyone know for certain that
sequestrine works? Alternatively, does anyone know the definitive
answer as to whether it's iron or aluminium that's necessary for blue
hydrangeas?

Everything I've ever read says that hydrangeas need both an acid soil
(pH 5.2-5.5 is ideal) and aluminium to be blue. Aluminium sulphate
will provide both but it has its drawbacks - use too much and it'll
damage the roots and it can be very damaging to young plants (under
2-3 years old).The recommended approach seems to be to use aluminium
sulphate in one year and sequestered iron in the next.

Cheers, Jake
================================================== ==========
URGling from the less wet end of Swansea Bay where it's ....
wet!

www.rivendell.org.uk

I think ammonium sulphate is more widely available and, according to
some, also works on hydrangeas. Judging by the smell, the ammonium part
seems to be released when mixed with water.