its the stuff you use for removing limescale on taps and things its basically acid so its good for finding out if something is acid or alkaline you can also use ph down you can get that from aquatic shops
if it bubbles up when you put it on the white stuff you then know its alkaline but don't use it in the pot
if you put a perforated bag of peat or acidic compost in a bucket of rain water it will reduce the ph of the water to around 5 you can then water with that until the white stuff disappears might take awhile but its better than changing it instantly
atb
tim
