most likely the minerals in the actual pot then. Its called efflorescence. On buildings it gradually erodes the brick so you occasionally wash it off. Totally harmless to people and presumably plants. On a pot I wouldnt think there is anything to worry about as it would take years to damage it enough to make it unusable, so either leave it or wash it off if you find it unsightly.
most likely the minerals in the actual pot then. Its called efflorescence. On buildings it gradually erodes the brick so you occasionally wash it off. Totally harmless to people and presumably plants. On a pot I wouldnt think there is anything to worry about as it would take years to damage it enough to make it unusable, so either leave it or wash it off if you find it unsightly.
I have to say , it doesn't bother me. I was more concerned it was harming the plant. I'm more than happy to leave it be. As my dear late Mother would have said " Never trouble trouble, until trouble troubles you"
It would be caused by a either a buildup of salts in the soil from plant food or your acid water drawing soluble minerals out of the terracotta. If the soil is staying damp the water carrying the dissolved salt pushes through the wall of the pot and evaporates leaving the salt behind. The salts and minerals attack the wall of the pot too which is why it's eroding away.
With brickwork the staining occurs because the salt reacts with the air and becomes insoluble so it doesn't wash off in the rain.
If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
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As my dear late Mother would have said " Never trouble trouble, until trouble troubles you"