It says in a text about fertilising hydrangea, that the soil needs to be acidic and contain sufficient plant-available aluminum. What does this mean?
All soils contain aluminium, particularly clay soils. But it is never pure - it is 'locked up' as part of a chemical compound. In some cases the plant can use the aluminum but often it is in an unusable, I.e. unavailable form. I am not a chemist but I do know that if your soil is acidic your Hydrangea is likely to have blue flowers; if the soil is alkaline then the flowers are likely to be pink. In my experience it is quite uncommon to have insufficient plant-available aluminium.
