Cumbria,
United Kingdom 
i have terracotta plant pots outside with a white residue on them which i can't remove. have tried spraying with moss/algae remover and scrubbing with kitchen cleaning liquids.
is the residue some sort of a reaction with the weather and can anyone suggest a way of removing it?
thanks in advance.
24 Apr, 2015
Answers
It is the same sort of Lime Scale as one gets on kitchen utensils in hard water areas.
24 Apr, 2015
Rain water is slightly acid and will disolve the lime scale. If you have a rain butt suspend the empty pot on a cord in the rainwater for a couple of days or so then scrub with a scrubbing brush. This is how I routinely clean my empty pots.
24 Apr, 2015
Previous question
It is salt build up. Remove all soil and plants and fill a bucket with 50% water and vinegar and scrub off the salts. it may take 2 attempts to remove it all. You can seal terraotta with various glue's to solve the problem. It is quite harmless.
24 Apr, 2015