Could be minerals from your water or the soil. Terra cotta is very porous and wicks water away from the soil, where it then evaporates and leaves the minerals behind.
this post was submitted on 13 Oct 2023
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Second that. Minerals from evaporated water.
Efflorescence: White Powder On Pots And What It Means
Have you noticed that your older terra cotta or clay pots have crusty or powdery white deposits on them? You are definitely not alone! A ton of our members ask us what it is and if it’s harmful to their plants or them. The long and the short answer is: No. In fact, it is kind of a look that is becoming popular, and you will see some store bought pots with factory made “patina.”
The water that most people give their plants will have minerals, such as calcium, in it. Fertilizers, as we know, contain minerals as well. If there are more of these minerals in the soil than the plant can use, they can sit there and accumulate (which can burn the roots if not flushed) or they can go through pots if the pots are made of a porous material. Terra cotta and clay pots are permeable, which means that water and minerals can actually pass through them. As the water evaporates, it will take this surplus pf minerals with it to the outside of the pot, where they dry out and form this white powder/crust that we all know. You can think of it as this crust blooming, hence the word efflorescence.
Thank you! :)